Hi Uncle Doug, this is probably the fourth time I have watched this and I think I finally have it. lol I am building a Fender 6G3 Brown Deluxe and I want to make it adjustable bias. I do hope that Rusty is still doing okay for such a senior doggy. We lost our boy Maxi two weeks ago as he was not doing will at all. He was thirteen and he had gotten to the point to where he had no quality of life anymore. I took him to the vet and made the decision to let him go. She was very compassionate and he took his last breath in my arms. I still cannot talk about it without getting very upset. So I hope that you and Rusty have plenty of good times left together.
Best of luck with your adjustable bias modification. I looked at a schematic and it appears that you will be replacing the 22K bias resistor with a resistor (probably around 10K Ohms) and a pot (around 20K). I'm sorry to hear about Maxi. We had a Collie named Max who presented exactly the same situation about a year ago, with the same result. I completely understand and share in your grief. Losing a family pet is almost as traumatic as losing a family member. Rusty and I send our best regards to you all.
Another great video Doug! Quite often I get Fender Princetons in for repair...I try t always install matched pairs of 6V6's to make life a little easier, When Fender came out with the Silver Face line, they laid an egg with it...That was the series of amps that earned Fender a bad rep at the time. They used the bias balance circuit rather than a true adjustable bias...I remember doing modifications on these amps to bring them back to the original Black Face design...Keep up the great videos!!!
Hi Uncle Doug! I am a robotic technician and a weekend rock star Ha!!! I own 4 tube amps and wanted to learn how they work Your passion is contagious!!!! Thank you so much for sharing your knowledge to the world Yannick
As I'm watching this video, I noticed your up to 931 subscribers. Certainly if you make it to 1000, it will be a milestone....your on your way to being a RUclips legend!
Uncle Doug ! Your projects are priceless works of art and technology, your tutorials are incredibly informative and interesting and you are a gift to all us tube amp heads in the world. I want to thank you for all your videos that I have watched and re-watched over and over again with great interest and enthusiasm. Peace ! JM
UD, your suggestion to bend a piece of coat hanger to attach a bias pot to the power transformer screw post -- was brilliant! I have now installed a bias pot in my 1966 BF Princeton Reverb thanks to your great video. The original bias spec is 27K but the resistor measured 25.2K ohms. I was not able to get the PC below 27 mA on any pair of 6V6s, so the idle PD was over 11.5 watts. Once I installed the 10K linear bias pot in series with the 25K resistor, I increased the resistance and reduced the PC to 19.7 mA and the PD to 8.4 watts. The amp sounds awesome and the tube life should be normal now!!
Wow....thanks, DB. You made my day. I'm glad to hear that the coat hanger pot mount worked so well. It really is a very simple, effective way to solve the problem. It also sounds like your biasing job was a total success, with both excellent tone and much longer tube life ahead. Your experience provides the best possible reason for making this conversion.....and your next biasing job should take all of 3 or 4 minutes to complete. Congratulations !!
Uncle Doug I did the bias pot wiring in an alternate way. But first, I used a Fender style 10K linear pot. The kind that is normally mounted in the chassis with a mounting hole cut, however I used your coat hanger mount as a “bracket” and avoided drilling the hole. Back to the wiring. I connected the 25K resister to the left terminal of the bias pot. The bias pot’s back and terminals are facing up. I added a jumper wire from the left to the center terminal. The right terminal goes to the intensity pot’s left terminal. This way if the center wipe fails the one resistor is still there, thus this avoids adding an extra resistor. Thanks again for your video that inspired me to add grid biasing to my Princeton!
Uncle Doug! Thanks so much for these videos! I've added your bias circuit to several 6V6 push-pull amps, including 2 Princetons (one a '64, and the other a '69-70 Princeton Reverb), but I went one step further on the last one. I made a pot bracket for the next transformer stud and mounted a 250 ohm Ohmite pot to balance the grids themselves, because of varying output transformer impedance either side of center tap, and of course a "matched pair" these days is only apparently matched on a tube tester rather than graphed as they were a few decades back. One further easily reversed mod I've been doing is placing a 1% 1 ohm 2 watt resister (got a 100 of them from vendor on Ebay) in each cathode circuit, much easier current measurement when replacing tubes! I suspect the 250 ohm pot could be replaced with a 50 or 100 ohm pot too, probably closer to what's actually needed. Geez, right on the money current balance and setting the 70% max tube rating. I thought you'd appreciate this bit of OCD amp service!
camera decided not to link to computer, pictures to follow ... further think through: 250 ohm pot did nothing. thorough rummage through parts lead me to 150K 1% resistors to replace the 220k grid resistors and then used a 100k pot (finding them all to be +/- 10% with most being about 90k, finding ONE to be 110k and using that)this gives CLOSE to the 220k original grid resistors of which Fender used +/- 20% so the new ones are certainly in the ball park. I didn't check what the originals actually were. THIS actually works well with minimal fiddling and can balance unmatched tubes, but given what these amps now sell for and matched tubes sell for 50 bucks on Ebay, its not worth it not to use them. I wonder if I could match a 6W6 with a 6V6 ...
Hi Doug, thank you for getting back to me. Yeah there is a schematic for my amp on the net for the Blues Deville 4x10 amp. Well yeah everything that I've read on the net says to do it like that. I always have reservations about a lot of things you read on the net. I just don't want to blow anything like my power transformer or valves up doing something wrong. I was having some time ago a problem with my 1965 Fender Blackface Bandmaster amp red plating. After exhausting every avenue on what the problem could be, I finally took out the bias pot only to find that it was burnt and ruined. Replaced it and have never had a problem since. I did make the mistake of trying to use KT66 power valves in the Bandmaster and it was playing fine for a couple of days, then I went to turn it on the other day a nd it blew a fuse and took out one of the screen resistors. I replaced it and put 6L6's back in and It's back up running. I won't be trying that again. I just thought that maybe you could look at the Deville 1993 schematic for me as I'm not good or experienced enough to read these schematics with any competency. Do you think that I should use a 50K linear pot or a 25K linear pot. Which wound be best. It's just that to install a pot in this PCB board, it's going to be a pain in the butt trying to get that board out to solder these wires in this very small space. Thanks Doug. I wish I had your knowledge on these things. Your videos never cease to amaze me. Thanks so much. Michael Newell from Niagara Falls, Ontario, Canada P.S. There's just one last question that I have for you. I presently have a 100uf@ 100 volts in my bias supply of my Fender Blackface Bandmaster. It calls for 25uf@50 volts. Is that value that I am using ok or should it be replaced with something smaller like a47uf @ 100 volts? What does that bias supply electrolytic cap actually do?
The 25K linear pot will be better, Michael. I avoid working on PCB amps for just that reason (among others).....inaccessibility. The bias cap smooths out the rectified DC for the output tube grids. You are better off using what the schematic suggests: 25uf @ 50V. The circuit designers knew what was best.
Uncle Doug, I continue to watch your videos all the time and am learning so much. I wasn't sure which video to post this comment on since I couldn't find a specific soldering tutorial video. Re: the amp I asked about the questionable-looking capacitor on a week or two ago, this is my first hands-on attempt with desoldering and then soldering in a new component. I've already removed the capacitor in question but in doing so already learned more which leads to this question. At a minimum I have two components to replace. I'm hoping to do this without catastrophe, burning up components, or damaging nearby components with the iron, and so I am practicing and intend to continue practicing a ton before then.....on a secret resource that I never had the heart to take to the dumpster.... The story is almost too tragic (and long) to print but I got paid very well by the insurance company and it is what it is so here is what I am practicing on. For about 6 years I was the steward of a high powered Tweed Twin clone that an old pal (who built these amps) had built around 1992. It became Danny Gatton's amp in the last year or so of his life. After Danny passed it went back to the builder. When I got out of school, it was a gift from a combination of the builder (who could have charged way more), my parents (who supported me), and something I tended to keep under-wraps but cherished for its history. Its last hoorah was at the Ryman in April of 2010 with Vince Gill (who'd graciously given me the chance to come out to play together) so it had a good run. After the evening, it got trucked back to Soundcheck, where it was submerged under the Cumberland River for 5 days beginning on May 1st. In the aftermath of the flood, there were a few survivors. The first really good amp I ever bought when I was about 19 years old (you did a video on the exact same model, serial number 00060. Mine was 00353) had a cover on it that my mother had made with a sort of vinyl-material. Somehow that cover was the difference between that amp making it back and Danny's Twin. Everything went to dry out for about 18 months after coming out of the triage area. In August of September of 2011, I was at my guitar builder friend's house (who'd let it all dry out in his workshop) for his daughter's birthday party, and assuming everything was toast (all insurance claims were settled) we tried turning on the old one. Of course I know so much more now than I ever did then. Surely we should have at least brought it up slowly on a Variac or something, but expecting nothing, we turned it on and low and behold, the tone was incredible. Within days it was in the shop for a restoration, and it lives to this day (I lost the original speaker but not transformers....actually I still have the speaker but never got it re-coned). Hoping the Danny amp might have made it out alive too, I very excitedly called the builder and informed him of this Fender miracle. I don't think he held out much hope for the Twin either. A day or two later I finally worked up the nerve to turn it on....all it made was one big "pop" and then nothing. The builder had moved onto other builds and didn't want it back, 5 or 6 years later Todd Sharp said they could try to rebuild it, but the settlement had already been spent and I didn't hold out much hope or want to pour the cost of a complete rebuild into trying to restore whatever remained. In 2021 -- 11 years later --- I finally got it out of my friend's garage and brought it back to the house and stuck it in my basement. I could never throw it out, even though my Dad kept telling me to get it out of my hair. That amp is how I practiced getting a chassis out of the cabinet and how I've begun to practice desoldering for starters. I've gotten a few things out and soldered back in and learned along the way that those eyelet boards have a certain amount of mass at the eyelet. I've been watching that Pace soldering tutorial for years (literally, since I built that 1176 compressor out of a kit). Here's what I'm finding out. With heat-sink clips on and applying a little bit of fresh solder, with the biggest chisel tip I have for the iron, I can get solder melted in maybe 3-4 seconds at the highest temperature on my Hakko station, and then use the solder sucker for the first removal. Then I can heat again to remove the sweat-joints and remove the component (I got this sequence from a Rob Robinette post on the TDRPI forums so if I'm doing any of this wrong, at least this was my attempt at understanding the process). Then I've got to work a bit more with desoldering wick (SuperWick) to clear the eyelet hole completely. I'm doing this at about 875 F. If I don't have the iron that hot, it takes too long for the solder to melt. I burned up a resistor the other evening practicing and realize all the soldering gurus say to get in and out in 2-3 seconds so the fact that it takes me more already makes me nervous but so far I've only torpedoed one resistor in my efforts. Of course I realize the real problem is relative thermal mass. Those brass eyelets that go through the board, combined with anywhere from 2 to 4 component leads joined together in the same eyelet. Applying some fresh solder to the joint before removal seemed to improve things in today's practice-run. Here's the real question. Is there a standard recommended order of events and approach when working with eyelet boards for component swapping? I almost felt like giving up, taking a trip to see the builder or just surrendering to my local tech shop once I realized that with 3-4 things joining up in the same eyelet, it seems I could be jeopardizing everything connected at that point if I don't go into this knowing what I'm doing. Is it bad to heat twice instead of just once? In other words, to heat and suck, then heat and pull out the component? Then invariably I have to heat and use wick to clear the hole of remaining solder. I haven't had an eyelet pop out or noticeably disintegrated anything on my practice board yet, but the main question is if this is a normal series of events when working on eyelet board construction. Based on the Pace soldering videos, I'm completely in over my head for this taking longer than 2-3 seconds total. But I'm wondering if that same advice applies with tube amp eyelet boards, as long as heat is not applied to a component for too long. Is the start/stop/start/release thing okay? When I burned up the resistor on the practice board, I knew it because it measured about 5-6k lower until I kept the iron on the lead for way way way too long (and I knew it was too long, maybe anywhere from 5-7 seconds) when re-soldering back into place. My hesitation to surrender and give up is that I'm afraid if I don't learn now then it might not ever happen. But between using a disgusting relic of the Cumberland River to practice on and a lot of conscientious care, I wanted to ask someone and hope this note was not too long. (Of course it was but hope it was at least not boring or too heartbreaking. I figured Danny liked working on his cars. Maybe he'd approve of me using the amp to practice working on repairs, so that I don't become a drunk chimpanzee when the lights are up and it's time to do this for real on an otherwise good amp). (*I am editing this a few hours later to add that I was using a medium sized chisel tip which would have been large for a pedal or circuit board....but I thought increasing the size of the tip might help so I have ordered a set of genuine Hakko tips for my station, with a couple much larger tips than I currently have. When they arrive, I'll try things out with the test-board again to see if I can't get better flow with faster times, and perhaps a lower temperature).
Greetings, Sean. I think you are obsessing too much on the soldering process. I personally use a soldering gun, probably at the same high temperature of your iron, and simply heat the eyelet until the solder melts and then, with needle-nosed pliers, pull the component lead out of the hole.....leaving the solder in place. Then, when I have the new component leads trimmed and bent to fit, heat up the eyelet until the solder melts and push the new lead into the hole. Once the solder hardens, I pull on the lead to be sure it is soldered in place.....and move on. If there is a shortage of solder on the eyelet, I may add some fresh solder to make a dome. Both processes, removal and installation, take about 5 to 10 seconds each. Resistors and capacitors are resistant to heat, and I have never damaged one during the soldering process. Solid-state components like transistors are heat sensitive and require heat-sinking of the leads. I hope this helps. Good luck with your project.
P.S. Having just watched several of your YT videos I can honestly say that if your soldering skills ever approach your guitar-playing skills, you will be immensely skilled at soldering :)
@@UncleDoug Thank you so much, both for your reply and for the kind compliment about my guitar playing :-). That certainly helps a lot and makes a lot of sense. In fact, I just practiced your technique on the flood-amp (just removing and reinstalling the same component) and was able to confirm with my trusty Fluke that the resistors measured the same both before and after. There is one other quick thing I've been thinking about.... On Fender wiring-diagrams where there are a few eyelets connected under-the-board, are there ever any precautions or things to watch out for to avoid having a connection come loose when swapping out a component on top? It's not an issue with my current project, as neither of the spots I'll be working on are connected....but it is something I wonder about from time to time. Thanks as always.
@@SeanWeaver I guess it could happen, Sean, but in many, many years of working on Fender eyelet boards, I have never experienced the problem. Again, best of luck with your continued self-education endeavors.
@@UncleDoug Thank you so much. That you take the time to respond to comments and have answered all of my questions (even the paranoid/obsessive ones) means and lot. I'm so appreciative and continue to learn so much from each of your videos.
Excellent. Clear, concise information that is coming in handy with my current build. And the added commentary here was a great benefit as well. Thank you for all you do Uncle Doug.
Wow, just read the footnotes on protecting the bias circuit from pot failure. I haven't experienced this but I get the point. A recent prototype build of mine is not behaving as expected; this may be the answer. Thanks as always.
@@UncleDoug Thanks, This is a test bed project. I'm experimenting with selectable negative feedback, tone stack modifications, preamp tube placement, and some control location alternatives. The first pass sounded good, but the 6V6s tubes were running very hot, and the amplifier was drawing 1.5 amperes at 120 volts. Also, grid current seemed high to me, but no red plating. Yes, I may have bitten off more than I can chew. Be well, HR
Excellent video, *Doug*! As usual fantastic explanation and leading all straight to the point...:) Here are my two cents: If you gonna do this, you may want to make the negative grid bias rock solid, as you already plan to desolder some components and the amp is upon your workbench anyway. If you connect just the wiper of the bias pot to the grids, what happend when this wiper becomes loose or does not making any good connenction inside the pot anymore.... You know this: you´ll burn your tubes. Because the grids then have a bias voltage near zero....oops... To prevent this put a single resistor, may with the same resistance or slightly higher, in *parallel* to the wiper pin and the negative-voltage pin of the filter cap. *Not* ground! So if the pot´s wiper loose its contact to the resistance-path that added resistor provides lowest grid bias possible to the tubes. So they will survive and all you have to do is change the pot - not the tubes as well :) I´ve serviced a lot of Amps with blown output tubes due to a defective grid bias pot. Some times even the OPT was melted, or the pri. winding was shorted. Well known for this are the ´braun´ CSV 60 Amps with PL 500 output tubes just because the tubes are so rugged, they can stand zero grid bias for minutes while cooking the pri windings and even set the OPT on fire. Have fun, and thanx for this great video!
You have brought up a good point, TC. We have all seen pots in which the wiper made poor or inconsistent contact with the perimeter resistance; which, in this case would eliminate the bias voltage and PC would soar. It does appear that a 10K resistor connecting the wiper to the negative input from the diode would provide an alternate route for the maximum grid bias voltage (= minimum PC) to reach the tubes in case of wiper failure. Please acknowledge that I have correctly interpreted your recommendation and I will test it in the PR circuit.
This is by far more true to the newer pots, the old ones found in vintage Amps are much more reliable than today´s brothers. I would suggest 15kOhm up to 33kOhm sould do it. To make it *complete clear* [ :) ] you should connect the resistor where you put your pencil at 05:59 (where the diode and the Cap conected together) and the whiper. If you put it prior to the diode you´ll add a *huge* amount of hum to your amp, you´ll hear it and definitively shut down your amp very quick....;)
I will increase the resistance as you suggest. I intended to simply jumper between the input (right) and wiper (center) lugs on the pot, rather than extending out to the diode/cap junction. Is there a reason you prefer the junction?
I installed a 25K protective resistor between the wiper (center lug) and the negative voltage input (right) lug, and the circuit works fine. I also added a note to the video describing this modification. If the pot fails, the output tubes will now be protected. Thanks again for your input.
Thanks for the quick reply! Yes I've watched those videos, I saw you had put them in a link to someone else's question. It was just that I saw you using a multimeter in this video measuring current so was curious. I'm in the middle of making a DE amp for an electronics course final project, with the power amp based on the Fender Princeton. I have made notes from the videos you refer to for when I come to bias it. Your videos have helped me a lot to understand the inner workings of guitar amplifiers so thank you for taking your time to make them!
Just got done doing it to my '79. Sorta jackleg, but I live in a condo and half my tools are at a storage facility. :( Anyway, very clear and concise! Thanks a lot for taking the time!
The best tube amp instructor/editor/photographer/best boy on the web bar none. My only question on this very well done video is can the pot or knob possibly ground to the chassis? The pot looks like its only a few mm from the chassis. Hopefully someday soon Doug will do a video on how to set up and use a scope/signal gen to troubleshoot and bias a tube amp.
Thanks so much, Mr. G. The pot is well grounded through the metal bracket that holds it. I have made several videos using an oscilloscope, please see my video list. The scope is not used for biasing.
Love it! might just add this mod to my Princeton. If you ever get your hands on another I'd love to see how the cathode biasing mod works! I hear it really changes the sound. thanks for everything!
The conversion to adjustable bias is rather easy and useful for the Princeton, Frankie. Cathode biasing is generally not necessary or all that helpful.
Once again.. thank you for sharing your knowledge with us. I am about 2 week away from digging into my Princeton... and im definitely going to do this. P.s. Seeing that board is warped makes me feel better about mine....is pretty wavy lol.
I love all your videos .. very informative and well explained in a format that anyone with any degree of skill can interpret.Ive built three marshall style amps,and have had a wonderfull time doing it ,the videos you have put up here have provided me with extra information that came in very handy while figuring out some problems i had with circuits.. thanks again for your efforts .. PS im also a big fan of Rusty
You're quite welcome, F9. Rusty and I are pleased to hear that the videos are clear and helpful, and want to wish you the best.....or should I say "beast" (from Rusty :)
Hi uncle Doug, I am a faithful spectator and I am looking forward to every video of you. I'm sorry for my horrible english - I'm 60 years old and never had english lessons - (russian in school ;-)) This is not a criticism your work - but I have a question about the timecode 10:50 to 10:55 of your video: You are talking about -20 miliampere anode current but the TEK4 shows - in my opinion, only - 0,20 miliampere - if the measuring range, "mA" right in the window of the TEK4, is correct - or am I wrong? And again: thank you for the opportunity to learn from you Erwin
Greetings, Erwin. Your English is far better than my Russian ;) You make an excellent point. The readout of the TEK4 does appear to be 0.20mA rather than 20mA. I'm not sure how I had it connected or if the range was correct. I know that the PC was 20mA, so my guess is the range on the TEK4 was not correct.
Hello Uncle Doug , thanks for the answer - it is as it is and I knew that with 0.20 mA the amplifier would not really work - but the two 6V6's would have a very, very long and above all quiet life ;-) wouldn't they? ,.. or not-I still have a lot to learn... Many greetings from Hamburg
Hi Doug, Many thanks for another great video. There seems to be a good deal of discussion regarding how hot or cool you bias your tubes and how this effects the sound of the amp. I would be interested in understanding if this is a "perceived" difference or exactly how much does the frequency plot change vs bias settings. I believe it would make an interesting video.
Yes, indeed. The biasing procedure really does only take a couple minutes to perform now, leaving me more time to cater to Rusty's contractual benefits. I must admit, however, that frequent steak dinners do seem to make his camera work a bit steadier.
Hello Doug, you talked about matched tubes but in other videos you said that the unmatched tubes would give the amp a little character tone. thanks........
Another very clear and informative video Uncle Doug. I had a couple novice questions after watching. At 8:20, you say "this is a negative voltage circuit the wiring to of the capacitors and the diode are backwards." I expect most of your subscribers know how you could determine this from just looking at the circuit itself, but I wonder how you would know this either from the circuit or the schematic. Also, you note that the tubes aren't matched and the pot biasing with affect them differently--better to have a matched set. I wonder how tubes are tested to see if they match without putting them in the amp? My takeaway from this video is that if tubes are matched, the adjustable grid bias works the same for both and the only reason Fender didn't do it in the first place is simply cost. Is that so? Thank you and Rusty the Wonder Dog again for your time and highly enjoyable video.
Greetings, Carl. The high voltage circuit in amplifiers and radios is positive direct current (DC) and often called B+. The tube bias circuit, however, is negative DC. Since it's charge is the opposite of the B+ circuit, certain components, like diodes and electrolytic capacitors that have polarity (i.e. they must be installed in the correct direction) are installed "backwards" in the negative bias circuit compared to when they are installed in the B+ circuit. Failure to do this can cause a lot of problems. The polarity of these components is clearly marked on the schematic for the Princeton Reverb. The 50uf capacitor is + to ground (instead of -), and the diode band is on the 100K resistor side (not on the tube grid side). Output tubes are matched by putting them in a circuit and measuring their plate current (PC). Tubes are like fingerprints, they each draw a unique amount of PC......generally more when they are new and less as they age. After measuring the PC of a bunch of tubes, those with very similar PC values are put together in "matched pairs" and sold for a premium price. If you have matched tubes, then a single bias adjustment will work for the pair. If the tubes are badly mismatched, each one needs to be individually biased to neutralize the difference. If you have a bunch of vintage output tubes, you can do your own matching (by testing the PC of each) prior to biasing them. I agree that Fender probably omitted the adjustable bias in the Princeton amp to reduce production cost. I hope this all makes sense. Rusty and I thank you for your input and wish you the best !!!
Thanks Uncle Doug. I'll look at the Princeton schematic and follow along with your description. Please assure Rusty I'm keeping my fingers out of harm's way until I'm better educated. Paper is much safer than high voltage components.
Aloha Uncle Doug, My cousin found this tool that calculates frequency responses for Fender tone stacks, giving the option of different capacitance and resistance choices, using stock and other common values. Thought you and Rusty might find it of interest. To get it to work properly you have to download the app. demonstrations.wolfram.com/FenderToneCircuit/
What a good and doable modification. Wouldn't be better to use a trimpot? I'm afraid the pot could move with vibration while moving the amp. Thanks so much!
Glad you liked it, C. Use a trim pot if you wish, but vibration has never been an issue with the other pots in an amp circuit, so why worry about a bias pot ?
I am now considering making this mod to my Princeton reverb. Also, many thanks for the heads up on the stock 50uf capacitor being rather low in terms of voltage rating. I shall check it and swap it out to be safe I think.
You could add a balance pot network to balance out tube mismatch from ageing also,i need a balance pot network. And i love seeing rusty,i know he's no longer with us,but all the same,i love dogs. You see ,your dog has a different temperament to your wife(not your wife ,just in general), because the later you are out, the warmer the welcome you receive when you return home. Love the vids,i'm a tech and do make amps,but i don't get to see inside or work on older amps,plus the mistakes fender,marshall made,like not adding variable/balanceable bias circuits. Subbed and like every vid.
Thanks, John. Some Fender amps came with the balance control you suggested. Unfortunately, not many come with both the variable bias AND the balance adjustment.
I'm in scotland so i would use pounds sterling,but the term "dollar driven" still applies ,considering what two or three pots and a couple of resistors(all bought in mass bulk too i might add),fender,marshall to name a few are mostly cheaply built RCA copies,and custom builders (like you and i) can be asked to justify a higher price rate,and for something that would equal the build quality of old valve/tube hifi gear from early years that still work to-day with very little done other than maybe caps and tube/valve jobs. The modern pcb amp "made in (3rd world cess pit) are over priced garbage badly built with bad design criteria. kindest regards, jh.
Doug, I must say awesome video! I noticed you directly measured PC with your dmm, meaning you put it in series with the actual current. I’d rather not do this. Just wanted to point out that you’re still able to use the calculation method you presented in your “biasing double ended amps” video, correct?
Thanks, Nathan. I understand your hesitation. A good friend of mine who's been an electrical engineer for many years burned up a Fluke DMM doing this same procedure....so it's definitely hazardous. I did it strictly for the video, so the actual PC would show on the screen. Yes, you can use the voltage drop across the output transformer primary method......it will work just fine.
I study electrical engineering at ASU and they've always told us to use the calculation method for that reason. It was nice being able to see the current measurements on screen though! I just wanted to point out to others in the comment section. Thanks for the reply, best wishes to you and Rusty!
Hello Uncle Doug, i installed the bias control on my princeton reverb amp and it works just fine! just one question, on the end of the video you demonstrate how smoothly the adjustments works by measuring mA. Did you disconnect a wire, or do you use a tube adaptor with external wiring? do i have to recalculate each time i adjust the value to get the right PD? greetings Benny
+BAMPSANDPEDALS To directly measure Plate Current, I was using the OPT shunt method: You attach your leads to either side of one half of the OPT primary (the half connected to the tube you are biasing), then using the Ammeter function of your multi-meter, you measure the current shunted around the winding and through your meter (due to it's near-zero resistance). If this is confusing, then you might study up on the method using Google searches for more info, or simply do what you are doing. There is some risk to this method and I don't advise anyone else to use it unless they are experienced, knowledgeable, and comfortable with it. Be careful. Good luck.
So using this method no current is flowing through the transformer primary, and it is measuring current essentially without the transformer in circuit? I've noticed most transformers actually show different resistance (I know, NOT impedance) either side to center tap so balancing an amp might also involve swapping tubes inside to outside? I've seen others putting a 1 ohm resister cathode to ground on each output tube and measuring the voltage across each resister. Using Ohm's law, knowing voltage drop will give current flow. Do you consider this advisable? would you need the bypass electrolytic across this 1 ohm as well?
Yes, the current takes a short cut through the meter, allowing you to measure it. Yes, the two primary windings of the OPT almost always have different resistance, which complicates the biasing procedure a little because it affects the current running through each tube, even if the tubes are perfectly matched. For this reason, each output tube must be biased separately, if possible. Yes, swapping tubes is a common practice. You can install the higher current tube with the higher resistance winding to try to balance them out. I don't like the 1 Ohm method. Even a slight error in its resistance can throw the result off quite a bit (i.e. 10% or more). The electrolytics are no issue since current does not run through them.
It's hard to judge on my crappy computer audio setup, but even on this ragged system, that Fender Princeton AMP sounds really sweet. It seems to have a real rich fullness to it. Thanks for taking us through the Grid Bias mod. I presume that in person, you can hear the difference in a maximized bias setup or you wouldn't bother. (Or maybe just to save the tubes?)
Any Princeton that doesn't sound great is simply not working properly, Trebor. They are like polished gems in a quarry of rock ;) Yes, in person, all the differences are more apparent.
@@UncleDoug If you saw/heard how crappy my audio system is on the old desktop, you would have even more appreciation for their qualities! Thanks again for all of your wonderful videos. It 's like dinner and a show.
In cathode bias situation, it's always better to use a separate resistor per power tube. I experimented it myself and it reduces the hum to at least nothing, especially with not properly matched tubes. On the other hand, I widely prefer my power tubes to operate in Push/Pull.
The original construction doesn't have any voltage regulation on the biasing line. It's simple, and it works. When the mains voltage changes (rises for example), the plate voltage rises, but the negative bias voltage (to be exact - it's absolute value) rises too, which limits the plate current more, keeping the plate dissipation equilibrium more-or-less. If I was to modify the amp myself, I would add a second diode, to have a full-wave rectifier and less ripple on the filtering capacitor, but this would mean changing a couple of resistors too to get the voltages back to the same level. But that's just my obsession with hum, and dislike for half-wave rectification :) But then again, maybe this slight built-in 60Hz tremolo (due to some ripple in the biasing voltage) adds to the character of the amp :) Best regards
An interesting analysis, Stan. It is a little worrisome to apply a half-wave-rectified bias voltage to your output grids, but I must say that hum has never been a problem with the Princeton Reverb. Thanks for your input.
Hi there Uncle! How's rusty doing? Thanks, as always, for sharing with us yourself and your knowledge -I hope you know me and a whole lot of folks appreciate it. The video, superb. One question though. While measuring the negative Vdc you set the minimum at -38Vdc. Correct me if I'm wrong here but was it not -34Vdc what the schematics suggested?
+Fuga Mante Greetings, Fuga. Rusty is doing very well, especially for an 18-year-old pup. The schematic value is a ballpark estimate and varies greatly with different output tubes. As you could see when I actually biased the tubes, the range of values I chose worked quite well, with lots of adjustment left in both directions.
Hi Uncle Doug! Nice Videos! Where do you measure the Plate Current? I'm trying to BIAS a Fender Deluxe Reverb and I was trying to do this grid biasing. Thanks!
Greetings, Gus. I have posted a two-part video that shows how to bias a double-ended amp, such as the DR, step-by-step: ruclips.net/video/w9B0Rhr_Y-E/видео.html and ruclips.net/video/pjKYiSr497w/видео.html I hope they are helpful. Please be very careful and follow all safety procedures. Good luck :)
I'm trying to make the readings like you said on the first videos about bias but It's strange. My resistence reading is measuring -634 ohms / -03.8 Voltage Drop / resulting in a plate current of 0,0063 with a plate voltage of 660 resulting in 4,18 W. I'm bising a Deluxe Reverb 6V6... My mutimeter is manual maybe Im selecting the wrong reading... Don't know...
Gus, your readings are quite different from those I recently got from a DR. OPT Center Tap to pin 3 of Left 6V6: Resistance = 216.6 ohms, Voltage Drop = 4.59V, PC = 21.2mA, PD = 9.95W. The readings for the right 6V6 were quite similar. I think there may be some problems with the way you are taking the measurements. Perhaps it's your multimeter that is at fault. You should watch the video again carefully, and maybe seek the help of an experienced amp tech to guide you during this first attempt at tube biasing. Good luck.
I consider them to be AB class, BB.....but, to be honest, the designation doesn't matter that much to me. I just focus on achieving the optimum operating parameters for good tone and component longevity.
Doug, fantastic video here, as all of yours are (full credit to Rusty also). I have an Ampeg Jet 20, which is a Korean faux reissue supposedly based on the Fender 6G3 Deluxe circuit. It seems well enough built and has a tagboard construction so no nasty PCB stuff to fiddle with here. To get variable bias, I want to replace the 22K bias resistor and happen to have a nice 10K CTS bias pot sitting here. Could I just put the pot (using just one side of the pot and wiper) in series with another fixed resistor, rather than doing it the way you have done? I'm thinking perhaps around 15K on the R (hence 15-25 sweep) would then give me an OK range...or am I missing something e.g. re the suggested value, and are there disadvantages to this approach? There is a guy on the internet who did something similar, but modded his with a 100K trimpot (weeny thing) in parallel with a 51K resistor. NB on the Ampeg the bias is on the main board, not a separate one like good old Leo. I thought about doing something similar, but prefer the idea of the CTS metal pot mounted outboard as per your approach. I can see the idea of his to have parallel VR (100K) and fixed R (51K) but of course I only have 10K to work with on my pot. RIchard
twerpeater Greetings and thanks, Richard. I understand your situation and see no problem in trying what you suggest. I would use patch cords to set up the series circuit, i.e. pot in series with a 17K fixed resistor.....try it out to see if it works.....and then either modify a bit, or hard wire it into the circuit if you are pleased with the result. Monitor your plate current at all times to be sure you don't harm the tubes while experimenting. Good luck.
A Tweed Deluxe would definitely reduce my sorrow over the loss of the Ampeg (even though the Ampeg was great amp). It sounds like a pretty good trade to me, Javi, but it's a shame you couldn't have them both.
Thank you so much for this informative video. I do have a question, in the video it appears that you put an insulator of some sort on the 27K resistors leads on the bias board. Is there a reason for this ?
Hello Uncle Doug, I watched one of your vids where you designed a custom amp with a Princeton type push pull output. You mentioned that altering the resistor value at the input to the intensity pot would create havoc with the intensity function. What would be the changes we would see? I chose to change my bias to your suggestion of individual cathode bias, which I settled at 470 ohm each and have 9 watts each. Dead quiet, and with the celestion alnico blue 12" and the reduced capacitance to reduce the over active bottom end. Love from England.
Hello, Great video. please excuse my ignorance, I'm not very familiar with electronics. After watching your video would it be possible to add two adjustment knobs so you can Match Bias both tubes ? Thank you for your video.
I'm considering this mod for a Mojotone Princeton kit that I built, but I can't seem to find a bracket to mount the pot on. Any clues? Thanks so much for all the videos! I am a big fan and feel like I'm back in school with one of my favorite teachers soaking up all the great info.
Some more suggestions: If you put in two seperate pots, one for each tube - may along with a 47Ohm/5W resistor in each of the kathode path of the output tubes. It is now possible to insert any 6V6 tube you got - if it is not dead ;) You loose some output power but you are no longer need to match your output tubes. As this circuit does it for you. Simply match the cathode current by adjusting the pots you just inserted and way you go...
How about replacing the two 220K grid resistors with a 500K pot (input bias voltage connected to the wiper) and then adjusting it to dispense different amounts of bias voltage to the tubes until their plate currents are balanced? The only downside, as you warned in another comment, is that you face the possibility of a loss of pot wiper continuity and tube bias.
You can do this as well, the result would be the same in first place and as you described it But the driver stage will see different loads when the whiper is not in center position. Let say the whiper will be in 2/3 of a 500k pot. one half would be 166k and the other 333k resistance presented to the driver stage. If the whiper would fully CCW or CW turned you short out your driver stage completely (for AC). If you see the whole schematic from AC topology. DC coulped is all correct, but AC would be differ a lot. Be as symetric as you can, your amp will thank it with good (=clean) sound, add single side a little DC-offset in the driver and your amp will produce even harmonics. Vary single side driver amplification and your amp will produce even harmonics which will differ upon volume settings.
I can see that turning the pot to either extreme would cause problems. How about a 100K pot with 200K resistors between the pot output lugs and the tube grids? This would provide balance capability without jeopardizing the tubes.
Hi Uncle Doug. Your videos are very informative, as always. I have a question about your adjustable bias circuit. If the wiper on the pot were to fail or become dirty, is it possible there could be a meltdown of the output tubes? I'm wondering if a bias circuit could be devised where the pot is only part of the shunt element? Thus, a failure would result in a more negative bias voltage.
Thanks, A61. The current flow to the grids is negligible, so the adjustment pot is completely unstressed. Bear in mind that Fender has been using bias pots in hundreds of thousands of amps built over the last 60+ years, and to the best of my knowledge, pot failure has simply never been an issue. Indeed, there are all sorts of more likely failure points within amp circuits.....I deal with them daily......but so far, not a single failed bias pot.
Thanks for another excellent video! So just so my neophyte brain can get this straight, before the mod this amp was cathode biased and you would aim for a plate dissipation of close to 100%. Now it’s considered grid biased and you’d want the dissipation to be around 70% correct? I’ve been asking a lot of questions lately and you have been very kind to answer each one, I’ll slow down for a bit now 😂 Thanks!
Very good info Doug, Thank you very much. I'm having one hell of a time trying to get my first attempt adding Tremolo to a 6V6 push /pull build working. I've converted/built over 50 amps and finally decided to add tremolo to one. I'm now debating using one or both halves of the 12AX7 that I added. Wonder which sounds best. I'm guessing the one that changes the power tube voltage as opposed to the preamp tube.
Uncle Doug - I'm very new to tube amp repair and your videos are very helpful. Hope you make more soon. I've watched this video several times and think I've finally got it. I wish there was a camera shot from the other side of the new pot. It seems that you either cut the yellow wire (connected to the vib pot) at the eyelet board or unsoldered it from under the eyelet board. Can't really tell. Also you soldered a black wire to the eyelet board at negative end of diode where the yellow wire used to be. Finally, you unsoldered the 22K from the board and replaced it with a 27K with some kind of tubing and soldering one end at the grounded end and the other end to the new pot. What I can't be see but indicated in your screen note is the installation of a 25K resistor between center pot lug the right lug. Is all this correct? When you say left or right lug of the pot, I assume that is with the pot oriented with the shaft up. So now here's the real question. If the original power cord is replaced, the ground switch is no longer necessary. If the ground switch is removed, there is an available hole in the back of the chassis where a trim pot (no shaft) could be placed. This would allow for tube biasing without removing the chassis if you use a tube socket type bias device such as QuadStage BiasPro. Probably to do this right a new eyelet should be installed in the board to attach the free end of the 27K resistor and a wire to run to the pot. Or the ground switch could be replaced with a standby switch. Standby switch or bias pot? What are you thoughts?
Bob, I made this video quite a while ago and the specifics, such as wire color, etc. now elude me. Your best bet, if any of my connections are not clear to you, would be to consult a Fender schematic and/or layout diagram for the circuit you are working on. I explain the theory of the modification in the video and this should enable its transference to your circuit. Re your second paragraph, yes, you could replace the bypassed ground switch with the biasing knob......however, you must be sure that nothing can alter the setting after it is made. This is generally done by cutting a slot in the pot shaft so a screwdriver can turn it, and not installing a knob that can be accidentally rotated. It should be noted that tube bias is based on Plate Dissipation, not simply PC and PV (upon which it is based), so any bias device must provide both measurements so that the PD can be calculated.
Hi Uncle Doug. Another informative video. I was wondering if you could put a VR in the cathode in a doubler circuit.? Would that work to control cathode bias. Just wondering.
Hi Doug, thanks for another great video! Just a quick question, how are you measuring the current? Are you putting the amp meter in between the pin of the tube's anode and its wire or using specialist current measuring socket?
You're welcome, Cam. I explain the method in a couple videos, but will offer a brief explanation here. In a DE amp like the PR, with the amp off, I measure the resistance (R) of each half of the output transformer primary winding (first 6V6 plate to CT, CT to second 6V6 plate). Then while the amp is on, you measure the voltage drop (V) across each of these two resistances to get the PC for each tube (C = V/R). This is the safest, best method I have found for this measurement.
It's a wonder to listen to!! Excellent teacher! Say Uncle, can I send you my tube amp schematic (fixed bias) to check if a similar mod would be possible?
If the amount of -DC sent to the grids is regulated by a fixed resistor to ground, you can substitute a smaller fixed resistor plus a pot to make the bias voltage variable.
Hi Doug, Great video as always. Just a quick question. With regard to your bias voltage range, were the results shown here with output tube's installed or not? The reason I ask, is I just finished a quasi 6G3 build (minus Tremolo Section) , and my negative bias voltage varied significantly once the output tubes were installed. Understandably being due to the load applied to the power supply. Like your amp, my adjustable fixed grid bias was derived from the HT winding of the Power Transformer. Regards, Dan
As I recall, Dan, the bias voltage range shown on the meter was obtained without the 6V6's in place. My reason for this was to set the bias voltage at its highest setting (and PC at its lowest) before installing the tubes, to protect them. I never measured it with the tubes installed, focusing instead on PC and PV, but would assume that it would be lower.
Quick question about replacing the diode... what amp did you go with? The aa764 schematic doesn't give us a value for the diode.. just that there is one lol. I'm about to order all the parts.. 47uf (lol you had me going trying to find that 50uf) capacitor.. pots.. and new power tubes (im pretty sure ive got a 6v6 that's bad.. not a transformer after doing a bit more research).. but the value for the diode eludes me. Thanks again for the help.
Greetings, P6. Sorry for rounding off the cap value ;) Re diodes, about the only thing you need to worry about is voltage tolerance, and the 1N4007 diode is generally used (1000V @ 1A) in the bias circuit. Good luck with your project.
Hi Uncle Doug. I have to ask something. If the negative side of the diode is the banded side, why the hell on the schematic the diode was written + to negative and - to positive?
You ask an interesting question, SW. Traditionally, the banded end of a diode is the cathode or negative end....yet in Fender schematics, the banded end is marked + and the unbanded end is - . I believe that the seemingly backwards Fender designation is simply to make it clear that negative DC is passed by the diode and utilized for grid biasing. If the diode were to be marked conventionally, it might look like the grid-biasing DC was + .
Uncle Doug, I have a question, I don’t know if is possible… but i just thinking about make two grid bias pot and disconnect 270k resistor in order that each one receive an individual grid bias pot… in order to Match unmatched tubes… thank you for your very educative videos
It's more common for there to be an overall bias pot, then a second (balance) pot to distribute the -DC bias voltage to each tube grid, Jonathan. But you could experiment with other configurations.
@@UncleDoug the idea was came when I see this picture where is a kind a multiple variable grid bias (or at least I think so) www.tetrode.co.uk/sweep3c.jpg I just try and hope I won’t burn my house, thank you uncle Doug
dear Uncle, I've completed my Reverb fixing, and it was performing fine, then I decided to check for plate dissipation, according to your video guides. I measured voltage drop across center tap of OT and the other two ends, each for each power tubes. I had about -4V DC, with a plate voltage of about 399V DC. This would mean a PD of about 10/11 Watts. That seemed a bit to high to me, so i decided to install the small 10K pot to adjust bias. If was responding ok, I swept it so to have around 8 W of PD. After that I was putting back the ampli in his case, then I was horrified to discover that no sound was coming out from the speaker. I did not make any evident mistakes, no smoke, no shorts....I removed the pot and restored original connections but stilll no sound, just a little hum. So it is possible that I have killed the 6v6 during my tests? I'm lost at the moment, going to follow the whole signal path to see whats wrong.
You didn't provide enough information for me to check your calculations, D, but I doubt that you destroyed your 6V6's while reducing their bias. Check the bias voltage on the grids.....you may have inadvertently increased it to the point of stopping the current flow. BE SURE you are properly calculating your PD.....remember that PC is voltage drop divided by OPT winding resistance.
@@UncleDoug ok Uncle, all sorted out...the culprit was one of the copper wires connecting components on the preamp to the chassis under the pots, which was disconnected leaving all those component with floating ground. Anyway, I've set the bias pot as follows: on one of the 6v6 plate voltage is 405 V to ground, resistance of that side of OT is 146.6, Voltage drop across it is 2.95 ohms, so PD should be 8.14 W, am I right? The other tube has similar values. Am I too cold? The amp tends to distort a bit at high volumes on input 1. Is that desiderable or normal? Thank you again
@@dreamlessfull Your calculations are correct. If your circuit is grid biased, 8.14W is ideal. If it is cathode biased, this is way too cool.....and should be more in the 11W range.
@@UncleDoug yes, being a '67 blackface princeton reveverb, it is grid biased, as the cathode is tied to ground. Thank you again for your time and your invaluable teaching!
Hi Uncle Doug, Thanks for all the great videos. I've learned a lot by watching them. As the proud owner of a 1966 BF Princeton Reverb, I want to go through adding adjustable grid bias to my amp via your procedure. However, I have a question. In your 12/9/2013 video about biasing double-ended output tubes, you caution against using a grid bias pot due to its effect on the vibrato circuit. In this video, performing that modification is detailed. Just wondering if you had a change of heart about the effect on vibrato or perhaps made a New Years resolution to become less of a vibrato hog? Thanks again!!
I guess I found that the good provided by the bias pot outweighed any possible bad effect on the tremolo.....and that the "bad" was virtually indistinguishable.
@@UncleDoug Cool, kind of what I figured. Like most things in life, a balancing act. I learned a lot about electronics by virtue of an intense, 5 Trimester Electronic Technician course at DeVry completed in 1987 but since I didn't go into a career of component level trouble shooting, I'm terribly out of practice. Your videos are a great refresher and combine nicely with my love of guitars and amps. Thank you
Hi uncle Doug. We chatted a few weeks ago about our favorite amp the Princeton Reverb. I watch this video over and over again and just realized that you made your bias adjustment based on plate current. I biased to the -34vdc on the schematic. That doesn't give me a accurate measurement of the tube voltage and dissipation. Would you recommend that I go back in and do the same rather than trust the bias voltage from the schematic?
The -34VDC figure is simply an estimate and varies depending on the age of the output tubes and condition of the circuit, Tele. To accurately set the bias for a specific amp circuit, you need to measure both the PV and PC, then calculate the Plate Dissipation, which should be around 8W or slightly higher for grid biased 6V6's.
Hi Doug, I hate like he'll to bug you, but I just wanted to ask you a question. You the only one that I know of that could give me an answer to my problem. I have a Fender 1993 Blues Deville 4x10 amp. It has a nonadjustable bias. The cathode resistor is 27K. I've read that you can take a 25K linear potentiometer and run a wire from one side of the holes where the 27K resistor was and connect it to both the left and middle wiper. Then you connect another wire from the other hole where the 27K resistor was in series with a 15K resistor to the ground prong the 3rd leg. Would this be the correct way to hook up an adjustable bias pot? I want to make sure that I am doing this right. Thanks from Michael Newell from Niagara Falls, Ontario, Canada
I much prefer wiring schematics to verbal descriptions, Michael, but I think your description is correct. There is one way to be sure. Wire it this way, adjust the pot for maximum resistance to ground, attach DC voltmeter probes: red to the hot bias wire (goes to the tube grids) and black to ground, turn on the amp, adjust the pot until the voltmeter reads -49.6VDC.....then measure the plate current and plate dissipation. Adjust the pot until these values are correct.
Hi Uncle Doug, sorry to hear about Rusty, really looked forward to seeing him in your videos. Thanks for all your work and sharing your vast knowledge and skills. I have a question on this circuit....Does the Intensity pot vary the 6V6s bias when the vibrato is shut off? This has me perplexed thinking that its possible to vary the output tubes idling bias by changing the vibrato intensity control please help me understand.I can understand how the vibrato would modulate the bias to get the vibrato effect but once it is shut off does it still change the tubes bias?
The bias is set for the output tubes with the tremolo shut off, otherwise you couldn't get a stable reading to base the adjustment on. Once you turn on the tremolo, the bias varies above and below the set bias point to give the tremolo effect.
@@UncleDoug Thanks, the only thing i can come up with is that the grids don't pull any current so the inline resistance of the Intensity pot doesn't change the negative bias voltage no matter the setting on the Intensity pot?
Doug, I've just built my first Princeton Reverb and spent all evening trouble shooting it. I got it working but had to revert the bias to standard. The pot is still in place so I would like to get the adjust working. What range should the negative voltage be from the pot in one direction to the other? I was only getting a few volts if I turned it slowly but if I turned it quickly it would give me more but then settle a few volts more than it started. It's just to help me trouble shoot it. Many thanks. You have inspired me. I have now built a 5F1, 5E3 and the Princeton Reverb..... I have five boxes with parts building up for five other amps.......
I should have watched to the end of the video again! I only watched it to the circuit diagram this time but when I came back to ask you I let it run and you show the range!! :-)
Yes, simply because it allowed for direct measurement for video purposes. This method is somewhat dangerous however, both for humans and test equipment, so I tend to preach the gospel of indirect measurement via voltage drop across OPT windings or cathode bias resistors.
Hi Doug, first I really enjoy these videos and thanks for that. It appears that you are still responding to this older video so I have a couple of questions for you if you feel inclined to answer. Twenty years ago I purchased what I can best identify as a mid 70's SF Princeton Reverb. Some months into using it I smoked the power transformer. I have no idea how. I had it replaced by a qualified tech. It has worked fine (with limited usage) ever since. I got concerned when you spoke of Russian Roulet when it comes to the original pair of 50/50 capacitors. Is it possible that this was the cause of my burnt transformer? You didn't say what could happen if this pair is not changed. Thanks, TW (This mod looks so simple I might go ahead and do it - I'm sure Rusty is still your helper, just at a different level))
If you are referring to the 50uf @ 50V cathode bypass caps, they alone could not have caused the failure of your power transformer. The originals can be easily and inexpensively replaced with the higher spec new caps. You should also check your output tube bias.....which CAN cause the PT to fail.
Hi Doug. Do you think this will work on a single ended Champ style build with an adjustable bias with the cathode trem? I have an adjustable bias installed so I can run all different power tubes. 6L6 , EL34, KT's , etc. Thanks.
Are you sure that your power transformer can fulfill the current demands of the tubes you listed? Can your SE output transformer handle the high power output of these tubes? Finally, each output tube you listed has different load resistance and thus will reflect a different output impedance.....how will you cope with this?
@@UncleDoug Oh yes. I made certain of that Doug. What do you think about the bias? It drifts up about 5- 7 mA with all controls on zero after running a little while. As far as the load resistance goes I have multiple taps. to choose from ; )
@@williswet Is the tremolo activated? If so, even if it's set to zero intensity, the PD will be altered by it. You can pull the oscillation tube to check this.
Hi UD! The BFPR is indeed a great Fender amp. In addition to lacking an adjustable grid bias control, there is another difference. All of Fender’s large amps have two resistors on each of the power tube sockets. A 1.5K resistor between pin 1 and pin 5. Also a 470 ohm, 1 watt resistor between pin 4 and pin 6 (unused pin used as a mounting terminal). I have been told that this helps the output tube stability and improves tube life. Do you agree? Are these resistors worth adding to the Princeton Reverb amp? Thanks.
Greetings, DR. Pin 5 is the grid in both the 6V6 and 6L6 and Pin 1 is unused....the only resistor I see in this position is the 220K bias-input resistor. The reason the larger amps use a 470 Ohm resistor on the screen input (Pin 4) is because they also have filter chokes. Circuits using resistors instead of filter chokes, like the PR, do not need additional screen resistance.
I was watching your series on biasing and watched this one again, and have a question about the Tremolo Intensity pot in the circuit. If this pot controls the bias of the amp by changing the Plate Voltage, isn't that going to be screwing with the power dissipation every time you use the Tremolo and change that setting? Doesn't make any sense to me. Could you clarify what's going on for me? Thanks as always.
The bias pot has no effect on the plate voltage. It regulates the -DC bias voltage on the tube grid. Plate dissipation varies greatly depending on the strength of the input signal......that's why it's set at 70% of max PD. Watch my other videos on tremolos, tube biasing, etc. for full explanations.
Uncle Doug, sorry about the confusion. I wasn't talking about the new bias pot, but the Tremolo Intensity Pot Center wiper lug that connects to the Grid resistors. The schematic originally shows the B+ going through the top of the 27K resistor, to the Tremolo Intensity Pot Center lug, and then up to the connection with the two 220 K grid resistors that connect to the Grids. So before you installed the new bias pot, wasn't the Tremolo Intensity Pot affecting the bias when the Tremolo Intensity Pot was adjusted?
Yes, the Tremolo Intensity Pot does indeed alter the tube bias somewhat, establishing the threshold at which point the tremolo effect becomes audible.@@wsscott72
Hey uncle doug, great videos you have over here! I have a background in electrics and playing guitar (both terribly), but i am mew to valve circuitry. I have decided after watching your build from scratch series to build my own princeton with this mod attatched. the Aa1164 schematic im using/modding has 5u4gb tube, and i was wondering if i could use a 5ar4 instead for some more headroom and less sag, can it be a straight swap or do i need some more modding? Many thanks!
Greetings, Craig, and thanks for the nice comments. The 5AR4 or GZ34 are both much better rectifiers than the 5U4. They can be substituted with no circuit alterations, other than making sure that the output tube bias is properly adjusted after the substitution.
If the pot fails due to contamination in a way that it creates a large resistance greater than 10k from dirt on the wiper, the tubes will redplate due to an absence of bias supply. I sometimes wonder if running the pot in parallel with that resistor to ground is safer. That way if it fails there still will be bias supplied to the tubes. I sometimes fret over all that bias supply depending on that pot not getting "scratchy". Also, wouldn't changing the value of that 27k resistor to a higher value give you a sweep in both directions instead of just biasing hotter?
As in all industrial made Amps one can see, that the company needs to get some profit out of them. So the little brothers of the Big´uns need some budget-law cuts. In here it is the grid bias. They use the pot as well to provide the following stage (1/2Ax7) an AC-load. clever ;) Pots, in those days, cost a dollar each for a company. And about 4-8 for a replacing one. So saving component count really was a big deal.
Removed the 470k at the 7025. Like taking a blanket off the bassman. Get this , someone had put a 1 meg in the neg feedbk loop . The one off the speaker jack. The amp is lively but you think that value is too high?
I'm glad to hear that the 470K removal worked well for you. Re the NFB, I would think that a 1 meg resistor would effectively eliminate any negative feedback, but if the amp sounds good to you, that's all that really matters. You could substitute a 1 meg potentiometer for the 1 meg resistor. This would allow you to select the amount of negative feedback that you like best, instead of settling for a fixed value.
How do you "match" the tube current to balance them? Is this really an issue? I thought that both tubes should be adjusted so they are operating in a true complementary fashion. Wouldn't this reduce notch or crossover distortion? Is this something that is added to the cathode circuits for each tube?
+Erik Thorsen In most cases, especially with fixed bias circuits, you simply use two tubes that have the same current flow.....i.e. a matched pair. If the tubes are cathode biased, you could bias each one individually, with a carefully selected bias resistor for each, to provide identical plate dissipation. If their PD is unequal, then it's like two people rowing a boat in an unequal manner.......efficiency and possibly tone is degraded.
I know it looks cool and high quality an' all that but that aluminium knob is overkill, but since I get your point somewhat, I'm thinking even More Over the Top. I'm thinking a miniature that looks like an actual era correct fender knob. Do those toy battery powered amp have them? If I find some I'll send them along for future fun.
I'm not a doctor, but I play one with my stage name. So yes, as a matter of fact, I do more than occasionally Prescribe or Order Overkill for entertainment use. As both a Real person and character, you surely you understand more than I the finer points of internet knob close-up photography for for recreational use. Peace be upon you and Happy Thanxgiven
I have a question, my silver faced princeton reverb has an erratic tremolo speed pot. When I increase the the speed at about 8 it will cancel the tremolo. I installed 2 parallel 1meg resistors and a 0.2 cap like your video shows but the pot still acts up. can I replace the 3meg pot for a 1meg. In the speed control?
Hey Doug, thanks again for this video! I have a simple question about the capacitor that is inline with the 27k resistor. Does the end of that also connect to the new 10k bias pot? From the angle the video was shot, I'm unable to tell. Sorry, if you already answered this question, I got dizzy reading all of the questions and replies. Hey Rusty; what's up??!!
You may be referring to the filter cap for the -DC bias circuit, William. It's negative end connects to the circuit and the positive end goes to ground.
@@UncleDoug thank you for this reply, UD! However, I'm still not seeing it-- do you leave the cap in place where it was originally or do you connect it to the same point on the pot like the resistor? Saying "it connects to the circuit" is still a little confusing as, well, the whole amp is a circuit. Thank you from a non-EE slack-jawed yokel.
You are the best teacher ever. I love watching you explain and then perform your work. You are so articulate and informative.
Wow....thanks so much, Sean.
Hi Uncle Doug, this is probably the fourth time I have watched this and I think I finally have it. lol I am building a Fender 6G3 Brown Deluxe and I want to make it adjustable bias. I do hope that Rusty is still doing okay for such a senior doggy. We lost our boy Maxi two weeks ago as he was not doing will at all. He was thirteen and he had gotten to the point to where he had no quality of life anymore. I took him to the vet and made the decision to let him go. She was very compassionate and he took his last breath in my arms. I still cannot talk about it without getting very upset. So I hope that you and Rusty have plenty of good times left together.
Best of luck with your adjustable bias modification. I looked at a schematic and it appears that you will be replacing the 22K bias resistor with a resistor (probably around 10K Ohms) and a pot (around 20K). I'm sorry to hear about Maxi. We had a Collie named Max who presented exactly the same situation about a year ago, with the same result. I completely understand and share in your grief. Losing a family pet is almost as traumatic as losing a family member. Rusty and I send our best regards to you all.
😎
hi uncle doug, hope you are well and the hands are holding up ,
loved the clear concise content presented here ,,as always,
chapeau !
Thanks so much, William. We're doing quite well :)
Another great video Doug! Quite often I get Fender Princetons in for repair...I try t always install matched pairs of 6V6's to make life a little easier, When Fender came out with the Silver Face line, they laid an egg with it...That was the series of amps that earned Fender a bad rep at the time. They used the bias balance circuit rather than a true adjustable bias...I remember doing modifications on these amps to bring them back to the original Black Face design...Keep up the great videos!!!
Watched this video a few times. Never got tired of it. R.I.P Rusty
Glad you enjoyed it, Sean. Thanks :)
Hi Uncle Doug!
I am a robotic technician and a weekend rock star Ha!!!
I own 4 tube amps and wanted to learn how they work
Your passion is contagious!!!!
Thank you so much for sharing your knowledge to the world
Yannick
You're welcome, Yannick. We're glad the videos are helpful :)
As I'm watching this video, I noticed your up to 931 subscribers. Certainly
if you make it to 1000, it will be a milestone....your on your way to being a RUclips legend!
Almost 100k now!
Uncle Doug ! Your projects are priceless works of art and technology, your tutorials are incredibly informative and interesting and you are a gift to all us tube amp heads in the world. I want to thank you for all your videos that I have watched and re-watched over and over again with great interest and enthusiasm. Peace ! JM
Wow.....You're welcome, Manny. We really appreciate all your very nice comments :)
UD, your suggestion to bend a piece of coat hanger to attach a bias pot to the power transformer screw post -- was brilliant! I have now installed a bias pot in my 1966 BF Princeton Reverb thanks to your great video. The original bias spec is 27K but the resistor measured 25.2K ohms. I was not able to get the PC below 27 mA on any pair of 6V6s, so the idle PD was over 11.5 watts. Once I installed the 10K linear bias pot in series with the 25K resistor, I increased the resistance and reduced the PC to 19.7 mA and the PD to 8.4 watts. The amp sounds awesome and the tube life should be normal now!!
Wow....thanks, DB. You made my day. I'm glad to hear that the coat hanger pot mount worked so well. It really is a very simple, effective way to solve the problem. It also sounds like your biasing job was a total success, with both excellent tone and much longer tube life ahead. Your experience provides the best possible reason for making this conversion.....and your next biasing job should take all of 3 or 4 minutes to complete. Congratulations !!
Uncle Doug I did the bias pot wiring in an alternate way. But first, I used a Fender style 10K linear pot. The kind that is normally mounted in the chassis with a mounting hole cut, however I used your coat hanger mount as a “bracket” and avoided drilling the hole. Back to the wiring. I connected the 25K resister to the left terminal of the bias pot. The bias pot’s back and terminals are facing up. I added a jumper wire from the left to the center terminal. The right terminal goes to the intensity pot’s left terminal. This way if the center wipe fails the one resistor is still there, thus this avoids adding an extra resistor. Thanks again for your video that inspired me to add grid biasing to my Princeton!
You're welcome, DB. I'm glad the project worked out so well. Thanks for sharing this with us.
Uncle Doug! Thanks so much for these videos! I've added your bias circuit to several 6V6 push-pull amps, including 2 Princetons (one a '64, and the other a '69-70 Princeton Reverb), but I went one step further on the last one. I made a pot bracket for the next transformer stud and mounted a 250 ohm Ohmite pot to balance the grids themselves, because of varying output transformer impedance either side of center tap, and of course a "matched pair" these days is only apparently matched on a tube tester rather than graphed as they were a few decades back. One further easily reversed mod I've been doing is placing a 1% 1 ohm 2 watt resister (got a 100 of them from vendor on Ebay) in each cathode circuit, much easier current measurement when replacing tubes! I suspect the 250 ohm pot could be replaced with a 50 or 100 ohm pot too, probably closer to what's actually needed. Geez, right on the money current balance and setting the 70% max tube rating. I thought you'd appreciate this bit of OCD amp service!
Your modifications all make excellent sense, Mr. B. Thanks for sharing this with us.
camera decided not to link to computer, pictures to follow ... further think through: 250 ohm pot did nothing. thorough rummage through parts lead me to 150K 1% resistors to replace the 220k grid resistors and then used a 100k pot (finding them all to be +/- 10% with most being about 90k, finding ONE to be 110k and using that)this gives CLOSE to the 220k original grid resistors of which Fender used +/- 20% so the new ones are certainly in the ball park. I didn't check what the originals actually were. THIS actually works well with minimal fiddling and can balance unmatched tubes, but given what these amps now sell for and matched tubes sell for 50 bucks on Ebay, its not worth it not to use them. I wonder if I could match a 6W6 with a 6V6 ...
Hi Doug, thank you for getting back to me. Yeah there is a schematic for my amp on the net for the Blues Deville 4x10 amp. Well yeah everything that I've read on the net says to do it like that. I always have reservations about a lot of things you read on the net. I just don't want to blow anything like my power transformer or valves up doing something wrong. I was having some time ago a problem with my 1965 Fender Blackface Bandmaster amp red plating. After exhausting every avenue on what the problem could be, I finally took out the bias pot only to find that it was burnt and ruined. Replaced it and have never had a problem since. I did make the mistake of trying to use KT66 power valves in the Bandmaster and it was playing fine for a couple of days, then I went to turn it on the other day a nd it blew a fuse and took out one of the screen resistors. I replaced it and put 6L6's back in and It's back up running. I won't be trying that again. I just thought that maybe you could look at the Deville 1993 schematic for me as I'm not good or experienced enough to read these schematics with any competency. Do you think that I should use a 50K linear pot or a 25K linear pot. Which wound be best. It's just that to install a pot in this PCB board, it's going to be a pain in the butt trying to get that board out to solder these wires in this very small space. Thanks Doug. I wish I had your knowledge on these things. Your videos never cease to amaze me. Thanks so much. Michael Newell from Niagara Falls, Ontario, Canada P.S. There's just one last question that I have for you. I presently have a 100uf@ 100 volts in my bias supply of my Fender Blackface Bandmaster. It calls for 25uf@50 volts. Is that value that I am using ok or should it be replaced with something smaller like a47uf @ 100 volts? What does that bias supply electrolytic cap actually do?
The 25K linear pot will be better, Michael. I avoid working on PCB amps for just that reason (among others).....inaccessibility. The bias cap smooths out the rectified DC for the output tube grids. You are better off using what the schematic suggests: 25uf @ 50V. The circuit designers knew what was best.
Uncle Doug, I continue to watch your videos all the time and am learning so much. I wasn't sure which video to post this comment on since I couldn't find a specific soldering tutorial video.
Re: the amp I asked about the questionable-looking capacitor on a week or two ago, this is my first hands-on attempt with desoldering and then soldering in a new component. I've already removed the capacitor in question but in doing so already learned more which leads to this question. At a minimum I have two components to replace. I'm hoping to do this without catastrophe, burning up components, or damaging nearby components with the iron, and so I am practicing and intend to continue practicing a ton before then.....on a secret resource that I never had the heart to take to the dumpster....
The story is almost too tragic (and long) to print but I got paid very well by the insurance company and it is what it is so here is what I am practicing on. For about 6 years I was the steward of a high powered Tweed Twin clone that an old pal (who built these amps) had built around 1992. It became Danny Gatton's amp in the last year or so of his life. After Danny passed it went back to the builder. When I got out of school, it was a gift from a combination of the builder (who could have charged way more), my parents (who supported me), and something I tended to keep under-wraps but cherished for its history. Its last hoorah was at the Ryman in April of 2010 with Vince Gill (who'd graciously given me the chance to come out to play together) so it had a good run. After the evening, it got trucked back to Soundcheck, where it was submerged under the Cumberland River for 5 days beginning on May 1st.
In the aftermath of the flood, there were a few survivors. The first really good amp I ever bought when I was about 19 years old (you did a video on the exact same model, serial number 00060. Mine was 00353) had a cover on it that my mother had made with a sort of vinyl-material. Somehow that cover was the difference between that amp making it back and Danny's Twin. Everything went to dry out for about 18 months after coming out of the triage area. In August of September of 2011, I was at my guitar builder friend's house (who'd let it all dry out in his workshop) for his daughter's birthday party, and assuming everything was toast (all insurance claims were settled) we tried turning on the old one. Of course I know so much more now than I ever did then. Surely we should have at least brought it up slowly on a Variac or something, but expecting nothing, we turned it on and low and behold, the tone was incredible. Within days it was in the shop for a restoration, and it lives to this day (I lost the original speaker but not transformers....actually I still have the speaker but never got it re-coned).
Hoping the Danny amp might have made it out alive too, I very excitedly called the builder and informed him of this Fender miracle. I don't think he held out much hope for the Twin either. A day or two later I finally worked up the nerve to turn it on....all it made was one big "pop" and then nothing. The builder had moved onto other builds and didn't want it back, 5 or 6 years later Todd Sharp said they could try to rebuild it, but the settlement had already been spent and I didn't hold out much hope or want to pour the cost of a complete rebuild into trying to restore whatever remained.
In 2021 -- 11 years later --- I finally got it out of my friend's garage and brought it back to the house and stuck it in my basement. I could never throw it out, even though my Dad kept telling me to get it out of my hair. That amp is how I practiced getting a chassis out of the cabinet and how I've begun to practice desoldering for starters.
I've gotten a few things out and soldered back in and learned along the way that those eyelet boards have a certain amount of mass at the eyelet. I've been watching that Pace soldering tutorial for years (literally, since I built that 1176 compressor out of a kit). Here's what I'm finding out.
With heat-sink clips on and applying a little bit of fresh solder, with the biggest chisel tip I have for the iron, I can get solder melted in maybe 3-4 seconds at the highest temperature on my Hakko station, and then use the solder sucker for the first removal. Then I can heat again to remove the sweat-joints and remove the component (I got this sequence from a Rob Robinette post on the TDRPI forums so if I'm doing any of this wrong, at least this was my attempt at understanding the process). Then I've got to work a bit more with desoldering wick (SuperWick) to clear the eyelet hole completely.
I'm doing this at about 875 F. If I don't have the iron that hot, it takes too long for the solder to melt. I burned up a resistor the other evening practicing and realize all the soldering gurus say to get in and out in 2-3 seconds so the fact that it takes me more already makes me nervous but so far I've only torpedoed one resistor in my efforts. Of course I realize the real problem is relative thermal mass. Those brass eyelets that go through the board, combined with anywhere from 2 to 4 component leads joined together in the same eyelet. Applying some fresh solder to the joint before removal seemed to improve things in today's practice-run.
Here's the real question. Is there a standard recommended order of events and approach when working with eyelet boards for component swapping? I almost felt like giving up, taking a trip to see the builder or just surrendering to my local tech shop once I realized that with 3-4 things joining up in the same eyelet, it seems I could be jeopardizing everything connected at that point if I don't go into this knowing what I'm doing. Is it bad to heat twice instead of just once? In other words, to heat and suck, then heat and pull out the component? Then invariably I have to heat and use wick to clear the hole of remaining solder. I haven't had an eyelet pop out or noticeably disintegrated anything on my practice board yet, but the main question is if this is a normal series of events when working on eyelet board construction.
Based on the Pace soldering videos, I'm completely in over my head for this taking longer than 2-3 seconds total. But I'm wondering if that same advice applies with tube amp eyelet boards, as long as heat is not applied to a component for too long. Is the start/stop/start/release thing okay? When I burned up the resistor on the practice board, I knew it because it measured about 5-6k lower until I kept the iron on the lead for way way way too long (and I knew it was too long, maybe anywhere from 5-7 seconds) when re-soldering back into place.
My hesitation to surrender and give up is that I'm afraid if I don't learn now then it might not ever happen. But between using a disgusting relic of the Cumberland River to practice on and a lot of conscientious care, I wanted to ask someone and hope this note was not too long. (Of course it was but hope it was at least not boring or too heartbreaking. I figured Danny liked working on his cars. Maybe he'd approve of me using the amp to practice working on repairs, so that I don't become a drunk chimpanzee when the lights are up and it's time to do this for real on an otherwise good amp).
(*I am editing this a few hours later to add that I was using a medium sized chisel tip which would have been large for a pedal or circuit board....but I thought increasing the size of the tip might help so I have ordered a set of genuine Hakko tips for my station, with a couple much larger tips than I currently have. When they arrive, I'll try things out with the test-board again to see if I can't get better flow with faster times, and perhaps a lower temperature).
Greetings, Sean. I think you are obsessing too much on the soldering process. I personally use a soldering gun, probably at the same high temperature of your iron, and simply heat the eyelet until the solder melts and then, with needle-nosed pliers, pull the component lead out of the hole.....leaving the solder in place. Then, when I have the new component leads trimmed and bent to fit, heat up the eyelet until the solder melts and push the new lead into the hole. Once the solder hardens, I pull on the lead to be sure it is soldered in place.....and move on. If there is a shortage of solder on the eyelet, I may add some fresh solder to make a dome. Both processes, removal and installation, take about 5 to 10 seconds each. Resistors and capacitors are resistant to heat, and I have never damaged one during the soldering process. Solid-state components like transistors are heat sensitive and require heat-sinking of the leads. I hope this helps. Good luck with your project.
P.S. Having just watched several of your YT videos I can honestly say that if your soldering skills ever approach your guitar-playing skills, you will be immensely skilled at soldering :)
@@UncleDoug Thank you so much, both for your reply and for the kind compliment about my guitar playing :-). That certainly helps a lot and makes a lot of sense. In fact, I just practiced your technique on the flood-amp (just removing and reinstalling the same component) and was able to confirm with my trusty Fluke that the resistors measured the same both before and after. There is one other quick thing I've been thinking about....
On Fender wiring-diagrams where there are a few eyelets connected under-the-board, are there ever any precautions or things to watch out for to avoid having a connection come loose when swapping out a component on top? It's not an issue with my current project, as neither of the spots I'll be working on are connected....but it is something I wonder about from time to time. Thanks as always.
@@SeanWeaver I guess it could happen, Sean, but in many, many years of working on Fender eyelet boards, I have never experienced the problem. Again, best of luck with your continued self-education endeavors.
@@UncleDoug Thank you so much. That you take the time to respond to comments and have answered all of my questions (even the paranoid/obsessive ones) means and lot. I'm so appreciative and continue to learn so much from each of your videos.
Thank you so much for breaking it down....explaining it to us mortals
You're welcome, BB. I'm glad you enjoyed it :)
Excellent. Clear, concise information that is coming in handy with my current build. And the added commentary here was a great benefit as well. Thank you for all you do Uncle Doug.
You're welcome, Kevin. That's good to hear :)
Wow, just read the footnotes on protecting the bias circuit from pot failure. I haven't experienced this but I get the point. A recent prototype build of mine is not behaving as expected; this may be the answer.
Thanks as always.
I hope it helps, Howard. Good luck.
@@UncleDoug
Thanks,
This is a test bed project. I'm experimenting with selectable negative feedback, tone stack modifications, preamp tube placement, and some control location alternatives.
The first pass sounded good, but the 6V6s tubes were running very hot, and the amplifier was drawing 1.5 amperes at 120 volts. Also, grid current seemed high to me, but no red plating.
Yes, I may have bitten off more than I can chew.
Be well,
HR
Excellent video, *Doug*!
As usual fantastic explanation and leading all straight to the point...:)
Here are my two cents:
If you gonna do this, you may want to make the negative grid bias rock solid, as you already plan to desolder some components and the amp is upon your workbench anyway.
If you connect just the wiper of the bias pot to the grids, what happend when this wiper becomes loose or does not making any good connenction inside the pot anymore....
You know this: you´ll burn your tubes.
Because the grids then have a bias voltage near zero....oops...
To prevent this put a single resistor, may with the same resistance or slightly higher, in *parallel* to the wiper pin and the negative-voltage pin of the filter cap. *Not* ground!
So if the pot´s wiper loose its contact to the resistance-path that added resistor provides lowest grid bias possible to the tubes. So they will survive and all you have to do is change the pot - not the tubes as well :)
I´ve serviced a lot of Amps with blown output tubes due to a defective grid bias pot. Some times even the OPT was melted, or the pri. winding was shorted. Well known for this are the ´braun´ CSV 60 Amps with PL 500 output tubes just because the tubes are so rugged, they can stand zero grid bias for minutes while cooking the pri windings and even set the OPT on fire.
Have fun, and thanx for this great video!
You have brought up a good point, TC. We have all seen pots in which the wiper made poor or inconsistent contact with the perimeter resistance; which, in this case would eliminate the bias voltage and PC would soar. It does appear that a 10K resistor connecting the wiper to the negative input from the diode would provide an alternate route for the maximum grid bias voltage (= minimum PC) to reach the tubes in case of wiper failure. Please acknowledge that I have correctly interpreted your recommendation and I will test it in the PR circuit.
This is by far more true to the newer pots, the old ones found in vintage Amps are much more reliable than today´s brothers.
I would suggest 15kOhm up to 33kOhm sould do it.
To make it *complete clear* [ :) ] you should connect the resistor where you put your pencil at 05:59 (where the diode and the Cap conected together) and the whiper.
If you put it prior to the diode you´ll add a *huge* amount of hum to your amp, you´ll hear it and definitively shut down your amp very quick....;)
I will increase the resistance as you suggest. I intended to simply jumper between the input (right) and wiper (center) lugs on the pot, rather than extending out to the diode/cap junction. Is there a reason you prefer the junction?
There is no reason. I just wanted to made it complete clear.
In serious HiFi audio gear, it differs but here it´s the same thing.
I installed a 25K protective resistor between the wiper (center lug) and the negative voltage input (right) lug, and the circuit works fine. I also added a note to the video describing this modification. If the pot fails, the output tubes will now be protected. Thanks again for your input.
Thanks Doug I will be considering doing this to a 1961 Princeton Amp I am working on. You explain everything in an essay to understand way
You're welcome, AL. Best of luck with the project.
Thanks for the quick reply! Yes I've watched those videos, I saw you had put them in a link to someone else's question. It was just that I saw you using a multimeter in this video measuring current so was curious. I'm in the middle of making a DE amp for an electronics course final project, with the power amp based on the Fender Princeton. I have made notes from the videos you refer to for when I come to bias it. Your videos have helped me a lot to understand the inner workings of guitar amplifiers so thank you for taking your time to make them!
You're welcome, Cam. I'm glad the videos were helpful. Best of luck with your course and final project :)
thanks for all your great videos Doug and Rusty they are a great help to every one .Your a man with great talent
cheers Tony.
You're welcome, Tony. Thanks :)
Just got done doing it to my '79. Sorta jackleg, but I live in a condo and half my tools are at a storage facility. :(
Anyway, very clear and concise! Thanks a lot for taking the time!
+crustycabs You're welcome, CC.....glad the video was helpful :)
Wow no comments for a long time, nice ! I have a 1968 silverface prince, I learned a lot thanks
Glad to hear it, Javier :)
The best tube amp instructor/editor/photographer/best boy on the web bar none. My only question on this very well done video is can the pot or knob possibly ground to the chassis? The pot looks like its only a few mm from the chassis. Hopefully someday soon Doug will do a video on how to set up and use a scope/signal gen to troubleshoot and bias a tube amp.
Thanks so much, Mr. G. The pot is well grounded through the metal bracket that holds it. I have made several videos using an oscilloscope, please see my video list. The scope is not used for biasing.
Thank you so much for this extremely understandable and informative video.
You're welcome, LM. Glad you enjoyed it :)
Love it! might just add this mod to my Princeton. If you ever get your hands on another I'd love to see how the cathode biasing mod works! I hear it really changes the sound. thanks for everything!
The conversion to adjustable bias is rather easy and useful for the Princeton, Frankie. Cathode biasing is generally not necessary or all that helpful.
Once again.. thank you for sharing your knowledge with us. I am about 2 week away from digging into my Princeton... and im definitely going to do this. P.s. Seeing that board is warped makes me feel better about mine....is pretty wavy lol.
I love all your videos .. very informative and well explained in a format that anyone with any degree of skill can interpret.Ive built three marshall style amps,and have had a wonderfull time doing it ,the videos you have put up here have provided me with extra information that came in very handy while figuring out some problems i had with circuits.. thanks again for your efforts .. PS im also a big fan of Rusty
You're quite welcome, F9. Rusty and I are pleased to hear that the videos are clear and helpful, and want to wish you the best.....or should I say "beast" (from Rusty :)
Ok, got the feedback loop fixed. Made it like AA864. switched OT wires. All good. Might try cathode follower. Hoping it isn't too loose. Thanks again!
You're welcome, DS. I'm glad it all worked out well.
Fabulous video. Great step-by-step instructions.
Thanks so much, Joe :)
Thank you very much for a very nicely presented informative procedure!
You're welcome. Thanks for your nice comment :)
So good! I'm going to do this very mod to my Princeton once I figure out the red plating thing.
added to favorites for my '66 VC. wow, it'd be cool to have this baby too...
Glad you liked it, Steve :)
Thank you very much, Uncle Doug!
Glad you enjoyed it, Nicholas.
Hi uncle Doug, I am a faithful spectator and I am looking forward to every video of you. I'm sorry for my horrible english -
I'm 60 years old and never had english lessons - (russian in school ;-)) This is not a criticism your work - but I have
a question about the timecode 10:50 to 10:55 of your video: You are talking about -20 miliampere anode current but the
TEK4 shows - in my opinion, only - 0,20 miliampere - if the measuring range, "mA" right in the window of the TEK4, is correct -
or am I wrong?
And again: thank you for the opportunity to learn from you
Erwin
Greetings, Erwin. Your English is far better than my Russian ;) You make an excellent point. The readout of the TEK4 does appear to be 0.20mA rather than 20mA. I'm not sure how I had it connected or if the range was correct. I know that the PC was 20mA, so my guess is the range on the TEK4 was not correct.
Hello Uncle Doug , thanks for the answer - it is as it is and I knew that with 0.20 mA the amplifier would not really work - but the two 6V6's would have a very, very long and above all quiet life ;-) wouldn't they? ,.. or not-I still have a lot to learn... Many greetings from Hamburg
Hi Doug, Many thanks for another great video. There seems to be a good deal of discussion regarding how hot or cool you bias your tubes and how this effects the sound of the amp. I would be interested in understanding if this is a "perceived" difference or exactly how much does the frequency plot change vs bias settings. I believe it would make an interesting video.
I already posted a video showing the correllation between bias and tone, Geoff. Check it out.
Much easier than replacing resistors 'til you get it right. Rusty's camera work was very steady. :))
Yes, indeed. The biasing procedure really does only take a couple minutes to perform now, leaving me more time to cater to Rusty's contractual benefits. I must admit, however, that frequent steak dinners do seem to make his camera work a bit steadier.
Hello Doug, you talked about matched tubes but in other videos you said that the unmatched tubes would give the amp a little character tone.
thanks........
I'm just alerting the audience to the fact that there are alternatives to perfectly matched tubes, Jim.
Another very clear and informative video Uncle Doug. I had a couple novice questions after watching. At 8:20, you say "this is a negative voltage circuit the wiring to of the capacitors and the diode are backwards." I expect most of your subscribers know how you could determine this from just looking at the circuit itself, but I wonder how you would know this either from the circuit or the schematic. Also, you note that the tubes aren't matched and the pot biasing with affect them differently--better to have a matched set. I wonder how tubes are tested to see if they match without putting them in the amp? My takeaway from this video is that if tubes are matched, the adjustable grid bias works the same for both and the only reason Fender didn't do it in the first place is simply cost. Is that so? Thank you and Rusty the Wonder Dog again for your time and highly enjoyable video.
Greetings, Carl. The high voltage circuit in amplifiers and radios is positive direct current (DC) and often called B+. The tube bias circuit, however, is negative DC. Since it's charge is the opposite of the B+ circuit, certain components, like diodes and electrolytic capacitors that have polarity (i.e. they must be installed in the correct direction) are installed "backwards" in the negative bias circuit compared to when they are installed in the B+ circuit. Failure to do this can cause a lot of problems.
The polarity of these components is clearly marked on the schematic for the Princeton Reverb. The 50uf capacitor is + to ground (instead of -), and the diode band is on the 100K resistor side (not on the tube grid side).
Output tubes are matched by putting them in a circuit and measuring their plate current (PC). Tubes are like fingerprints, they each draw a unique amount of PC......generally more when they are new and less as they age. After measuring the PC of a bunch of tubes, those with very similar PC values are put together in "matched pairs" and sold for a premium price. If you have matched tubes, then a single bias adjustment will work for the pair. If the tubes are badly mismatched, each one needs to be individually biased to neutralize the difference. If you have a bunch of vintage output tubes, you can do your own matching (by testing the PC of each) prior to biasing them.
I agree that Fender probably omitted the adjustable bias in the Princeton amp to reduce production cost. I hope this all makes sense. Rusty and I thank you for your input and wish you the best !!!
Thanks Uncle Doug. I'll look at the Princeton schematic and follow along with your description. Please assure Rusty I'm keeping my fingers out of harm's way until I'm better educated. Paper is much safer than high voltage components.
Aloha Uncle Doug,
My cousin found this tool that calculates frequency responses for Fender tone stacks, giving the option of different capacitance and resistance choices, using stock and other common values. Thought you and Rusty might find it of interest. To get it to work properly you have to download the app.
demonstrations.wolfram.com/FenderToneCircuit/
Thanks, Carl. I will check it out :)
What a good and doable modification. Wouldn't be better to use a trimpot? I'm afraid the pot could move with vibration while moving the amp. Thanks so much!
Glad you liked it, C. Use a trim pot if you wish, but vibration has never been an issue with the other pots in an amp circuit, so why worry about a bias pot ?
I am now considering making this mod to my Princeton reverb. Also, many thanks for the heads up on the stock 50uf capacitor being rather low in terms of voltage rating. I shall check it and swap it out to be safe I think.
+lexzbuddy You're welcome, LB. I think you'll like the adjustable bias control. The higher voltage rating on the cap is definitely advisable.
You could add a balance pot network to balance out tube mismatch from ageing also,i need a balance pot network.
And i love seeing rusty,i know he's no longer with us,but all the same,i love dogs.
You see ,your dog has a different temperament to your wife(not your wife ,just in general),
because the later you are out,
the warmer the welcome you receive when you return home.
Love the vids,i'm a tech and do make amps,but i don't get to see inside or work on older amps,plus the mistakes fender,marshall made,like not adding variable/balanceable bias circuits.
Subbed and like every vid.
Thanks, John. Some Fender amps came with the balance control you suggested. Unfortunately, not many come with both the variable bias AND the balance adjustment.
I'm in scotland so i would use pounds sterling,but the term "dollar driven" still applies ,considering what two or three pots and a couple of resistors(all bought in mass bulk too i might add),fender,marshall to name a few are mostly cheaply built RCA copies,and custom builders (like you and i) can be asked to justify a higher price rate,and for something that would equal the build quality of old valve/tube hifi gear from early years that still work to-day with very little done other than maybe caps and tube/valve jobs.
The modern pcb amp "made in (3rd world cess pit) are over priced garbage badly built with bad design criteria.
kindest regards,
jh.
Nice work!
Thanks, Rich :)
Hey Uncle Doug. I just cracked open the case in my 64 Princeton NR and my smoothing cap is a Mallory 25mfd / 50v!
+jorma Spaziano If you are speaking of the bias circuit, then you should change the cap to a 50uf @ 100V.
I'm having a bit of a time finding that value. Where did you get yours? Thanks Uncle Doug!
Antique Electronic Supply is an excellent source.
Thanks Uncle Doug! Great video!
You're welcome, V :)
Doug, I must say awesome video!
I noticed you directly measured PC with your dmm, meaning you put it in series with the actual current. I’d rather not do this. Just wanted to point out that you’re still able to use the calculation method you presented in your “biasing double ended amps” video, correct?
Thanks, Nathan. I understand your hesitation. A good friend of mine who's been an electrical engineer for many years burned up a Fluke DMM doing this same procedure....so it's definitely hazardous. I did it strictly for the video, so the actual PC would show on the screen. Yes, you can use the voltage drop across the output transformer primary method......it will work just fine.
I study electrical engineering at ASU and they've always told us to use the calculation method for that reason. It was nice being able to see the current measurements on screen though! I just wanted to point out to others in the comment section. Thanks for the reply, best wishes to you and Rusty!
You're welcome, Nathan. Best of luck with your studies.
Enjoy your videos ! You make them interesting !
Thanks, Kal. I'm glad you like them.
Hello Uncle Doug, i installed the bias control on my princeton reverb amp and it works just fine! just one question, on the end of the video you demonstrate how smoothly the adjustments works by measuring mA. Did you disconnect a wire, or do you use a tube adaptor with external wiring? do i have to recalculate each time i adjust the value to get the right PD? greetings Benny
+BAMPSANDPEDALS To directly measure Plate Current, I was using the OPT shunt method: You attach your leads to either side of one half of the OPT primary (the half connected to the tube you are biasing), then using the Ammeter function of your multi-meter, you measure the current shunted around the winding and through your meter (due to it's near-zero resistance). If this is confusing, then you might study up on the method using Google searches for more info, or simply do what you are doing. There is some risk to this method and I don't advise anyone else to use it unless they are experienced, knowledgeable, and comfortable with it. Be careful. Good luck.
So using this method no current is flowing through the transformer primary, and it is measuring current essentially without the transformer in circuit? I've noticed most transformers actually show different resistance (I know, NOT impedance) either side to center tap so balancing an amp might also involve swapping tubes inside to outside? I've seen others putting a 1 ohm resister cathode to ground on each output tube and measuring the voltage across each resister. Using Ohm's law, knowing voltage drop will give current flow. Do you consider this advisable? would you need the bypass electrolytic across this 1 ohm as well?
Yes, the current takes a short cut through the meter, allowing you to measure it. Yes, the two primary windings of the OPT almost always have different resistance, which complicates the biasing procedure a little because it affects the current running through each tube, even if the tubes are perfectly matched. For this reason, each output tube must be biased separately, if possible. Yes, swapping tubes is a common practice. You can install the higher current tube with the higher resistance winding to try to balance them out. I don't like the 1 Ohm method. Even a slight error in its resistance can throw the result off quite a bit (i.e. 10% or more). The electrolytics are no issue since current does not run through them.
Thanks for sharing. Timeless good technique.
Thanks, Jon :)
It's hard to judge on my crappy computer audio setup, but even on this ragged system, that Fender Princeton AMP sounds really sweet. It seems to have a real rich fullness to it. Thanks for taking us through the Grid Bias mod. I presume that in person, you can hear the difference in a maximized bias setup or you wouldn't bother. (Or maybe just to save the tubes?)
Any Princeton that doesn't sound great is simply not working properly, Trebor. They are like polished gems in a quarry of rock ;) Yes, in person, all the differences are more apparent.
@@UncleDoug If you saw/heard how crappy my audio system is on the old desktop, you would have even more appreciation for their qualities! Thanks again for all of your wonderful videos. It 's like dinner and a show.
@@treborheminway1196 You're welcome, Trebor :)
I wonder if it's possible to set bias for each output tube? Love seeing the older videos with Rusty.
It can be done. I a few of their larger amp circuits, Fender provided a Bias Balance Control.
In cathode bias situation, it's always better to use a separate resistor per power tube. I experimented it myself and it reduces the hum to at least nothing, especially with not properly matched tubes. On the other hand, I widely prefer my power tubes to operate in Push/Pull.
Excellent suggestion, Paul. I believe the manufacturers started the single-resistor method simply to reduce build cost and time.
The original construction doesn't have any voltage regulation on the biasing line. It's simple, and it works. When the mains voltage changes (rises for example), the plate voltage rises, but the negative bias voltage (to be exact - it's absolute value) rises too, which limits the plate current more, keeping the plate dissipation equilibrium more-or-less.
If I was to modify the amp myself, I would add a second diode, to have a full-wave rectifier and less ripple on the filtering capacitor, but this would mean changing a couple of resistors too to get the voltages back to the same level. But that's just my obsession with hum, and dislike for half-wave rectification :)
But then again, maybe this slight built-in 60Hz tremolo (due to some ripple in the biasing voltage) adds to the character of the amp :)
Best regards
An interesting analysis, Stan. It is a little worrisome to apply a half-wave-rectified bias voltage to your output grids, but I must say that hum has never been a problem with the Princeton Reverb. Thanks for your input.
love the circuit i can adapted it to my amps Doug i just get rid of the intensity control
Hi there Uncle! How's rusty doing? Thanks, as always, for sharing with us yourself and your knowledge -I hope you know me and a whole lot of folks appreciate it.
The video, superb. One question though. While measuring the negative Vdc you set the minimum at -38Vdc. Correct me if I'm wrong here but was it not -34Vdc what the schematics suggested?
+Fuga Mante Greetings, Fuga. Rusty is doing very well, especially for an 18-year-old pup. The schematic value is a ballpark estimate and varies greatly with different output tubes. As you could see when I actually biased the tubes, the range of values I chose worked quite well, with lots of adjustment left in both directions.
Hi Uncle Doug! Nice Videos! Where do you measure the Plate Current? I'm trying to BIAS a Fender Deluxe Reverb and I was trying to do this grid biasing. Thanks!
Greetings, Gus. I have posted a two-part video that shows how to bias a double-ended amp, such as the DR, step-by-step: ruclips.net/video/w9B0Rhr_Y-E/видео.html and ruclips.net/video/pjKYiSr497w/видео.html I hope they are helpful. Please be very careful and follow all safety procedures. Good luck :)
I'm trying to make the readings like you said on the first videos about bias but It's strange. My resistence reading is measuring -634 ohms / -03.8 Voltage Drop / resulting in a plate current of 0,0063 with a plate voltage of 660 resulting in 4,18 W. I'm bising a Deluxe Reverb 6V6... My mutimeter is manual maybe Im selecting the wrong reading... Don't know...
Reading the voltage in pin 5 (grid) is reading -35 but I can't see in mA to see the bias. Thanks for your help
Uncle Doug Hi Doug! Thanks! I saw your message now. I saw the videos and I'm going to buy a better multimeter to try again. Thanks!
Gus, your readings are quite different from those I recently got from a DR. OPT Center Tap to pin 3 of Left 6V6: Resistance = 216.6 ohms, Voltage Drop = 4.59V, PC = 21.2mA, PD = 9.95W. The readings for the right 6V6 were quite similar.
I think there may be some problems with the way you are taking the measurements. Perhaps it's your multimeter that is at fault. You should watch the video again carefully, and maybe seek the help of an experienced amp tech to guide you during this first attempt at tube biasing. Good luck.
Hi Doug
As your goal is for for 8.4 Watts idle power dissipation does engaging the tremolo disrupt that goal
It probably does modulate the bias up and down a bit, WC, but not to a harmful degree.
Doug, are the 60's and 70's Princeton reverbs considered to be Class A or Class AB push pull amps ? Your videos are the best available. Thanks
I consider them to be AB class, BB.....but, to be honest, the designation doesn't matter that much to me. I just focus on achieving the optimum operating parameters for good tone and component longevity.
Doug, fantastic video here, as all of yours are (full credit to Rusty also).
I have an Ampeg Jet 20, which is a Korean faux reissue supposedly based on the Fender 6G3 Deluxe circuit. It seems well enough built and has a tagboard construction so no nasty PCB stuff to fiddle with here.
To get variable bias, I want to replace the 22K bias resistor and happen to have a nice 10K CTS bias pot sitting here. Could I just put the pot (using just one side of the pot and wiper) in series with another fixed resistor, rather than doing it the way you have done? I'm thinking perhaps around 15K on the R (hence 15-25 sweep) would then give me an OK range...or am I missing something e.g. re the suggested value, and are there disadvantages to this approach?
There is a guy on the internet who did something similar, but modded his with a 100K trimpot (weeny thing) in parallel with a 51K resistor. NB on the Ampeg the bias is on the main board, not a separate one like good old Leo. I thought about doing something similar, but prefer the idea of the CTS metal pot mounted outboard as per your approach. I can see the idea of his to have parallel VR (100K) and fixed R (51K) but of course I only have 10K to work with on my pot.
RIchard
twerpeater Greetings and thanks, Richard. I understand your situation and see no problem in trying what you suggest. I would use patch cords to set up the series circuit, i.e. pot in series with a 17K fixed resistor.....try it out to see if it works.....and then either modify a bit, or hard wire it into the circuit if you are pleased with the result. Monitor your plate current at all times to be sure you don't harm the tubes while experimenting. Good luck.
I used to have that Ampeg, had to sell it to fund a Tweed Deluxe... I miss it, it was a great amp.
A Tweed Deluxe would definitely reduce my sorrow over the loss of the Ampeg (even though the Ampeg was great amp). It sounds like a pretty good trade to me, Javi, but it's a shame you couldn't have them both.
Thank you so much for this informative video. I do have a question, in the video it appears that you put an insulator of some sort on the 27K resistors leads on the bias board. Is there a reason for this ?
Insulation is often placed over the exposed leads of capacitors and resistors to avoid short circuits.
Hello Uncle Doug, I watched one of your vids where you designed a custom amp with a Princeton type push pull output. You mentioned that altering the resistor value at the input to the intensity pot would create havoc with the intensity function. What would be the changes we would see? I chose to change my bias to your suggestion of individual cathode bias, which I settled at 470 ohm each and have 9 watts each. Dead quiet, and with the celestion alnico blue 12" and the reduced capacitance to reduce the over active bottom end. Love from England.
I don't recall the "create havoc" comment, Ian. I'm glad the individual cathode biasing worked well for you.
Beautiful job!
Thanks, Michael :)
Hello, Great video. please excuse my ignorance, I'm not very familiar with electronics. After watching your video would it be possible to add two adjustment knobs so you can Match Bias both tubes ?
Thank you for your video.
Thanks, Daryl. Yes, indeed. In fact some Fender amps came with this dual adjustment feature.
Great Video.
Thanks, Franky. Rusty and I are glad you liked it....:)
I'm considering this mod for a Mojotone Princeton kit that I built, but I can't seem to find a bracket to mount the pot on. Any clues?
Thanks so much for all the videos! I am a big fan and feel like I'm back in school with one of my favorite teachers soaking up all the great info.
I make my own brackets out of old steel or aluminum strap, John. It's available at Home Depot.
Some more suggestions:
If you put in two seperate pots, one for each tube - may along with a 47Ohm/5W resistor in each of the kathode path of the output tubes. It is now possible to insert any 6V6 tube you got - if it is not dead ;)
You loose some output power but you are no longer need to match your output tubes. As this circuit does it for you. Simply match the cathode current by adjusting the pots you just inserted and way you go...
How about replacing the two 220K grid resistors with a 500K pot (input bias voltage connected to the wiper) and then adjusting it to dispense different amounts of bias voltage to the tubes until their plate currents are balanced? The only downside, as you warned in another comment, is that you face the possibility of a loss of pot wiper continuity and tube bias.
You can do this as well, the result would be the same in first place and as you described it
But the driver stage will see different loads when the whiper is not in center position.
Let say the whiper will be in 2/3 of a 500k pot.
one half would be 166k and the other 333k resistance presented to the driver stage.
If the whiper would fully CCW or CW turned you short out your driver stage completely (for AC). If you see the whole schematic from AC topology. DC coulped is all correct, but AC would be differ a lot.
Be as symetric as you can, your amp will thank it with good (=clean) sound, add single side a little DC-offset in the driver and your amp will produce even harmonics. Vary single side driver amplification and your amp will produce even harmonics which will differ upon volume settings.
I can see that turning the pot to either extreme would cause problems. How about a 100K pot with 200K resistors between the pot output lugs and the tube grids? This would provide balance capability without jeopardizing the tubes.
Uncle Doug This will work well, as the AC loads differs not that much anymore.
Thanks, TC. I really appreciate your expertise and input.
Hi Uncle Doug. Your videos are very informative, as always. I have a question about your adjustable bias circuit. If the wiper on the pot were to fail or become dirty, is it possible there could be a meltdown of the output tubes? I'm wondering if a bias circuit could be devised where the pot is only part of the shunt element? Thus, a failure would result in a more negative bias voltage.
Thanks, A61. The current flow to the grids is negligible, so the adjustment pot is completely unstressed. Bear in mind that Fender has been using bias pots in hundreds of thousands of amps built over the last 60+ years, and to the best of my knowledge, pot failure has simply never been an issue. Indeed, there are all sorts of more likely failure points within amp circuits.....I deal with them daily......but so far, not a single failed bias pot.
Thanks for another excellent video! So just so my neophyte brain can get this straight, before the mod this amp was cathode biased and you would aim for a plate dissipation of close to 100%. Now it’s considered grid biased and you’d want the dissipation to be around 70% correct? I’ve been asking a lot of questions lately and you have been very kind to answer each one, I’ll slow down for a bit now 😂 Thanks!
Every double-ended Princeton I've ever seen is grid-biased, Tony. The mod simply makes the biasing adjustable.
@@UncleDoug Thanks for answering another beginner question, it’s all becoming much clearer now lol. You rock Doug! 🤘🏻
@@tonyhills2112 We're glad to hear it, Tony. Good luck :)
Very good info Doug, Thank you very much. I'm having one hell of a time trying to get my first attempt adding Tremolo to a 6V6 push /pull build working. I've converted/built over 50 amps and finally decided to add tremolo to one. I'm now debating using one or both halves of the 12AX7 that I added. Wonder which sounds best. I'm guessing the one that changes the power tube voltage as opposed to the preamp tube.
Either version works well if properly designed, Jimi. One triode is generally for the oscillator loop and the other can be used as a cathode follower.
Thank you kindly. That was awesome!
Winky Dee You're welcome, WD. Glad you liked it.
Uncle Doug Rusty cracks me up!
He is a very talented pup.....if you don't think so, just ask him :))
Uncle Doug - I'm very new to tube amp repair and your videos are very helpful. Hope you make more soon.
I've watched this video several times and think I've finally got it. I wish there was a camera shot from the other side of the new pot. It seems that you either cut the yellow wire (connected to the vib pot) at the eyelet board or unsoldered it from under the eyelet board. Can't really tell. Also you soldered a black wire to the eyelet board at negative end of diode where the yellow wire used to be. Finally, you unsoldered the 22K from the board and replaced it with a 27K with some kind of tubing and soldering one end at the grounded end and the other end to the new pot. What I can't be see but indicated in your screen note is the installation of a 25K resistor between center pot lug the right lug. Is all this correct? When you say left or right lug of the pot, I assume that is with the pot oriented with the shaft up.
So now here's the real question. If the original power cord is replaced, the ground switch is no longer necessary. If the ground switch is removed, there is an available hole in the back of the chassis where a trim pot (no shaft) could be placed. This would allow for tube biasing without removing the chassis if you use a tube socket type bias device such as QuadStage BiasPro. Probably to do this right a new eyelet should be installed in the board to attach the free end of the 27K resistor and a wire to run to the pot. Or the ground switch could be replaced with a standby switch. Standby switch or bias pot? What are you thoughts?
Bob, I made this video quite a while ago and the specifics, such as wire color, etc. now elude me. Your best bet, if any of my connections are not clear to you, would be to consult a Fender schematic and/or layout diagram for the circuit you are working on. I explain the theory of the modification in the video and this should enable its transference to your circuit.
Re your second paragraph, yes, you could replace the bypassed ground switch with the biasing knob......however, you must be sure that nothing can alter the setting after it is made. This is generally done by cutting a slot in the pot shaft so a screwdriver can turn it, and not installing a knob that can be accidentally rotated. It should be noted that tube bias is based on Plate Dissipation, not simply PC and PV (upon which it is based), so any bias device must provide both measurements so that the PD can be calculated.
Rusty seems to be a very self-sacrificing dog. Great vid!
He is indeed a dedicated, hard working hound dog. Thanks, Spinn !!
I’d assume that these measurements can be made by using Uncle Doug’s bias-probe as described in another excellent video tutorial 😁
Yes they can, KC. Thanks !!
Hi Uncle Doug. Another informative video. I was wondering if you could put a VR in the cathode in a doubler circuit.? Would that work to control cathode bias. Just wondering.
I'm not sure that I understand your inquiry, Monty. The "doubler circuit" confuses me.
Sorry Uncle Doug I miss spoke. I was thinking a devider circuit on the cathode to balance the tubes. Its late. Sorry
Hi Doug, thanks for another great video!
Just a quick question, how are you measuring the current? Are you putting the amp meter in between the pin of the tube's anode and its wire or using specialist current measuring socket?
You're welcome, Cam. I explain the method in a couple videos, but will offer a brief explanation here. In a DE amp like the PR, with the amp off, I measure the resistance (R) of each half of the output transformer primary winding (first 6V6 plate to CT, CT to second 6V6 plate). Then while the amp is on, you measure the voltage drop (V) across each of these two resistances to get the PC for each tube (C = V/R). This is the safest, best method I have found for this measurement.
It's a wonder to listen to!! Excellent teacher! Say Uncle, can I send you my tube amp schematic (fixed bias) to check if a similar mod would be possible?
If the amount of -DC sent to the grids is regulated by a fixed resistor to ground, you can substitute a smaller fixed resistor plus a pot to make the bias voltage variable.
Hi Doug,
Great video as always. Just a quick question. With regard to your bias voltage range, were the results shown here with output tube's installed or not? The reason I ask, is I just finished a quasi 6G3 build (minus Tremolo Section) , and my negative bias voltage varied significantly once the output tubes were installed. Understandably being due to the load applied to the power supply. Like your amp, my adjustable fixed grid bias was derived from the HT winding of the Power Transformer.
Regards, Dan
As I recall, Dan, the bias voltage range shown on the meter was obtained without the 6V6's in place. My reason for this was to set the bias voltage at its highest setting (and PC at its lowest) before installing the tubes, to protect them. I never measured it with the tubes installed, focusing instead on PC and PV, but would assume that it would be lower.
Rusty is my hero!
John Pittman Thanks, John. I know that his countless groupies share your feelings :)
Quick question about replacing the diode... what amp did you go with? The aa764 schematic doesn't give us a value for the diode.. just that there is one lol. I'm about to order all the parts.. 47uf (lol you had me going trying to find that 50uf) capacitor.. pots.. and new power tubes (im pretty sure ive got a 6v6 that's bad.. not a transformer after doing a bit more research).. but the value for the diode eludes me. Thanks again for the help.
Greetings, P6. Sorry for rounding off the cap value ;) Re diodes, about the only thing you need to worry about is voltage tolerance, and the 1N4007 diode is generally used (1000V @ 1A) in the bias circuit. Good luck with your project.
very neat. I will be using your mod.
Sounds good, Jason. Let us know how it turns out.
Hi Uncle Doug. I have to ask something. If the negative side of the diode is the banded side, why the hell on the schematic the diode was written + to negative and - to positive?
You ask an interesting question, SW. Traditionally, the banded end of a diode is the cathode or negative end....yet in Fender schematics, the banded end is marked + and the unbanded end is - . I believe that the seemingly backwards Fender designation is simply to make it clear that negative DC is passed by the diode and utilized for grid biasing. If the diode were to be marked conventionally, it might look like the grid-biasing DC was + .
Uncle Doug, I have a question, I don’t know if is possible… but i just thinking about make two grid bias pot and disconnect 270k resistor in order that each one receive an individual grid bias pot… in order to Match unmatched tubes… thank you for your very educative videos
It's more common for there to be an overall bias pot, then a second (balance) pot to distribute the -DC bias voltage to each tube grid, Jonathan. But you could experiment with other configurations.
@@UncleDoug the idea was came when I see this picture where is a kind a multiple variable grid bias (or at least I think so)
www.tetrode.co.uk/sweep3c.jpg
I just try and hope I won’t burn my house, thank you uncle Doug
@@jonnathandiaz7139 Careful experimentation is always a good approach, Jonathan. Good luck.
dear Uncle, I've completed my Reverb fixing, and it was performing fine, then I decided to check for plate dissipation, according to your video guides. I measured voltage drop across center tap of OT and the other two ends, each for each power tubes. I had about -4V DC, with a plate voltage of about 399V DC. This would mean a PD of about 10/11 Watts. That seemed a bit to high to me, so i decided to install the small 10K pot to adjust bias. If was responding ok, I swept it so to have around 8 W of PD. After that I was putting back the ampli in his case, then I was horrified to discover that no sound was coming out from the speaker. I did not make any evident mistakes, no smoke, no shorts....I removed the pot and restored original connections but stilll no sound, just a little hum. So it is possible that I have killed the 6v6 during my tests? I'm lost at the moment, going to follow the whole signal path to see whats wrong.
You didn't provide enough information for me to check your calculations, D, but I doubt that you destroyed your 6V6's while reducing their bias. Check the bias voltage on the grids.....you may have inadvertently increased it to the point of stopping the current flow. BE SURE you are properly calculating your PD.....remember that PC is voltage drop divided by OPT winding resistance.
@@UncleDoug ok Uncle, all sorted out...the culprit was one of the copper wires connecting components on the preamp to the chassis under the pots, which was disconnected leaving all those component with floating ground. Anyway, I've set the bias pot as follows: on one of the 6v6 plate voltage is 405 V to ground, resistance of that side of OT is 146.6, Voltage drop across it is 2.95 ohms, so PD should be 8.14 W, am I right? The other tube has similar values. Am I too cold? The amp tends to distort a bit at high volumes on input 1. Is that desiderable or normal? Thank you again
@@dreamlessfull Your calculations are correct. If your circuit is grid biased, 8.14W is ideal. If it is cathode biased, this is way too cool.....and should be more in the 11W range.
@@UncleDoug yes, being a '67 blackface princeton reveverb, it is grid biased, as the cathode is tied to ground. Thank you again for your time and your invaluable teaching!
Hi Uncle Doug,
Thanks for all the great videos. I've learned a lot by watching them. As the proud owner of a 1966 BF Princeton Reverb, I want to go through adding adjustable grid bias to my amp via your procedure. However, I have a question. In your 12/9/2013 video about biasing double-ended output tubes, you caution against using a grid bias pot due to its effect on the vibrato circuit. In this video, performing that modification is detailed. Just wondering if you had a change of heart about the effect on vibrato or perhaps made a New Years resolution to become less of a vibrato hog?
Thanks again!!
I guess I found that the good provided by the bias pot outweighed any possible bad effect on the tremolo.....and that the "bad" was virtually indistinguishable.
@@UncleDoug Cool, kind of what I figured. Like most things in life, a balancing act. I learned a lot about electronics by virtue of an intense, 5 Trimester Electronic Technician course at DeVry completed in 1987 but since I didn't go into a career of component level trouble shooting, I'm terribly out of practice. Your videos are a great refresher and combine nicely with my love of guitars and amps. Thank you
You're welcome, UT.
Hi uncle Doug. We chatted a few weeks ago about our favorite amp the Princeton Reverb. I watch this video over and over again and just realized that you made your bias adjustment based on plate current. I biased to the -34vdc on the schematic. That doesn't give me a accurate measurement of the tube voltage and dissipation. Would you recommend that I go back in and do the same rather than trust the bias voltage from the schematic?
The -34VDC figure is simply an estimate and varies depending on the age of the output tubes and condition of the circuit, Tele. To accurately set the bias for a specific amp circuit, you need to measure both the PV and PC, then calculate the Plate Dissipation, which should be around 8W or slightly higher for grid biased 6V6's.
Hi Doug, I hate like he'll to bug you, but I just wanted to ask you a question. You the only one that I know of that could give me an answer to my problem. I have a Fender 1993 Blues Deville 4x10 amp. It has a nonadjustable bias. The cathode resistor is 27K. I've read that you can take a 25K linear potentiometer and run a wire from one side of the holes where the 27K resistor was and connect it to both the left and middle wiper. Then you connect another wire from the other hole where the 27K resistor was in series with a 15K resistor to the ground prong the 3rd leg. Would this be the correct way to hook up an adjustable bias pot? I want to make sure that I am doing this right. Thanks from Michael Newell from Niagara Falls, Ontario, Canada
I much prefer wiring schematics to verbal descriptions, Michael, but I think your description is correct. There is one way to be sure. Wire it this way, adjust the pot for maximum resistance to ground, attach DC voltmeter probes: red to the hot bias wire (goes to the tube grids) and black to ground, turn on the amp, adjust the pot until the voltmeter reads -49.6VDC.....then measure the plate current and plate dissipation. Adjust the pot until these values are correct.
Hi Uncle Doug, sorry to hear about Rusty, really looked forward to seeing him in your videos. Thanks for all your work and sharing your vast knowledge and skills.
I have a question on this circuit....Does the Intensity pot vary the 6V6s bias when the vibrato is shut off? This has me perplexed thinking that its possible to vary the output tubes idling bias by changing the vibrato intensity control please help me understand.I can understand how the vibrato would modulate the bias to get the vibrato effect but once it is shut off does it still change the tubes bias?
The bias is set for the output tubes with the tremolo shut off, otherwise you couldn't get a stable reading to base the adjustment on. Once you turn on the tremolo, the bias varies above and below the set bias point to give the tremolo effect.
@@UncleDoug Thanks, the only thing i can come up with is that the grids don't pull any current so the inline resistance of the Intensity pot doesn't change the negative bias voltage no matter the setting on the Intensity pot?
Doug, I've just built my first Princeton Reverb and spent all evening trouble shooting it. I got it working but had to revert the bias to standard. The pot is still in place so I would like to get the adjust working. What range should the negative voltage be from the pot in one direction to the other? I was only getting a few volts if I turned it slowly but if I turned it quickly it would give me more but then settle a few volts more than it started. It's just to help me trouble shoot it. Many thanks. You have inspired me. I have now built a 5F1, 5E3 and the Princeton Reverb..... I have five boxes with parts building up for five other amps.......
I should have watched to the end of the video again! I only watched it to the circuit diagram this time but when I came back to ask you I let it run and you show the range!! :-)
I'm glad you were able to resolve the problem, Darren. Best of luck with your amp.
Another really good video !! (:
Thanks, Kal.....glad you liked it :)
seems like you were measuring the tube current using the OT shunt method instead of using Ohms law as per your other video. Correct?
Yes, simply because it allowed for direct measurement for video purposes. This method is somewhat dangerous however, both for humans and test equipment, so I tend to preach the gospel of indirect measurement via voltage drop across OPT windings or cathode bias resistors.
Uncle Doug I see.
Hi Doug, first I really enjoy these videos and thanks for that. It appears that you are still responding to this older video so I have a couple of questions for you if you feel inclined to answer. Twenty years ago I purchased what I can best identify as a mid 70's SF Princeton Reverb. Some months into using it I smoked the power transformer. I have no idea how. I had it replaced by a qualified tech. It has worked fine (with limited usage) ever since. I got concerned when you spoke of Russian Roulet when it comes to the original pair of 50/50 capacitors. Is it possible that this was the cause of my burnt transformer? You didn't say what could happen if this pair is not changed. Thanks, TW (This mod looks so simple I might go ahead and do it - I'm sure Rusty is still your helper, just at a different level))
If you are referring to the 50uf @ 50V cathode bypass caps, they alone could not have caused the failure of your power transformer. The originals can be easily and inexpensively replaced with the higher spec new caps. You should also check your output tube bias.....which CAN cause the PT to fail.
Hi Doug. Do you think this will work on a single ended Champ style build with an adjustable bias with the cathode trem? I have an adjustable bias installed so I can run all different power tubes. 6L6 , EL34, KT's , etc. Thanks.
Are you sure that your power transformer can fulfill the current demands of the tubes you listed? Can your SE output transformer handle the high power output of these tubes? Finally, each output tube you listed has different load resistance and thus will reflect a different output impedance.....how will you cope with this?
@@UncleDoug Oh yes. I made certain of that Doug. What do you think about the bias? It drifts up about 5- 7 mA with all controls on zero after running a little while. As far as the load resistance goes I have multiple taps. to choose from ; )
@@williswet Is the tremolo activated? If so, even if it's set to zero intensity, the PD will be altered by it. You can pull the oscillation tube to check this.
@@UncleDoug It changes voltage on the 1st cathode since it's single ended.
Thanks Doug.
Hi UD! The BFPR is indeed a great Fender amp. In addition to lacking an adjustable grid bias control, there is another difference. All of Fender’s large amps have two resistors on each of the power tube sockets. A 1.5K resistor between pin 1 and pin 5. Also a 470 ohm, 1 watt resistor between pin 4 and pin 6 (unused pin used as a mounting terminal). I have been told that this helps the output tube stability and improves tube life. Do you agree? Are these resistors worth adding to the Princeton Reverb amp? Thanks.
Greetings, DR. Pin 5 is the grid in both the 6V6 and 6L6 and Pin 1 is unused....the only resistor I see in this position is the 220K bias-input resistor. The reason the larger amps use a 470 Ohm resistor on the screen input (Pin 4) is because they also have filter chokes. Circuits using resistors instead of filter chokes, like the PR, do not need additional screen resistance.
I was watching your series on biasing and watched this one again, and have a question about the Tremolo Intensity pot in the circuit. If this pot controls the bias of the amp by changing the Plate Voltage, isn't that going to be screwing with the power dissipation every time you use the Tremolo and change that setting? Doesn't make any sense to me. Could you clarify what's going on for me? Thanks as always.
The bias pot has no effect on the plate voltage. It regulates the -DC bias voltage on the tube grid. Plate dissipation varies greatly depending on the strength of the input signal......that's why it's set at 70% of max PD. Watch my other videos on tremolos, tube biasing, etc. for full explanations.
Uncle Doug, sorry about the confusion. I wasn't talking about the new bias pot, but the Tremolo Intensity Pot Center wiper lug that connects to the Grid resistors. The schematic originally shows the B+ going through the top of the 27K resistor, to the Tremolo Intensity Pot Center lug, and then up to the connection with the two 220 K grid resistors that connect to the Grids. So before you installed the new bias pot, wasn't the Tremolo Intensity Pot affecting the bias when the Tremolo Intensity Pot was adjusted?
Yes, the Tremolo Intensity Pot does indeed alter the tube bias somewhat, establishing the threshold at which point the tremolo effect becomes audible.@@wsscott72
Hey uncle doug, great videos you have over here! I have a background in electrics and playing guitar (both terribly), but i am mew to valve circuitry. I have decided after watching your build from scratch series to build my own princeton with this mod attatched. the Aa1164 schematic im using/modding has 5u4gb tube, and i was wondering if i could use a 5ar4 instead for some more headroom and less sag, can it be a straight swap or do i need some more modding? Many thanks!
Greetings, Craig, and thanks for the nice comments. The 5AR4 or GZ34 are both much better rectifiers than the 5U4. They can be substituted with no circuit alterations, other than making sure that the output tube bias is properly adjusted after the substitution.
If the pot fails due to contamination in a way that it creates a large resistance greater than 10k from dirt on the wiper, the tubes will redplate due to an absence of bias supply. I sometimes wonder if running the pot in parallel with that resistor to ground is safer. That way if it fails there still will be bias supplied to the tubes. I sometimes fret over all that bias supply depending on that pot not getting "scratchy". Also, wouldn't changing the value of that 27k resistor to a higher value give you a sweep in both directions instead of just biasing hotter?
As in all industrial made Amps one can see, that the company needs to get some profit out of them. So the little brothers of the Big´uns need some budget-law cuts. In here it is the grid bias. They use the pot as well to provide the following stage (1/2Ax7) an AC-load. clever ;) Pots, in those days, cost a dollar each for a company. And about 4-8 for a replacing one. So saving component count really was a big deal.
Removed the 470k at the 7025. Like taking a blanket off the bassman. Get this , someone had put a 1 meg in the neg feedbk loop . The one off the speaker jack. The amp is lively but you think that value is too high?
I'm glad to hear that the 470K removal worked well for you. Re the NFB, I would think that a 1 meg resistor would effectively eliminate any negative feedback, but if the amp sounds good to you, that's all that really matters. You could substitute a 1 meg potentiometer for the 1 meg resistor. This would allow you to select the amount of negative feedback that you like best, instead of settling for a fixed value.
How do you "match" the tube current to balance them? Is this really an issue? I thought that both tubes should be adjusted so they are operating in a true complementary fashion. Wouldn't this reduce notch or crossover distortion? Is this something that is added to the cathode circuits for each tube?
+Erik Thorsen In most cases, especially with fixed bias circuits, you simply use two tubes that have the same current flow.....i.e. a matched pair. If the tubes are cathode biased, you could bias each one individually, with a carefully selected bias resistor for each, to provide identical plate dissipation. If their PD is unequal, then it's like two people rowing a boat in an unequal manner.......efficiency and possibly tone is degraded.
I know it looks cool and high quality an' all that but that aluminium knob is overkill, but since I get your point somewhat, I'm thinking even More Over the Top. I'm thinking a miniature that looks like an actual era correct fender knob. Do those toy battery powered amp have them? If I find some I'll send them along for future fun.
Sometimes overkill is just what the doctor ordered :)
I'm not a doctor, but I play one with my stage name. So yes, as a matter of fact, I do more than occasionally Prescribe or Order Overkill for entertainment use. As both a Real person and character, you surely you understand more than I the finer points of internet knob close-up photography for for recreational use.
Peace be upon you and
Happy Thanxgiven
@@therugburnz I do indeed, RB. Likewise on the holiday good wishes :)
I have a question, my silver faced princeton reverb has an erratic tremolo speed pot. When I increase the the speed at about 8 it will cancel the tremolo. I installed 2 parallel 1meg resistors and a 0.2 cap like your video shows but the pot still acts up. can I replace the 3meg pot for a 1meg. In the speed control?
You can experiment with different pot values, Daniel. It's impossible for me to guess what value will work the best.
Hey Doug, thanks again for this video! I have a simple question about the capacitor that is inline with the 27k resistor. Does the end of that also connect to the new 10k bias pot? From the angle the video was shot, I'm unable to tell.
Sorry, if you already answered this question, I got dizzy reading all of the questions and replies.
Hey Rusty; what's up??!!
You may be referring to the filter cap for the -DC bias circuit, William. It's negative end connects to the circuit and the positive end goes to ground.
@@UncleDoug; thanks for the reply. Have a great day!
@@UncleDoug thank you for this reply, UD! However, I'm still not seeing it-- do you leave the cap in place where it was originally or do you connect it to the same point on the pot like the resistor? Saying "it connects to the circuit" is still a little confusing as, well, the whole amp is a circuit.
Thank you from a non-EE slack-jawed yokel.