Uncle Doug's videos are a masterclass collection, I love watching them and learning. And then there are quotes like these: "the last thing you want to do is die, while you're trying to bias your single ended amplifier"..
cmpmuller Greetings, CM, and thanks for the very kind words. Rusty and I are glad you enjoy the videos. I guess dying is indeed "the last thing" we'll ever do.....it would just be a shame to hasten the process ;)
I'm wondering if anyone has systematically assembled his videos to construct a primer on all things amplifier for the beginner ? I have begun such an endeavour, but RUclips seems to revel in scattering stuff all over the map. Here's what I've got so far (unedited): UNCLE DOUG NOTES How Tube Amps work ‑ HOW THE AMP WORKS 1,2 ruclips.net/video/x5SSKX74DKg/видео.html ruclips.net/video/901iaPVVzY0/видео.html TONE STACKS 1,2, ruclips.net/video/BatwDYFJ9ug/видео.html ruclips.net/video/uZHM5BwPLRw/видео.html CATHODE BYPASS CAPACITORS ruclips.net/video/HrkYUPmEUhk/видео.html OSCILOSCOPE BASICS 1,2 ruclips.net/video/ueOup-XBexU/видео.html ruclips.net/video/UswkMHK27F8/видео.html OSCILOSCOPE AC/DV TRACING, RMS OUTPUT MEASURING ruclips.net/video/PmVq7NLv31s/видео.html TUBE REVERB 1,2,3,4 ruclips.net/video/EZFuT4liLAw/видео.html ruclips.net/video/3KEiQBaYZDc/видео.html ruclips.net/video/wTwTcjMYnAY/видео.html ruclips.net/video/pjTpiQG-ov4/видео.html IISOLATION TRANSFORMERS ruclips.net/video/5SjVm-rF39A/видео.html TRANSFORMER IMPEDANCE RATIOS ruclips.net/video/5jUitplchok/видео.html CAPACITORS IN AUDIO CIRCUITS ruclips.net/video/l-u8J-yh9ZA/видео.html ruclips.net/video/9UULx3-hHv4/видео.html ruclips.net/video/YXChioNC5NY/видео.html RESISTORS ruclips.net/video/LBtoarc6Nls/видео.html ruclips.net/video/8ruznIHuh7M/видео.html ruclips.net/video/QFHcmST7AZ0/видео.html ruclips.net/video/ORjsSfab278/видео.html ruclips.net/video/TvQWX0M6hJY/видео.html POWER TRANSFORMERS ruclips.net/video/MXgG-0MIBGA/видео.html PT BASIC DESIGN AND FUNCTION ruclips.net/video/hJsPQdAhTkQ/видео.html GUITAR AMP VACUUM TUBES ruclips.net/video/Xo4nVjj4NSE/видео.html ruclips.net/video/Zh_51RP4V6I/видео.html PHASE INVERTERS ruclips.net/video/U6By31V9fDo/видео.html BUILD A CURRENT LIMITER ruclips.net/video/wRFRwOnLsZI/видео.html HUM AND ITS POSSIBLE SOURCE ruclips.net/video/GrVtX0QGNls/видео.html BIASING DOUBLE-ENDED AMPLIFIER OUTPUT TUBES, PART 1: BASIC EXPLANATION & METHODS ruclips.net/video/w9B0Rhr_Y-E/видео.html
I am going through these videos slowly (about one every 2-3 days) and they're helping me get through these days, while educating me at the same time. You're a great teacher, and I appreciate you sharing your knowledge. Thanks Uncle Doug! :)
Uncle Doug, I completed this procedure successfully today without incident, thanks to your excellent teaching. When I measured 450 volts after switching on my Vibro Champ, I felt a good amount of respect, but I didn't quake in my boots because I had watched your video. I did the measurements for several 6V6 tubes and found the newer ones (JJ) were running much hotter than an old GE tube (which was at 12.5 W with the original biasing). I also liked the overall sound better with the GE, so left it in there. Once I have to switch to JJs, I will have to rebias the amp because they're running at 15.5 - 16 watts of plate dissipation. I know how to do that now. The measurements were interesting. I also took the opportunity to glue the tolex back down and polish the front, using methods I learned from your videos. The amp is shiny and sounds great. I'm using a step-down transformer with this US-made amp, since I'm in Europe. I ran into some interesting polarity issues with that. If one turns the (Schuko) plug the wrong way around, the step-down transformer outputs 234 volts instead of 117. Not very nice. I believe many people aren't aware of this. I got myself a model with a polarity check so this doesn't happen anymore (it didn't hit the amp, I measured first). What is the result of running an amp made for 117 volts on 234 volts instead? I wonder.
Greetings, KD. A PD of 12.5W is still excessive and 16W is crazy. I tend to keep the cathode-biased 6V6's at around 11W. If 115VAC input can produce 450 plate volts, then 234V would yield close to 900 PV......which would breach all the capacitors and destroy the tubes.
@@UncleDoug I guess I might change the resistor then. I suspect previous owners in Europe didn't pay any attention to the plug orientation, so I'm surprised the amp still works and nothing looks destroyed inside... I should probably measure all the caps, though.
@@kneedeepinthedoomed Plug orientation is a safety risk but does not threaten the circuit components, KD. Increasing the bias resistor value will result in better tone (in my opinion) and longer tube life.
And this is when the light came on for me - no pun intended. You can read forums for hours, observing well intended people argue back and forth. It becomes a battle in my mind between trying to grasp the information itself and then sorting thru the emotion to see if there's a consensus of some kind buried in the arguments of the "experts." Save yourself weeks, maybe months, of forum scrubbing and please just watch this video. The Air Force paid good money to send me thru electronic principles and I still have a massive 3 ring binder full of my notes, but none of that helped me as much as this video. Bravo - well done my friend! Thank YOU AGAIN! *And my Champ thanks you too. 👍
ermmm...the plate voltage you measured was across the valve and the cathode resistor combined, as you connected the metre between the plate and ground, surely the valve is dissipating (296V - 14.84V) x 0.0415A = 11.69 watts.
You're right, Rod. It's more accurate to measure the PV between the plate and cathode, rather than the plate and ground. The error is minimal (0.4W), and the plate-to-ground measurement provides a slight margin of safety (for beginners), but it is more accurate. Thanks for your input.
Uncle Doug-I don’t have my RCA Radio Tube book yet, so these tutorials are essential to first pique one’s interest, then draw us in with some basics that don’t clutter our heads and obscuring our understanding in the process. Very skillfully presented. Getting close to pulling the trigger on the book!
Thanks, B, but judging from the fact that I always get some thumbs down.....granted they are from slobbering cretins.....would indicate that not everyone is a fan :)
Doug, all of your videos are great but this one is fantastic! I can't wait to use what you've shown here in my own builds. Oh yes, great advice: "Don't die while biasing your tubes. It's a real downer!"
I am new to vintage tube amps and found this lesson to be of great value. I always wondered how one would modify an amp to have more or less "head-room". I always wondered what caused an amp to distort with manipulation of the electrical components. Now I know, thanks to Uncle Doug. I really appreciate your lessons, videos and topics covered.
Holy moley UD, I went to the URL you suggested and boy do I have a lot more studying to do. But at least I'm seeing a little light the further down this rabbit hole I go. Or should I say rabbit tube? It's like an alternate universe down here. The more you learn the more you need to learn... magic... And a little trippy too.
Indeed, if you really want to go into detail on topics like this, the depth of information is almost unlimited. You need to ask yourself.....how much do I really want and need to know ?
@@UncleDoug I've said this before, and I'll say it again. Your advice is always spot on. At this point in time this is a perfect fit for me. I hobble around some, but I will soon get my ankles fused and after that I should be walking around normal. No worries :) until then I have some vintage test gear to work on and plan on repairing electronics for a living. Doing some of that already, mostly computers, but there is something about the sound of my guitar playing through that crazy Eico Signal Tracer... sounds strange good. But your right, this bunny hole is bottomless. So I've decided to focus only mostly on the Fender 5F1 & the Eico 147A schematics. So that is why I am very grateful for your first rate "tubetorials". Keep 'em comin! BTW I will be happily throwin down some well spent clams on your patreon as soon as I am able! Thanks again!
Great intro video. The KISS [Keep It Simple, Shtupid] principle is a very good one. When is comes to math for electronics, it cannot be too simple for moi. Thanks to your well-presented tutorials, I am beginning to grasp and actually understand the mysteries and vagaries of audio electronics. Thank ya ver' much, Unca Doug Now, on to double-ended output biasing , where the fun really is.
@@UncleDoug I had A-Fib for six years. THAT is like an earthquake going off in your heart, and it is horrifically miserable. Got that fixed. Now I just have six blown discs in my lower back. It's been a good past 15 years ...
Just wanted to thank you for your awesome videos! An electrical engineer friend of mine (guitarist, electronics tinkerer) recommended your channel as the best way to learn tube amp electronics--he was totally right. You're a fantastic educator.
What a fabulous lesson! I really like it because it offers constant safety reminders, an easy-to-follow, logical procedure, AND it explains how to voice (in part) an amp for more or less distortion. (Hope I'm not over-simplifying). Thanks Uncle Doug ! I'm feeling even more ready for my amp build with Bruce Egnater on Saturday coming.
@@UncleDoug I'll do my best to remember your tip about one hand only in the chassis. I can be impetuous and would hate it if my darling wife had to travel to a Detroit suburb to collect my ashes.
Wow 👍 I picked one of those up at the goodwill store for 7 dollars awhile back soldered the speaker wire's back on and cleaned the controls and it sounds great
I just found you the other day. Im loving learning about tube amps. I would never try this at home but its fun to know how the things work. I have a marshall DLC5 , fender blues jr and a boogie mk5 35. cheers
very valuable information here ,my therory and basic electronic training is starting to come back and make sense ,been 28 years but thats ok wont work on any of my projects until im well informed and have proper instruments to do it safely and proper
Uncle Doug once again great video thank you so much I hope you know how much you are appreciated for what you're doing for us noobs out here you are the coolest of the coolest
Very educational video on biasing an amp. Thanks, I now feel comfortable in checking/adjusting tube bias on my BF and SF VibroChamp's, A good way to think of the bias resistor, for me, is to think of a dimmer switch for a light bulb. Increase resistance dimmer bulb= lower output, decrease resistance brighter bulb= higher output.
@@UncleDoug I have a question for ya. My Blackface VC is alot darker sounding than my early 70's Silverface VC. The Silverface does have a Pyle Driver speaker, model MH810, in it. Other than that and new Tung Sol tubes both have all original components. Is that normal between the BF and SF VC?
Well I finally gave the Blackface VibroChamp a good going over to find out why it is so dark sounding. The problem is the speaker cone is shot, kaput, finished. Guess it's time to order a Weber 8A125
wish you had a tube amp builders class... oh wait, this is it. good video... coincidentally I had a scare with a small arc.. discharge your caps folks....
Very comprehensive and clear description. I had the miss conception that biasing was for matching pairs of power tubes and therefore, single power tube amps did not require biasing. I own 2 amps and don't use them regularly, should biasing be checked periodically or when replacing tubes. I am assuming the sound quality will indicate when tubes need replacing.
Thanks, Theo. To be honest, most cathode-biased output tubes (which are in virtually all single-ended amps) are probably not in any great danger from incorrect biasing, since they tend to "self-correct" and protect themselves from harm. It's a good idea to check their bias every year or two (or whenever you change output tubes) to see if the biasing resistor has changed value over time (especially if they are old amps), but otherwise, they tend to be "low maintenance". Fixed bias output tubes (with grounded cathodes) are another story. They do need to be checked at regular intervals and definitely rebiased whenever a tube or tubes are replaced. If you have a double-ended amp and are not sure what type of biasing it utilizes, I can tell you.
With the way you explain things it does make alot of sense. Thanks. I'v seen this vid. Of yours before But as a matter of study and learning I watch it again. Who's afraid of alittle Elesstrigerty ha. Don't touch! Oooh ah Pow boom yyooowoo I learned that lesson the hard Way back when I was 18 yrs old on an old Radio that I was repairing it didnt stop My heart but it was really beeting hard fast For about 5 or 10 minutes the shock Was painful. Guess I got lucky. Don't Forget REO spit. wagon. Dwight.
Very well done video! I learned something new. I thought by biasing the output tubes hotter, or with more plate current would actually cause amp to break up or distort / flattop a sine-wave at a lower volume level. Thanks for your explanation. I’m next looking forward towards seeing how you bias a tube amp with two or four output tubes with 4, 2 or 1 variable bias resistor in a amp that doesn’t use a fixed cathode bias resistor for output tube(s). 🙂👍
Thanks, Mike. The chart in the video was a bit misleading and I corrected it with graphics that are not visible on all viewing devices. Hot bias does induce earlier breakup, and cool bias increases headroom. I apologize for the confusion. This was one of my earlier videos. My knowledge and techniques have improved with time and my more recent videos are far more astute. I have posted a video showing double-ended biasing, variable resistor fixed biasing, installing of a variable bias pot, etc. I hope you enjoy them.
Very helpful. I am writing notes as I watch to keep this all straight in my head. Have been away from tube amps for a while, so it's good to refresh. I learn more each time I visit.
Thanks, Necro (Rusty is a little nervous about your nickname :) and you're welcome. I'm glad the videos have been helpful and interesting. Please stay tuned for Part 2 on the topic of tube biasing.
@@UncleDoug well you're here now and digital communication allows us to be in contact. I quit working as a technician 30 years ago. Digital technology made myself a thousands of other guys redundant. I worked maintaining telephone exchanges which were mostly relay switch operated. I also worked on big dc power supplies and have melted a few tools - lol
Awesome info , I have a bit of learning of electronics and this is much more straightforward than other biasing issues, also more understandable, thanks.
Uncle Doug for President! ... if only our politicians could explain things so perfectly. I think I'm really beginning to understand how guitar tube amps work ....
I'm confused now: 4:55 bias too cold = too much headroom, lots of clean before clipping 18:20 "this amp is biased very hot so it has lots of clean headroom" and in the picture "and distort at high volumes".
Greetings Uncle Doug, I was confused about how the plate voltage was measured, as I thought it would be between the plate and cathode, rather than plate to ground. I see you answered the same question below to MrAletube, so plate to cathode. Comparing plate to ground versus plate to cathode the difference seems significant in my 5F1 (363 vs 344V), and depending on which value used yields 12.4W to 13.2W (6V6GT). Bilbo, Sam, myself, and even Gandalf are wondering which is the more accurate way to know what the plate dissipation is--as we're creeping up on dissipation limits in our quest for the Ring of Tone. Forgive us for asking about to a very old video--news doesn't travel fast in shire.
This was an early video and the PV measurement was kept as simply as possible......but the most accurate method is plate to cathode, as you said. I am finding that excellent tone is readily available with very conservative biasing, i.e. 9W to 10W for a cathode-biased 6V6.
Hi Doug, this is the first comment I have ever posted. I wanted to thank you for this video and the follow-up video. I have been playing guitar for 30yrs and during the last few years learning about amps. I had found it very hard to find a thorough demonstration of the biasing procedure even though I understood the theory around it. Its really good that someone with your experience is posting this type of experience. Warm regards John C
Thanks Uncle Doug, this is the clearest explanation of cathode biasing of single ended amps that I've found so far. I really like the way that you explain things. I know have a much better understanding of biasing and it's relationship with headroom.
Well I have watched this a few times and I'm starting to get it. My 6V6 on the 9002 is perfect and removing the strange air gap resistor caused the thing to just start working. I am amazed, I actually fixed this thing... ;) Thank you Doug
Apparently my little Monoprice 15w is set cold. It's got a test point and adjustment pot inside, from what I understand. Headed to watch the double ended vids next in hopes of learning to check and adjust mine. The last post I found at TDPRI said this guy's was around -5.86v and he set it to -12v but I'm not just going to accept that as correct until I learn how this all works and whether it actually is correct for 2 EL84s Edit: so I found the schematic and it plainly says 'set to -12.5V' at the bias pot. So I guess that settles that?
Hello Uncle Doug. Thank you for this excellent tutorial. I gave you attribution for this in a video I did where I used this method to check the bias on a IF Amp tube in a '53 Chevy Radio (Part 6). Just wanted to let you know I let everyone know where I got the technique, and a link to this video. Thanks again. [edit: spelling]
Very Very interesting serie of video's. Very clearly explained. Thank you! I consider building a guitar tube amp. I wonder where I can find the wiring diagrams of vintage tube amps, as the ones you are using? Greetings from Brussels in semi-lockdown ...
Hi Uncle Doug, Thank you for the video. I am an electrician and guitar player. Loved watching ohm's law in action. Your safety measures and explanation were awesome. You have renewed my fascination with electricity. Thanks again. You have a new fan.
So, if 12w is the maximum for that tube. And you decided to sell this amp and lower the head room. Would use the 70% guide line to accomplish the task and there-by increase the tube life. Or is the 70% rule applied differently?
+david jones You're welcome, David. As demonstrated in the video, tube (or valve) plate dissipation must be accurately adjusted for best tone and tube life. Simply not glowing red hot, called "red-plating", is a bit too general an assessment, since the tube could be seriously underbiased, which seriously compromises tone.
I haven't read all comments. Maybe somebody noticed before me, but: If you measure current thrugh cathode resistor, you actually measure sum of all currents in the tube. Cathode emits electrones and they go somewhere -> as your tube is 6V6, it's tetrode so you have measured Ia+Ig2 (anode + grid #2 current). Not only plate current as you said. So you have to subtract: Ia=Ik - Ig2 As you don't have resistor in series with g2, disconnect it from supply and measure with amper-metre. Than subtract. g1 (control grid) doesn't pull any current as long as it's on lower potential than cathode. It will drain some current if we had cathode follower before it and driving grid with positive signal. (Not for guitar but hi-fi) Plate dissipation: Pa = Ua * Ia -------> Ua=anode (plate) voltage=voltage between pins 3 & 8, not 3 and ground, as cathode is raised +14.84V above ground; Ia = Ik - Ig2 That's it.
+Ivan Horvat Yes, Ivan. The cathode current does include screen current and to be completely accurate, the SC should be subtracted and multiplied by the PV from plate to cathode to determine the PD.......but recall that this was a "basic" video and I chose to simplify the process. By using the cathode current times plate-to-ground PV, the PD that results will be a bit high, providing a safety factor for beginners. Thanks for your input.
Sometimes I watch some of your older videos again. They are entertaining informative and helpful. With safety in mind. I got shocked once and lucky to be alive past about 18 years old from one of my Ham radios uh never again +s my fingers. I learned from this video and successfully biased the 6V6 tube in my little champ. I promise I won't modify it any more or any one else's. But my champ is mine. It's done but I'll just maintain it as it is now. I hope you are still putting out a new video once in a while we all would like to see it out here on yt Land.
Note you can also use a Volt meter to discharge caps. Just measure the voltage til it's gone. Then you get the bonus of SEEING that the voltage is 0 before you work on the amp.
Occasionally, I still have my voltmeter attached to a tube socket in plate-voltage-reading mode while I am discharging the electrolytics with my grounded 100-Ohm probe. It is indeed interesting (and reassuring) to watch the voltage rapidly decline to a zero level.
I'm confused at the plate dissipation calculation. You measured the voltage from pin 3 to ground as opposed from pin 3 to pin 8. So, it seems that the wattage that you calculated would include that of the resistor and the tube, not just the tube. Is this correct?
EDIT nevermind, found the 100 ohm 10W resistor Hey Doug, Would a 10ohm 10W resistor work for discharging the death capacitor? We are talking about the capacitor right after the rectifier, correct? Or is this not always the case, I think it is on my schematic here at least. I'd say the capacitor right after the power transformer is what you refer as the death capacitor, correct? Is this the only capacitor you discharge, or maybe more?
Thanks, Jason. I restored 30's and 40's jukeboxes for many years and became interested in guitar amps about 6 or 7 years ago. I am entirely self-taught, with no formal training whatsoever.
Uncle Doug I can’t tell you how inspiring that is to hear. I’m not formally trained in any way, shape or form, but I’m a professional guitarist with a deep love and appreciation for amps and effects pedals and a growing interest in electronics. I’d really like to learn but I’m starting from scratch and I’m nearly 40 years old. I was afraid my age and lack of background experience might mean this won’t be possible. The fact that you’re self taught and so knowledgeable and just started Jess than 10 years ago gives me hope.
You're right, Mr. A, but since the screen grid current is generally only about 0.5mA and the difference between the plate-to-cathode and plate-to-ground voltage is generally negligible, I thought it would be better to ignore these complications and keep the presentation basic. These omissions result in a slightly more conservative bias setting, and since virtually all single-ended amps are cathode biased, the plate dissipation is prevented from increasing when the amp is driven. Bottom line, this method provides a reasonably accurate bias with no risk of harm to components. Thanks for your input :)
I am not into guitar amps; the plan is to build some HiFi tube amps, but principles stay the same - please correct if am wrong. However, your videos do help me understand what is going on in those circuits, how the electronic parts work and why. At school was I never really interested in physics (… it's just stuff that happens to some other stuff and it'll still happen without me understanding it …), but now, I want to know about it, and your videos do help a lot, they're epiphanic to me. Thank you very much for your effort in spreading knowledge in such a comprehensive and kind way. Please give your four legged companions an extra treat from me!
Mike Barney: Thanks so much, but your "Linked Comment" (?) arrived with no reply capability. Hopefully you will see this: I have posted a video on Cathode Bypass Caps that gives a detailed explanation: ruclips.net/video/HrkYUPmEUhk/видео.html
i concur, another awesome video! it's great to finally see one on cathode biasing, rather than the typical Fender trim-pot videos (which are a dime a dozen.) i started a new thread and reposted the video over on the TDPRI amp forum: i think it'll help out newcomers quite a bit, since SE cathode biased kits are so popular, as well as stuff like old Champs scored off Craigslist and that sort of thing. can't wait for the next one!
Thanks, Cat. It's always good to hear from you. I think the next installment will be cathode biasing with dual output tubes. It's much the same process, but with a couple of extra steps. I also hope your Melodier project is going well.
Uncle Doug my computer decided it needed a new hard drive, so i had to put off getting my transformers for a couple weeks. minor setback and no big deal though. who knows, maybe i'll get lucky and Triode will have some sale prices coming up (i can always hope ;) .
Transformers are always a major expense in any amp project, but if you get them from Triode, it's less painful than from other sources. Also, If you get them around Christmas, they'll be a nice gift (to yourself)......way better than a lousy pair of socks or underwear.
Hello Doug, I have some another questions, My Fender Vibrolux Reverb is consider what type of push and pull amplifier? Is it a class AB1 or AB2 ? Also you had mention that I was conservative on my bias current. I have a one RCA blackplate and a GE cleartop 6l6GC with 384 plate voltage. Im running the bias current at 42 on one tube and 45 on the other tube. How much more can I increase the bias? Thank you
I would say it's a plain old Class AB, Jim. By "bias current" you probably mean plate current. 42mA x 384PV = 16.1W PD. 45mA x 384V = 17.3W PD. Max PD for 6L6GC's in fixed bias (grounded cathodes) is around 21W, so you could increase your PC a bit more.....but I wouldn't recommend it if the amp sounds good as-is, especially if you want to use 5881's.
Hi Uncle Doug...the reel to reel donor circuit is from a unit that was sold commercially from 1953 forward till the 70's with other more elaborate and more expensiive.mdels in the line. I am somewhat overwhelmed by it's complexity because I am unsure of it's class. Although it has two 6V6's these are lain out iin seperate places in the chassis, maybe because of the motor mounting needs. All tubes are clustered across from each other in an almost circular fashion and maybe because one side with 5879 and one 6V6 acts as input buffering because of clean recording requirements. If you have time to look V-M Tape-O-Matic model 700 with that 5879 sharp cutoff pentode fiirst.. It also has two coils mounted,in the chassis and maybe a selenium rectifier also. I think I can build a Tweed 5E3 but what aboout a Super Champ so adding the second tube or even having both 6V6's "driven " . Forgive my ignorance,this thing is old with old technology that's still over my head a little bit. The champ was much simpler. Thanks for all you do,excellent teacher......Daniel
Daniel, it sounds like you might be better off simply harvesting the parts you need and building from scratch, rather than trying to repurpose an existing circuit which you don't fully understand.
Hi Uncle Doug! I checked the bias on my single ended amp that has a 50L6GT power tube. The bias resister measures 181.0 ohms. The voltage drop across the bias resister is 9.5 VDC. The voltage at pin 3 is 81.0 VDC. When I am done the calculations my plate dissipation comes out at 4.275 watts. The spec sheet for the 50L6GT says 10 watts max. Does my value seem like a normal bias reading?
Aerostoon Greetings, Aero. Using your measurements, I get a plate current of 52.5mA, which is a little high, and a plate dissipation of 4.25W, which is a little less than half of the 10W max. The culprit here is your plate voltage, which is less than half of the 200V maximum. This is a series-filament circuit, with no power transformer, so I tend to think that your values are probably very typical for a 50L6 in this type of circuit, in which the output tube is not driven very hard. The PC is already rather high, so you would not want to increase it in an effort to increase the PD. If the amp sounds OK, I would suggest leaving it alone.
All those ER shows are wrong. Defibrillation doesn't restart the heart. Defibrillation is (almost) the exact opposite; when a heart if malfunctioning, it is "fibrillating", meaning it is randomly twitching the muscles instead of evenly pumping. Defibrillation stops the heart from fibrillating, and then medical professionals perform chest compressions; those compressions are what restart the heart. If you try chest compressions on a fibrillating heart, it probably won't work, but a defibrillated heart is much more receptive to compressions. Back in the day, the fact that Hollywood consistently got this wrong was a minor issue, but now that portable defibrillators are becoming more and more common out in the world, this misinformation can actually kill people. Total tangent, but hopefully you can understand why I wrote it out.
I know this is really late, but while playing, doesn't the plate dissipation increase (especially if the tube is overdriven)? I built a single ended ECL82 amp (7 watt pentode) that dissipated 6.6 watts at quiescence. With the amp pushed HARD, those 19v across the cathode resistor spiked to 39v, for a staggering 14 watts of dissipation. I'm not sure how much of this is screen dissipation, but I do notice the ECL82 redplating while pushed. Is this normal?
In cathode biased circuits, the plate dissipation should not increase when the tube is driven, because this type of biasing is "self-regulating". In grid-biased amps, you should only bias to 70% of max PD to allow for a 30% increase in PD when the tube is driven. Regardless of the method by which the tube is (properly) biased, however, it should not red plate unless you are using some sort of overdrive pedal with a grid-biased output tube. In this case the tube should be biased lower than 70% to cope with the overdriven input.
Thank you, incredibly helpful. Still leaves some stuff out, i am experimenting with lower voltage stuff like running a 6K6 or 6V6 at about 150 Volt - so plate dissipation is way under the limit but how should i set that ? In the current amp it could be a 6K6 at 150 V 15mA or 130 V 30 mA - quite a difference - i decide by ear and stay somehow in the middle.
I have the 6156 version of the amp in your video. After watching the video again today I thought, what the heck, lets measure the plate dissipation. I followed your directions except I measured plate voltage across Pin 3 and 8, and came up with very similar numbers as you did. Plate dissipation is slightly lower at 11.28W. Here's the funny part. I've watched this video before but sometimes I like to watch informative videos like yours more than once. I have a schematic in the bottom of the amp. Guess what I wrote on it (last winter?)? Plate dissipation calculation values, except I used the method you used in the video. I came up with 11.87W that way. :)
Measuring the plate voltage between the plate and cathode, as you did, is the more accurate method, Michael, and will give you a slightly lower PD than measuring from plate to ground as I did in the video.
Can the output transformer method also be used with single ended amps like the fender champ? Or does it only work when you have pairs of tubes and a center tap on the transformer?
Counter intuitive. A loud clean amp destroys tubes and a loud dirty one preserves them... Real shame. That is when I'd love someone's amp to fail... (And the last thing I want to do would probably involve a Russian pole dancer)... (And wishful thinking)... Informative and clearly explained. Many thanks.
Hi Doug Good safety tips but your calculation are out. On a 6V6 the screen grid current is more likely to be 5mA, not the 2mA you suggest. Plus, the plate to cathode voltage is only 282V so the PD of the example valve is 282x36mA which is only 10.2 Watts which is perfect. Ultimately, increasing the plate voltage is the only true way to really increase headroom. Reducing the cathode resistor value to increase bias current will bias hotter. This produces earlier tube distortion and increases output transformer saturation, which also causes earlier onset of distortion.
ShawAmps Greetings, SA. The notes appearing in the video addressed the measurement of plate voltage from plate to cathode and the lower PV and PD that results. Since this is basic video, I did not feel that subtraction of screen grid current was an advisable complication. Thanks for your input.
Hello Doug, I have a Vibrolux reverb custom amp which I converted to an eyelet board and have wired it like the AA964 schematic. Two weeks ago I wanted to check the bias to the tubes and found it cold, so I lowered my resistor value at the bias pot so I could get a higher current value and when I powered the amp I blew a fuse. Well when inspecting the circuit and chassis I found a piece of heat shield by the heater and pilot light section. This came off from above, I guess I caught a piece when pulling the chassis in and out. I then troubleshoot and found the power tranny was shorted. I then installed a vibrolux tranny that was a Weber brand and had to remove the end bell where the wires come out of in order for the tranny to fit. The amp powered on and my plate voltage was now 384 as opposed to the 430 volts I had on previous transformer. I like the lower plate voltage. I am running about 42 ma and using the power dissipation formula comes out to 16 watts for 6L6GC tubes. With the plate voltage running at 384 can I use the 5881 tubes? Thank you for you time and videos
Both the plate current and plate voltage are rather conservative for 6L6's, so I would say, yes, 5881's would be safe in this circuit. Simply recheck the bias with the 5881's installed and verify that the PD is within acceptable limits. Good luck.
Hi Uncle Doug! I hope Rusty hasn't been lured away to Hollywood to a life of a debauched filmmaker. I expect he'll get caught up in some Tinseltown scandal with his photo in the National Inquirer. Aside from that I'm confused. This website proguitarshop.com/andyscorner/amp-bias-explained States, "under biased (otherwise known as biased too hot) is a stated when the idle current is set too high. This causes the signal to distort early..." . Does this not contradict what you said or maybe I didn't understand something? Thanks so much for your videos. I've learned so much!
Several years ago, I added notes to the video explaining that underbias increases headroom and overbias reduces it, Rick. Apparently these notes are not visible on the device you are using to watch the video. Watch it on your PC and you'll see them. Sorry for any confusion.
Hi Doug: I was about to correct you on a couple of points but I read the description and see that you cover them in your notes. Excellent tutorial and WOW what a clean little Gretsch! Usually the ones I've seen have been shall I say "faithful companions". Cheers Mike.
So if I understand correct, it is possible to scale down the power section of a cathode biased amp (W wise) by alternating the bias resistor and plate voltage? How is the voltage drop across the resistor come in mind in this? I’m asking because I’m curious about adding a power scale switch to an amp build
Hi Doug Damn it, the way you explain electronics is crystal clear. Two questions: 1-Have you ever got a high voltage shock working with these guys? 2-How can a tube amplifier be designed to sound good and have a way lower plate voltage? Cheers Pedro
Thanks, Pedro. Yes, I have been shocked a few times over the past 40+ years. Tubes must operate at the plate voltages for which they were designed; therefore, you would have to use an output tube(s) that was designed for low PV. See any Tube Manual to find one.
Doug I'm really enjoying these videos - very clearly presented and understandable. A couple of questions on this one: 1) I had the understanding that old tube amps with no ground were inherently unsafe but you are electing not to ground this one. Is it a matter of ensuring the 'death cap' is properly spec'd & functioning correctly? 2) Can you elaborate on the need for the lightbulb device? Cheers and thanks again! edit: I just found your video so #2 is answered :)
Thanks, Duane. With my own amps, if they are pristine originals, I keep them that way. When I sold the amp, however, I installed a 3-wire power cord and removed the "death cap".
I swapped my cathode resistor all the way from 270 to 400 Ohms and I still have 408 on the plate of my 5E3 build. It's all clean headroom up to almost 9 with a strat. I don't seem to be getting the bias right without losing all the famous tweed overdrive. right now it's 12W 33mA.
Wouldn't it be wise to just attach a variable resistor of the correct value so you could quickly adjust the desired headroom? I had a amp where I didn't like it's distortion and I wanted the cleanest amp possible by doing this, but at the time I didn't have much if any experience doing this, but I did check the ohms of the resistor and found the ohms was way off of the colors code or any available so I brought the ohms down to the next level and it cleaned up, but I wanted it totally clean so I kept lowering the ohms, increasing the resistance. Using this video as a example I wound up using a 270ohm it was a couple steps down from original and like I said I had no experience and I had no business touching my bread and butter, but it worked... My second amp was heavy metal all the way and I had it hooked up to a ABY switch the to a stereo chorus and reverb so I could use one or the other or even both amps at the same time... When I picked it sounded as clean as a piano, I had and still have the most bizarre neck pickup, it's a double single coil. It looks like a humbucker, but it's not. The box it came with said it was a double single coil wire for 500k It was the 80's when I did this so it's not as clear to remember every detail and my friend went his part bin and handed me a hand wound PAF and it came with a COA and schematic, this was over wound and screaming for a 1m volume pot not the usual 500k, but I had to leave it unless I installed a second pot. I truly found my sound. I purchased a cheap Ibanez around $450.00 but at all my gigs the fans ask me to use this over my Les Paul's...
I believe you mentioned in another video, power tubes tend to produce undesirable 5th harmonics when overdriven. So, I would suspect running COLD power tubes is undesirable ?? Thank you for all of your videos, they are fantastic ! !
Thank you very much for your videos. You explain things so clearly that even people that don't have English as their first language can understand everything very well.
I do the bias on several amps without the math you know by using Webers bias calculator for class A/B or class A by switching his chart to class A ! I use a plate voltage probe and a grid bias probe to set the Webers recommended value at slightly lower value because voltage in my location varies - 123VAC to 116VAC at home ! You put your tube in the probe and plug the socket of the probe into the amp , first the plate voltage probe then bias using the grid probe setting my DMM to the probe instructions which is critical !
Excellent video. I have one question. You calculated plate (+ screen grids) dissipation by multiplying the B+ Voltage with the current (calculated using Ohm's law). The cathode resistor however raises the cathode Voltage above ground so it has a positive Voltage. Shouldn't you substract the Voltage that falls over the cathode resistor from the B+ Voltage before you multiply by the current. E.g. when B+ is 400 Volts and the Voltage over the cathode resistor is 30 Volts, the Voltage the tube "sees" is 400 - 30 = 370 Volts. Or am I missing something?
You are correct, HPV. The plate dissipation calculation would be more accurate if the net plate voltage (to cathode, not to ground) had been used. This was a very basic video. More complex and accurate methods of PD measurement are covered in subsequent videos. Thanks for your inquiry.
@@UncleDoug Thanks for your reply. I highly appreciate your videos and the fact you are sharing your knowledge with others! Keep up the good work es 73!
excellent doug,,this is the best i have seen,,i worked in electronics production,but i forgot all this stuff,,now im ready to get in there and test my amp,,i want a warmer sound and mabey this amp is set different,,thankx again do more old school electronics,,no one knows much about this stuff in the computer age we live in.
I appreciate his ability to explain things in a way that’s interesting and easy to understand.
......and I appreciate your nice comment. :)
military training is best,,you know your shit ,you forgot more than i know
Uncle Doug's videos are a masterclass collection, I love watching them and learning. And then there are quotes like these: "the last thing you want to do is die, while you're trying to bias your single ended amplifier"..
cmpmuller Greetings, CM, and thanks for the very kind words. Rusty and I are glad you enjoy the videos. I guess dying is indeed "the last thing" we'll ever do.....it would just be a shame to hasten the process ;)
Uncle Doug toujours clair Net et précis! Thanks !!!!
c'est absolument la vérité
I'm wondering if anyone has systematically assembled his videos to construct a primer on all things amplifier for the beginner ? I have begun such an endeavour, but RUclips seems to revel in scattering stuff all over the map. Here's what I've got so far (unedited):
UNCLE DOUG NOTES How Tube Amps work
‑
HOW THE AMP WORKS 1,2
ruclips.net/video/x5SSKX74DKg/видео.html
ruclips.net/video/901iaPVVzY0/видео.html
TONE STACKS 1,2,
ruclips.net/video/BatwDYFJ9ug/видео.html
ruclips.net/video/uZHM5BwPLRw/видео.html
CATHODE BYPASS CAPACITORS
ruclips.net/video/HrkYUPmEUhk/видео.html
OSCILOSCOPE BASICS 1,2
ruclips.net/video/ueOup-XBexU/видео.html
ruclips.net/video/UswkMHK27F8/видео.html
OSCILOSCOPE AC/DV TRACING, RMS OUTPUT MEASURING
ruclips.net/video/PmVq7NLv31s/видео.html
TUBE REVERB 1,2,3,4
ruclips.net/video/EZFuT4liLAw/видео.html
ruclips.net/video/3KEiQBaYZDc/видео.html
ruclips.net/video/wTwTcjMYnAY/видео.html
ruclips.net/video/pjTpiQG-ov4/видео.html
IISOLATION TRANSFORMERS
ruclips.net/video/5SjVm-rF39A/видео.html
TRANSFORMER IMPEDANCE RATIOS
ruclips.net/video/5jUitplchok/видео.html
CAPACITORS IN AUDIO CIRCUITS
ruclips.net/video/l-u8J-yh9ZA/видео.html
ruclips.net/video/9UULx3-hHv4/видео.html
ruclips.net/video/YXChioNC5NY/видео.html
RESISTORS
ruclips.net/video/LBtoarc6Nls/видео.html
ruclips.net/video/8ruznIHuh7M/видео.html
ruclips.net/video/QFHcmST7AZ0/видео.html
ruclips.net/video/ORjsSfab278/видео.html
ruclips.net/video/TvQWX0M6hJY/видео.html
POWER TRANSFORMERS
ruclips.net/video/MXgG-0MIBGA/видео.html
PT BASIC DESIGN AND FUNCTION
ruclips.net/video/hJsPQdAhTkQ/видео.html
GUITAR AMP VACUUM TUBES
ruclips.net/video/Xo4nVjj4NSE/видео.html
ruclips.net/video/Zh_51RP4V6I/видео.html
PHASE INVERTERS
ruclips.net/video/U6By31V9fDo/видео.html
BUILD A CURRENT LIMITER
ruclips.net/video/wRFRwOnLsZI/видео.html
HUM AND ITS POSSIBLE SOURCE
ruclips.net/video/GrVtX0QGNls/видео.html
BIASING DOUBLE-ENDED AMPLIFIER OUTPUT TUBES, PART 1: BASIC EXPLANATION & METHODS
ruclips.net/video/w9B0Rhr_Y-E/видео.html
...thats a GOOD one all right-!!!
I am going through these videos slowly (about one every 2-3 days) and they're helping me get through these days, while educating me at the same time. You're a great teacher, and I appreciate you sharing your knowledge. Thanks Uncle Doug! :)
You're welcome, Kenji. Thanks for watching :)
Uncle Doug, I completed this procedure successfully today without incident, thanks to your excellent teaching. When I measured 450 volts after switching on my Vibro Champ, I felt a good amount of respect, but I didn't quake in my boots because I had watched your video. I did the measurements for several 6V6 tubes and found the newer ones (JJ) were running much hotter than an old GE tube (which was at 12.5 W with the original biasing). I also liked the overall sound better with the GE, so left it in there. Once I have to switch to JJs, I will have to rebias the amp because they're running at 15.5 - 16 watts of plate dissipation. I know how to do that now. The measurements were interesting. I also took the opportunity to glue the tolex back down and polish the front, using methods I learned from your videos. The amp is shiny and sounds great. I'm using a step-down transformer with this US-made amp, since I'm in Europe. I ran into some interesting polarity issues with that. If one turns the (Schuko) plug the wrong way around, the step-down transformer outputs 234 volts instead of 117. Not very nice. I believe many people aren't aware of this. I got myself a model with a polarity check so this doesn't happen anymore (it didn't hit the amp, I measured first). What is the result of running an amp made for 117 volts on 234 volts instead? I wonder.
Greetings, KD. A PD of 12.5W is still excessive and 16W is crazy. I tend to keep the cathode-biased 6V6's at around 11W. If 115VAC input can produce 450 plate volts, then 234V would yield close to 900 PV......which would breach all the capacitors and destroy the tubes.
@@UncleDoug I guess I might change the resistor then. I suspect previous owners in Europe didn't pay any attention to the plug orientation, so I'm surprised the amp still works and nothing looks destroyed inside... I should probably measure all the caps, though.
@@kneedeepinthedoomed Plug orientation is a safety risk but does not threaten the circuit components, KD. Increasing the bias resistor value will result in better tone (in my opinion) and longer tube life.
Years of confusion cleared up in 20 minutes.
That's good to hear, Jan :)
@@UncleDoug Amen to that.
And this is when the light came on for me - no pun intended. You can read forums for hours, observing well intended people argue back and forth. It becomes a battle in my mind between trying to grasp the information itself and then sorting thru the emotion to see if there's a consensus of some kind buried in the arguments of the "experts." Save yourself weeks, maybe months, of forum scrubbing and please just watch this video. The Air Force paid good money to send me thru electronic principles and I still have a massive 3 ring binder full of my notes, but none of that helped me as much as this video. Bravo - well done my friend! Thank YOU AGAIN! *And my Champ thanks you too. 👍
Wow....thanks so much, Chad. We're really glad to hear that the video was helpful :)
ermmm...the plate voltage you measured was across the valve and the cathode resistor combined, as you connected the metre between the plate and ground, surely the valve is dissipating (296V - 14.84V) x 0.0415A = 11.69 watts.
You're right, Rod. It's more accurate to measure the PV between the plate and cathode, rather than the plate and ground. The error is minimal (0.4W), and the plate-to-ground measurement provides a slight margin of safety (for beginners), but it is more accurate. Thanks for your input.
Uncle Doug-I don’t have my RCA Radio Tube book yet, so these tutorials are essential to first pique one’s interest, then draw us in with some basics that don’t clutter our heads and obscuring our understanding in the process. Very skillfully presented. Getting close to pulling the trigger on the book!
Don't wait, John.....the manuals are readily available on Ebay and not expensive.
How does one dislike Uncle Doug? Not cool. The man is a treasure.
Thanks, B, but judging from the fact that I always get some thumbs down.....granted they are from slobbering cretins.....would indicate that not everyone is a fan :)
This is what people need to learn especially if they rebuilt the amp or if it's been used heavily... Great video
Thanks, Mr. B :)
Doug, all of your videos are great but this one is fantastic! I can't wait to use what you've shown here in my own builds.
Oh yes, great advice: "Don't die while biasing your tubes. It's a real downer!"
Thanks, Jeff. Glad you enjoyed it. Be safe !! :)
I am new to vintage tube amps and found this lesson to be of great value. I always wondered how one would modify an amp to have more or less "head-room". I always wondered what caused an amp to distort with manipulation of the electrical components. Now I know, thanks to Uncle Doug. I really appreciate your lessons, videos and topics covered.
You're quite welcome, Daniel. It's great to hear that the videos are helpful :)
Holy moley UD, I went to the URL you suggested and boy do I have a lot more studying to do. But at least I'm seeing a little light the further down this rabbit hole I go. Or should I say rabbit tube? It's like an alternate universe down here. The more you learn the more you need to learn... magic... And a little trippy too.
Indeed, if you really want to go into detail on topics like this, the depth of information is almost unlimited. You need to ask yourself.....how much do I really want and need to know ?
@@UncleDoug I've said this before, and I'll say it again. Your advice is always spot on. At this point in time this is a perfect fit for me. I hobble around some, but I will soon get my ankles fused and after that I should be walking around normal. No worries :) until then I have some vintage test gear to work on and plan on repairing electronics for a living. Doing some of that already, mostly computers, but there is something about the sound of my guitar playing through that crazy Eico Signal Tracer... sounds strange good. But your right, this bunny hole is bottomless. So I've decided to focus only mostly on the Fender 5F1 & the Eico 147A schematics. So that is why I am very grateful for your first rate "tubetorials". Keep 'em comin! BTW I will be happily throwin down some well spent clams on your patreon as soon as I am able! Thanks again!
Thanks so much, FWR. Best wishes for a speedy recovery and good luck with all your projects.
Great intro video. The KISS [Keep It Simple, Shtupid] principle is a very good one.
When is comes to math for electronics, it cannot be too simple for moi. Thanks to your well-presented tutorials, I am beginning to grasp and actually understand the mysteries and vagaries of audio electronics.
Thank ya ver' much, Unca Doug
Now, on to double-ended output biasing , where the fun really is.
Thanks, Antonio. We're glad the videos are helpful.
"It's a real Downer when your heart stops!" Lol, I have to agree Uncle Doug!
Trust me, it's scary as heck, especially when you feel it struggling to regain its proper rhythm.
@@UncleDoug I had A-Fib for six years. THAT is like an earthquake going off in your heart, and it is horrifically miserable. Got that fixed. Now I just have six blown discs in my lower back. It's been a good past 15 years ...
Just wanted to thank you for your awesome videos! An electrical engineer friend of mine (guitarist, electronics tinkerer) recommended your channel as the best way to learn tube amp electronics--he was totally right. You're a fantastic educator.
Andy Reeves You're welcome, Andy. Rusty and I are really glad to hear that our videos have been helpful and informative. Thanks for watching :)
Thanks to you, i've build 2 amps for my self. One SE and one PP. You've opened a whole new world of me.
What a fabulous lesson! I really like it because it offers constant safety reminders, an easy-to-follow, logical procedure, AND it explains how to voice (in part) an amp for more or less distortion. (Hope I'm not over-simplifying). Thanks Uncle Doug ! I'm feeling even more ready for my amp build with Bruce Egnater on Saturday coming.
Thanks, Ian. I hope your amp building session is a total success.
@@UncleDoug I'll do my best to remember your tip about one hand only in the chassis. I can be impetuous and would hate it if my darling wife had to travel to a Detroit suburb to collect my ashes.
Yes, it would be a shame to have a good viewer, like you, incinerated, Ian. So for heaven's sake be careful :)
Thank you for taking the time to explain all this.
You're welcome, MP :)
Wow 👍 I picked one of those up at the goodwill store for 7 dollars awhile back soldered the speaker wire's back on and cleaned the controls and it sounds great
That has the be the best deal I've heard of in several years, DC. Congratulations !!
@@UncleDoug and it still has the price tag on it 👍 just couldn't bring myself to take it off
@@doctorcircuit5338 I don't blame you, DC. Why lose evidence of your major coup ??
One of the best clear vids I have ever seen. Thanks Doug
You're welcome, B :)
I just found you the other day. Im loving learning about tube amps. I would never try this at home but its fun to know how the things work. I have a marshall DLC5 , fender blues jr and a boogie mk5 35. cheers
Welcome aboard, Steve.
This is great my understanding expand 's every time I watch one of Uncle Doug's video 's
That's our goal, Dain. Thanks !!!
very valuable information here ,my therory and basic electronic training is starting to come back and make sense ,been 28 years but thats ok wont work on any of my projects until im well informed and have proper instruments to do it safely and proper
Take your time to recollect all the safe-handling rules, Joe......then best of luck with your projects.
Uncle Doug once again great video thank you so much I hope you know how much you are appreciated for what you're doing for us noobs out here you are the coolest of the coolest
You're welcome, MM. Thanks for your very nice comments.
Thank You! Love Your Teaching! You Are The Best!
Wow.....thanks, Greg.
Excellent. Love this stuff. Yes... I am just learning. Keep on Rock`en! From Sioux City,Iowa.
Thanks so much, Ronnie. It's great to hear that the video was helpful. Best regards from UD & Rusty :)
Very educational video on biasing an amp. Thanks, I now feel comfortable in checking/adjusting tube bias on my BF and SF VibroChamp's,
A good way to think of the bias resistor, for me, is to think of a dimmer switch for a light bulb. Increase resistance dimmer bulb= lower output, decrease resistance brighter bulb= higher output.
You're welcome, OAG. You're right, the Plate Current is indirectly related to the bias resistance.
@@UncleDoug
I have a question for ya. My Blackface VC is alot darker sounding than my early 70's Silverface VC. The Silverface does have a Pyle Driver speaker, model MH810, in it.
Other than that and new Tung Sol tubes both have all original components.
Is that normal between the BF and SF VC?
Well I finally gave the Blackface VibroChamp a good going over to find out why it is so dark sounding. The problem is the speaker cone is shot, kaput, finished. Guess it's time to order a Weber 8A125
Thank you, an excellent Video! Perfect timing too, as I am ready to start soldering a scratch built 5F1 with a solid state rectifier. Thanks again.
You're welcome, BT. I'm glad the video was of interest. Keep us posted on your project.....it sounds like an interesting one.
You are the man, Doug! Thanks..
Thanks, Carlos :)
wish you had a tube amp builders class... oh wait, this is it. good video... coincidentally I had a scare with a small arc.. discharge your caps folks....
Thanks, Jason.....and good advice :)
Ouch! my 40W (prowler) made an "impact" in the past; glad to be alive/learning now!!!, thx, bird.
You're welcome, Steve.
Very comprehensive and clear description. I had the miss conception that biasing was for matching pairs of power tubes and therefore, single power tube amps did not require biasing. I own 2 amps and don't use them regularly, should biasing be checked periodically or when replacing tubes. I am assuming the sound quality will indicate when tubes need replacing.
Thanks, Theo. To be honest, most cathode-biased output tubes (which are in virtually all single-ended amps) are probably not in any great danger from incorrect biasing, since they tend to "self-correct" and protect themselves from harm. It's a good idea to check their bias every year or two (or whenever you change output tubes) to see if the biasing resistor has changed value over time (especially if they are old amps), but otherwise, they tend to be "low maintenance". Fixed bias output tubes (with grounded cathodes) are another story. They do need to be checked at regular intervals and definitely rebiased whenever a tube or tubes are replaced. If you have a double-ended amp and are not sure what type of biasing it utilizes, I can tell you.
With the way you explain things it does make alot of sense.
Thanks.
I'v seen this vid.
Of yours before
But as a matter of study and learning
I watch it again.
Who's afraid of alittle
Elesstrigerty ha.
Don't touch! Oooh ah
Pow boom yyooowoo
I learned that lesson the hard
Way back when I was 18 yrs old on an old
Radio that I was repairing it didnt stop
My heart but it was really beeting hard fast
For about 5 or 10 minutes the shock
Was painful. Guess I got lucky. Don't
Forget REO spit. wagon.
Dwight.
Thanks for sharing, Dwight :)
Great content Sir! And thanks for the safety tips. Be safe.
Thanks, you too!
Very well done video! I learned something new. I thought by biasing the output tubes hotter, or with more plate current would actually cause amp to break up or distort / flattop a sine-wave at a lower volume level. Thanks for your explanation. I’m next looking forward towards seeing how you bias a tube amp with two or four output tubes with 4, 2 or 1 variable bias resistor in a amp that doesn’t use a fixed cathode bias resistor for output tube(s). 🙂👍
Thanks, Mike. The chart in the video was a bit misleading and I corrected it with graphics that are not visible on all viewing devices. Hot bias does induce earlier breakup, and cool bias increases headroom. I apologize for the confusion. This was one of my earlier videos. My knowledge and techniques have improved with time and my more recent videos are far more astute. I have posted a video showing double-ended biasing, variable resistor fixed biasing, installing of a variable bias pot, etc. I hope you enjoy them.
Very helpful. I am writing notes as I watch to keep this all straight in my head. Have been away from tube amps for a while, so it's good to refresh. I learn more each time I visit.
Excellent. Been eagerly watching many of your amp tech videos and learning a lot. Thanks so much for sharing the knowledge :)
Thanks, Necro (Rusty is a little nervous about your nickname :) and you're welcome. I'm glad the videos have been helpful and interesting. Please stay tuned for Part 2 on the topic of tube biasing.
Uncle Doug, I wish you'd been one of the instructors when I through tech school!!
Thanks bloke!!
I wish I could have been there for you, Vinny :)
@@UncleDoug well you're here now and digital communication allows us to be in contact. I quit working as a technician 30 years ago. Digital technology made myself a thousands of other guys redundant. I worked maintaining telephone exchanges which were mostly relay switch operated. I also worked on big dc power supplies and have melted a few tools - lol
@@ix-Xafra We retired guys have to keep busy :)
This is the best explanation on biasing I've ever found on YT.. and the whole Internet..
Thanks, Ivan. One change is that it's better to measure the plate voltage between the plate and cathode....not ground.
So the actual plate voltage is voltage at pin 3 minus the voltage drop value?
Yes, I guess you could look at it that way.
Awesome info , I have a bit of learning of electronics and this is much more straightforward than other biasing issues, also more understandable, thanks.
Uncle Doug for President! ... if only our politicians could explain things so perfectly. I think I'm really beginning to understand how guitar tube amps work ....
Thanks, TG. Stay tuned for more technical videos..... :))
I reckon he and Bernie...but seeing as UD is from Texas...uh..just an idle fantasy.
Uncle Doug, thanks for putting up this very helpful video. I refer to it often.
You're welcome, Denis.
Thx, very helpful video!!! Just one thing, 09:03 if you are not able to understand ohm's law, please do not open any electrical device
Thanks, Gunter :) Good advice.
The current you calculated is actually the sum of plate and screen grid.
The voltage you need is plate to cathode,not to ground.
+MrAletube You're right, Mr. A. I have since corrected this oversight. Thanks for your input.
I'm confused now:
4:55 bias too cold = too much headroom, lots of clean before clipping
18:20 "this amp is biased very hot so it has lots of clean headroom" and in the picture "and distort at high volumes".
You're right to be confused. The 4:55 comment is correct. I must have been hallucinating on solder fumes to make the 18:20 comment.
Greetings Uncle Doug, I was confused about how the plate voltage was measured, as I thought it would be between the plate and cathode, rather than plate to ground. I see you answered the same question below to MrAletube, so plate to cathode. Comparing plate to ground versus plate to cathode the difference seems significant in my 5F1 (363 vs 344V), and depending on which value used yields 12.4W to 13.2W (6V6GT). Bilbo, Sam, myself, and even Gandalf are wondering which is the more accurate way to know what the plate dissipation is--as we're creeping up on dissipation limits in our quest for the Ring of Tone. Forgive us for asking about to a very old video--news doesn't travel fast in shire.
This was an early video and the PV measurement was kept as simply as possible......but the most accurate method is plate to cathode, as you said. I am finding that excellent tone is readily available with very conservative biasing, i.e. 9W to 10W for a cathode-biased 6V6.
How many x ive watched, eye dunno but it makes a whee bit more progress towards my brain each time.
Glad to hear it, J ;)
Hi Doug, this is the first comment I have ever posted. I wanted to thank you for this video and the follow-up video. I have been playing guitar for 30yrs and during the last few years learning about amps. I had found it very hard to find a thorough demonstration of the biasing procedure even though I understood the theory around it. Its really good that someone with your experience is posting this type of experience. Warm regards John C
You're welcome, John. We're glad the videos were clear and helpful :)
Thanks Uncle Doug, this is the clearest explanation of cathode biasing of single ended amps that I've found so far. I really like the way that you explain things. I know have a much better understanding of biasing and it's relationship with headroom.
Thanks so much, David. I'm glad to hear that the video was informative and helpful.
Thanks Uncle Doug!
You're welcome, Liam.
Well I have watched this a few times and I'm starting to get it. My 6V6 on the 9002 is perfect and removing the strange air gap resistor caused the thing to just start working. I am amazed, I actually fixed this thing... ;) Thank you Doug
Congratulations, Anthony. This should give you confidence to continue your new career in amp repair :)
Ótimo vídeo mestre
Thanks, Jorge. Glad you liked it :)
Thanks Doug, helped save my StewMac Champ project. She was running way too hot from the get go.
I'm glad the video was helpful, Tom.
Apparently my little Monoprice 15w is set cold. It's got a test point and adjustment pot inside, from what I understand. Headed to watch the double ended vids next in hopes of learning to check and adjust mine.
The last post I found at TDPRI said this guy's was around -5.86v and he set it to -12v but I'm not just going to accept that as correct until I learn how this all works and whether it actually is correct for 2 EL84s
Edit: so I found the schematic and it plainly says 'set to -12.5V' at the bias pot. So I guess that settles that?
Hello Uncle Doug. Thank you for this excellent tutorial. I gave you attribution for this in a video I did where I used this method to check the bias on a IF Amp tube in a '53 Chevy Radio (Part 6). Just wanted to let you know I let everyone know where I got the technique, and a link to this video. Thanks again. [edit: spelling]
We're glad the hint worked for you, PF :)
excellent video clear and informative, plus some wry comments ......
Thanks, WC :)
Another great video! Please continue.
Thanks, DN. I will start production on a sequel, probably detailing the biasing process for dual output tubes. Stay tuned.......
Very Very interesting serie of video's. Very clearly explained. Thank you! I consider building a guitar tube amp. I wonder where I can find the wiring diagrams of vintage tube amps, as the ones you are using? Greetings from Brussels in semi-lockdown ...
Thanks, William. Virtually all schematics are readily available on the internet with a basic search. Good luck.
Thanks, another great video.
You're welcome, Michael :)
Hi Uncle Doug, Thank you for the video. I am an electrician and guitar player. Loved watching ohm's law in action. Your safety measures and explanation were awesome. You have renewed my fascination with electricity. Thanks again. You have a new fan.
Thanks, Steph. Welcome to our channel :)
So, if 12w is the maximum for that tube. And you decided to sell this amp and lower the head room. Would use the 70% guide line to accomplish the task and there-by increase the tube life. Or is the 70% rule applied differently?
All I had to do was watch the next video. Thanks UNCLE Doug. CB vs GB!
thanks Doug i would think if it the anode was not red hot it would be ok, we call a tube in the uk a valve, thanks Doug .
+david jones You're welcome, David. As demonstrated in the video, tube (or valve) plate dissipation must be accurately adjusted for best tone and tube life. Simply not glowing red hot, called "red-plating", is a bit too general an assessment, since the tube could be seriously underbiased, which seriously compromises tone.
I haven't read all comments. Maybe somebody noticed before me, but:
If you measure current thrugh cathode resistor, you actually measure sum of all currents in the tube. Cathode emits electrones and they go somewhere -> as your tube is 6V6, it's tetrode so you have measured Ia+Ig2 (anode + grid #2 current). Not only plate current as you said.
So you have to subtract: Ia=Ik - Ig2
As you don't have resistor in series with g2, disconnect it from supply and measure with amper-metre. Than subtract.
g1 (control grid) doesn't pull any current as long as it's on lower potential than cathode. It will drain some current if we had cathode follower before it and driving grid with positive signal. (Not for guitar but hi-fi)
Plate dissipation:
Pa = Ua * Ia -------> Ua=anode (plate) voltage=voltage between pins 3 & 8, not 3 and ground, as cathode is raised +14.84V above ground; Ia = Ik - Ig2
That's it.
+Ivan Horvat Yes, Ivan. The cathode current does include screen current and to be completely accurate, the SC should be subtracted and multiplied by the PV from plate to cathode to determine the PD.......but recall that this was a "basic" video and I chose to simplify the process. By using the cathode current times plate-to-ground PV, the PD that results will be a bit high, providing a safety factor for beginners. Thanks for your input.
13.48...the 330 ohm resistor has a 10% tolerance (silver 4th band)- that means that it can be as high as 363 ohms, and still be within tolerance.
Sometimes I watch some of your older videos again.
They are entertaining informative and helpful. With safety in mind. I got shocked once and lucky to be alive past about 18 years old from one of my Ham radios uh never again +s my fingers. I learned from this video and successfully biased the 6V6 tube in my little champ. I promise I won't modify it any more or any one else's. But my champ is mine. It's done but I'll just maintain it as it is now. I hope you are still putting out a new video once in a while we all would like to see it out here on yt Land.
Thanks for the nice comments, Dwight. Be careful !!
Safety is most important
Yep. You can't enjoy the tone if you're dead..
I have to say this was fantastic information delivered in the most professional and entertaining fashion. I'm hooked !
+thebuzztone Thanks so much, TB. We have posted over 135 videos so far, so there are lots for you to watch :)
Note you can also use a Volt meter to discharge caps. Just measure the voltage til it's gone. Then you get the bonus of SEEING that the voltage is 0 before you work on the amp.
Occasionally, I still have my voltmeter attached to a tube socket in plate-voltage-reading mode while I am discharging the electrolytics with my grounded 100-Ohm probe. It is indeed interesting (and reassuring) to watch the voltage rapidly decline to a zero level.
i actually do that always, its very pactical
Surely there's a meter available with adjustable built-in discharge capability?
I'm confused at the plate dissipation calculation. You measured the voltage from pin 3 to ground as opposed from pin 3 to pin 8. So, it seems that the wattage that you calculated would include that of the resistor and the tube, not just the tube. Is this correct?
Yes. This was an early and very basic video, Jimmy. Subsequent, more complex, videos have covered the biasing process in much greater detail.
EDIT nevermind, found the 100 ohm 10W resistor
Hey Doug,
Would a 10ohm 10W resistor work for discharging the death capacitor? We are talking about the capacitor right after the rectifier, correct? Or is this not always the case, I think it is on my schematic here at least. I'd say the capacitor right after the power transformer is what you refer as the death capacitor, correct? Is this the only capacitor you discharge, or maybe more?
Glad you found the correct resistor, Thijs. All electrolytics should be discharged. The D.C.is of no consequence.
Your videos are wonderful! What’s your background? How did you learn all this stuff and how old were you when you started?
Thanks, Jason. I restored 30's and 40's jukeboxes for many years and became interested in guitar amps about 6 or 7 years ago. I am entirely self-taught, with no formal training whatsoever.
Uncle Doug I can’t tell you how inspiring that is to hear. I’m not formally trained in any way, shape or form, but I’m a professional guitarist with a deep love and appreciation for amps and effects pedals and a growing interest in electronics. I’d really like to learn but I’m starting from scratch and I’m nearly 40 years old. I was afraid my age and lack of background experience might mean this won’t be possible. The fact that you’re self taught and so knowledgeable and just started Jess than 10 years ago gives me hope.
It's never too late to learn something new, Jason. All it takes is dedication. I hope you reach your goals.
The current you calculated is actually the sum of plate and screen grid.
The voltage you need is plate to cathode,not to ground.
You're right, Mr. A, but since the screen grid current is generally only about 0.5mA and the difference between the plate-to-cathode and plate-to-ground voltage is generally negligible, I thought it would be better to ignore these complications and keep the presentation basic. These omissions result in a slightly more conservative bias setting, and since virtually all single-ended amps are cathode biased, the plate dissipation is prevented from increasing when the amp is driven. Bottom line, this method provides a reasonably accurate bias with no risk of harm to components. Thanks for your input :)
I am not into guitar amps; the plan is to build some HiFi tube amps, but principles stay the same - please correct if am wrong. However, your videos do help me understand what is going on in those circuits, how the electronic parts work and why. At school was I never really interested in physics (… it's just stuff that happens to some other stuff and it'll still happen without me understanding it …), but now, I want to know about it, and your videos do help a lot, they're epiphanic to me. Thank you very much for your effort in spreading knowledge in such a comprehensive and kind way.
Please give your four legged companions an extra treat from me!
Thanks for the very nice note, EW. Yes, the fundamentals are all the same. We're glad the videos have been helpful. Best of luck with your projects.
XCLNT DEMO
⭐️⭐️⭐️⭐️⭐️
"SAFETY-1st"
I peed my pants when you said having your heart stop is not good !
(The non-shulant-way was STELLAR !!!) 🎯😎😇💭💭💭💭💭💭
Glad you enjoyed it, Jack :)
Mike Barney: Thanks so much, but your "Linked Comment" (?) arrived with no reply capability. Hopefully you will see this: I have posted a video on Cathode Bypass Caps that gives a detailed explanation: ruclips.net/video/HrkYUPmEUhk/видео.html
Sorry about that. Thanks for responding though!
You're welcome. I hope the video clears things up :)
i concur, another awesome video! it's great to finally see one on cathode biasing, rather than the typical Fender trim-pot videos (which are a dime a dozen.) i started a new thread and reposted the video over on the TDPRI amp forum: i think it'll help out newcomers quite a bit, since SE cathode biased kits are so popular, as well as stuff like old Champs scored off Craigslist and that sort of thing.
can't wait for the next one!
Thanks, Cat. It's always good to hear from you. I think the next installment will be cathode biasing with dual output tubes. It's much the same process, but with a couple of extra steps. I also hope your Melodier project is going well.
Uncle Doug my computer decided it needed a new hard drive, so i had to put off getting my transformers for a couple weeks. minor setback and no big deal though. who knows, maybe i'll get lucky and Triode will have some sale prices coming up (i can always hope ;) .
I never recall seeing any sort of sale prices from Triode. It's too bad that Musician's Friend doesn't sell transformers.....
yeah, probably just a bit of wishful thinking. the combined price of the ones i'm going to get is only around $75, so it's still a great price!
Transformers are always a major expense in any amp project, but if you get them from Triode, it's less painful than from other sources. Also, If you get them around Christmas, they'll be a nice gift (to yourself)......way better than a lousy pair of socks or underwear.
Hello Doug, I have some another questions, My Fender Vibrolux Reverb is consider what type of push and pull amplifier? Is it a class AB1 or AB2 ?
Also you had mention that I was conservative on my bias current. I have a one RCA blackplate and a GE cleartop 6l6GC with 384 plate voltage. Im running the bias current at 42 on one tube and 45 on the other tube. How much more can I increase the bias?
Thank you
I would say it's a plain old Class AB, Jim. By "bias current" you probably mean plate current. 42mA x 384PV = 16.1W PD. 45mA x 384V = 17.3W PD. Max PD for 6L6GC's in fixed bias (grounded cathodes) is around 21W, so you could increase your PC a bit more.....but I wouldn't recommend it if the amp sounds good as-is, especially if you want to use 5881's.
Fantastic class. And yes, I don't want to die:). Thanks Uncle Doug. And boy, do I hate math, but this was easy to follow.
You're welcome, Shaun.
Hi Uncle Doug...the reel to reel donor circuit is from a unit that was sold commercially from 1953 forward till the 70's with other more elaborate and more expensiive.mdels in the line. I am somewhat overwhelmed by it's complexity because I am unsure of it's class. Although it has two 6V6's these are lain out iin seperate places in the chassis, maybe because of the motor mounting needs. All tubes are clustered across from each other in an almost circular fashion and maybe because one side with 5879 and one 6V6 acts as input buffering because of clean recording requirements. If you have time to look V-M Tape-O-Matic model 700 with that 5879 sharp cutoff pentode fiirst.. It also has two coils mounted,in the chassis and maybe a selenium rectifier also. I think I can build a Tweed 5E3 but what aboout a Super Champ so adding the second tube or even having both 6V6's "driven " . Forgive my ignorance,this thing is old with old technology that's still over my head a little bit. The champ was much simpler. Thanks for all you do,excellent teacher......Daniel
Daniel, it sounds like you might be better off simply harvesting the parts you need and building from scratch, rather than trying to repurpose an existing circuit which you don't fully understand.
Hi Uncle Doug! I checked the bias on my single ended amp that has a 50L6GT power tube. The bias resister measures 181.0 ohms. The voltage drop across the bias resister is 9.5 VDC. The voltage at pin 3 is 81.0 VDC. When I am done the calculations my plate dissipation comes out at 4.275 watts. The spec sheet for the 50L6GT says 10 watts max. Does my value seem like a normal bias reading?
Aerostoon Greetings, Aero. Using your measurements, I get a plate current of 52.5mA, which is a little high, and a plate dissipation of 4.25W, which is a little less than half of the 10W max. The culprit here is your plate voltage, which is less than half of the 200V maximum. This is a series-filament circuit, with no power transformer, so I tend to think that your values are probably very typical for a 50L6 in this type of circuit, in which the output tube is not driven very hard. The PC is already rather high, so you would not want to increase it in an effort to increase the PD. If the amp sounds OK, I would suggest leaving it alone.
All those ER shows are wrong. Defibrillation doesn't restart the heart. Defibrillation is (almost) the exact opposite; when a heart if malfunctioning, it is "fibrillating", meaning it is randomly twitching the muscles instead of evenly pumping.
Defibrillation stops the heart from fibrillating, and then medical professionals perform chest compressions; those compressions are what restart the heart.
If you try chest compressions on a fibrillating heart, it probably won't work, but a defibrillated heart is much more receptive to compressions.
Back in the day, the fact that Hollywood consistently got this wrong was a minor issue, but now that portable defibrillators are becoming more and more common out in the world, this misinformation can actually kill people.
Total tangent, but hopefully you can understand why I wrote it out.
Thanks for sharing this interesting information with us, Keith. Tangent or not, facts are always welcome.
I know this is really late, but while playing, doesn't the plate dissipation increase (especially if the tube is overdriven)? I built a single ended ECL82 amp (7 watt pentode) that dissipated 6.6 watts at quiescence. With the amp pushed HARD, those 19v across the cathode resistor spiked to 39v, for a staggering 14 watts of dissipation. I'm not sure how much of this is screen dissipation, but I do notice the ECL82 redplating while pushed. Is this normal?
In cathode biased circuits, the plate dissipation should not increase when the tube is driven, because this type of biasing is "self-regulating". In grid-biased amps, you should only bias to 70% of max PD to allow for a 30% increase in PD when the tube is driven. Regardless of the method by which the tube is (properly) biased, however, it should not red plate unless you are using some sort of overdrive pedal with a grid-biased output tube. In this case the tube should be biased lower than 70% to cope with the overdriven input.
Thank you, incredibly helpful. Still leaves some stuff out, i am experimenting with lower voltage stuff like running a 6K6 or 6V6 at about 150 Volt - so plate dissipation is way under the limit but how should i set that ? In the current amp it could be a 6K6 at 150 V 15mA or 130 V 30 mA - quite a difference - i decide by ear and stay somehow in the middle.
Operating output tubes way below their optimum levels generally does not result in good tone, SF.
I have the 6156 version of the amp in your video. After watching the video again today I thought, what the heck, lets measure the plate dissipation. I followed your directions except I measured plate voltage across Pin 3 and 8, and came up with very similar numbers as you did. Plate dissipation is slightly lower at 11.28W. Here's the funny part. I've watched this video before but sometimes I like to watch informative videos like yours more than once. I have a schematic in the bottom of the amp. Guess what I wrote on it (last winter?)? Plate dissipation calculation values, except I used the method you used in the video. I came up with 11.87W that way. :)
Measuring the plate voltage between the plate and cathode, as you did, is the more accurate method, Michael, and will give you a slightly lower PD than measuring from plate to ground as I did in the video.
thank you Uncle.....
YW, Nino.
Can the output transformer method also be used with single ended amps like the fender champ? Or does it only work when you have pairs of tubes and a center tap on the transformer?
There are two distinctly different types of OPT, Bob......for single-ended and for double-ended amps. They are not interchangeable.
Counter intuitive. A loud clean amp destroys tubes and a loud dirty one preserves them... Real shame. That is when I'd love someone's amp to fail... (And the last thing I want to do would probably involve a Russian pole dancer)... (And wishful thinking)... Informative and clearly explained. Many thanks.
Thanks, Bill. Glad you enjoyed it.
Hi Doug Good safety tips but your calculation are out. On a 6V6 the screen grid current is more likely to be 5mA, not the 2mA you suggest. Plus, the plate to cathode voltage is only 282V so the PD of the example valve is 282x36mA which is only 10.2 Watts which is perfect. Ultimately, increasing the plate voltage is the only true way to really increase headroom. Reducing the cathode resistor value to increase bias current will bias hotter. This produces earlier tube distortion and increases output transformer saturation, which also causes earlier onset of distortion.
ShawAmps Greetings, SA. The notes appearing in the video addressed the measurement of plate voltage from plate to cathode and the lower PV and PD that results. Since this is basic video, I did not feel that subtraction of screen grid current was an advisable complication. Thanks for your input.
Hello Doug, I have a Vibrolux reverb custom amp which I converted to an eyelet board and have wired it like the AA964 schematic. Two weeks ago I wanted to check the bias to the tubes and found it cold, so I lowered my resistor value at the bias pot so I could get a higher current value and when I powered the amp I blew a fuse. Well when inspecting the circuit and chassis I found a piece of heat shield by the heater and pilot light section. This came off from above, I guess I caught a piece when pulling the chassis in and out. I then troubleshoot and found the power tranny was shorted. I then installed a vibrolux tranny that was a Weber brand and had to remove the end bell where the wires come out of in order for the tranny to fit. The amp powered on and my plate voltage was now 384 as opposed to the 430 volts I had on previous transformer. I like the lower plate voltage. I am running about 42 ma and using the power dissipation formula comes out to 16 watts for 6L6GC tubes. With the plate voltage running at 384 can I use the 5881 tubes?
Thank you for you time and videos
Both the plate current and plate voltage are rather conservative for 6L6's, so I would say, yes, 5881's would be safe in this circuit. Simply recheck the bias with the 5881's installed and verify that the PD is within acceptable limits. Good luck.
Hi Uncle Doug! I hope Rusty hasn't been lured away to Hollywood to a life of a debauched filmmaker. I expect he'll get caught up in some Tinseltown scandal with his photo in the National Inquirer. Aside from that I'm confused. This website proguitarshop.com/andyscorner/amp-bias-explained States, "under biased (otherwise known as biased too hot) is a stated when the idle current is set too high. This causes the signal to distort early..." . Does this not contradict what you said or maybe I didn't understand something? Thanks so much for your videos. I've learned so much!
Several years ago, I added notes to the video explaining that underbias increases headroom and overbias reduces it, Rick. Apparently these notes are not visible on the device you are using to watch the video. Watch it on your PC and you'll see them. Sorry for any confusion.
Hi Doug: I was about to correct you on a couple of points but I read the description and see that you cover them in your notes. Excellent tutorial and WOW what a clean little Gretsch! Usually the ones I've seen have been shall I say "faithful companions". Cheers Mike.
Thanks, Mike :)
So if I understand correct, it is possible to scale down the power section of a cathode biased amp (W wise) by alternating the bias resistor and plate voltage? How is the voltage drop across the resistor come in mind in this? I’m asking because I’m curious about adding a power scale switch to an amp build
You need to watch my video on cathode biased amp circuits, where all your questions will be answered.
Hi Doug
Damn it, the way you explain electronics is crystal clear.
Two questions:
1-Have you ever got a high voltage shock working with these guys?
2-How can a tube amplifier be designed to sound good and have a way lower plate voltage?
Cheers
Pedro
Thanks, Pedro. Yes, I have been shocked a few times over the past 40+ years. Tubes must operate at the plate voltages for which they were designed; therefore, you would have to use an output tube(s) that was designed for low PV. See any Tube Manual to find one.
Doug I'm really enjoying these videos - very clearly presented and understandable. A couple of questions on this one: 1) I had the understanding that old tube amps with no ground were inherently unsafe but you are electing not to ground this one. Is it a matter of ensuring the 'death cap' is properly spec'd & functioning correctly? 2) Can you elaborate on the need for the lightbulb device?
Cheers and thanks again!
edit: I just found your video so #2 is answered :)
Thanks, Duane. With my own amps, if they are pristine originals, I keep them that way. When I sold the amp, however, I installed a 3-wire power cord and removed the "death cap".
I swapped my cathode resistor all the way from 270 to 400 Ohms and I still have 408 on the plate of my 5E3 build. It's all clean headroom up to almost 9 with a strat. I don't seem to be getting the bias right without losing all the famous tweed overdrive. right now it's 12W 33mA.
I come up with a PD of about 13.5W for your amp, Patrick, which is still way too high. Try 500 or 600 Ohms.
Wouldn't it be wise to just attach a variable resistor of the correct value so you could quickly adjust the desired headroom? I had a amp where I didn't like it's distortion and I wanted the cleanest amp possible by doing this, but at the time I didn't have much if any experience doing this, but I did check the ohms of the resistor and found the ohms was way off of the colors code or any available so I brought the ohms down to the next level and it cleaned up, but I wanted it totally clean so I kept lowering the ohms, increasing the resistance. Using this video as a example I wound up using a 270ohm it was a couple steps down from original and like I said I had no experience and I had no business touching my bread and butter, but it worked... My second amp was heavy metal all the way and I had it hooked up to a ABY switch the to a stereo chorus and reverb so I could use one or the other or even both amps at the same time... When I picked it sounded as clean as a piano, I had and still have the most bizarre neck pickup, it's a double single coil. It looks like a humbucker, but it's not. The box it came with said it was a double single coil wire for 500k It was the 80's when I did this so it's not as clear to remember every detail and my friend went his part bin and handed me a hand wound PAF and it came with a COA and schematic, this was over wound and screaming for a 1m volume pot not the usual 500k, but I had to leave it unless I installed a second pot. I truly found my sound. I purchased a cheap Ibanez around $450.00 but at all my gigs the fans ask me to use this over my Les Paul's...
I have posted videos demonstrating how to convert fixed bias and cathode biased amp circuits to variable bias, Mr. B. You may find them informative.
I believe you mentioned in another video, power tubes tend to produce undesirable 5th harmonics when overdriven. So, I would suspect running COLD power tubes is undesirable ??
Thank you for all of your videos, they are fantastic ! !
You're welcome, Richard. See my recent video on the effects of tube bias on tone.
Thank you very much for your videos. You explain things so clearly that even people that don't have English as their first language can understand everything very well.
You're welcome, Ed. Thanks for your very nice comments.
I do the bias on several amps without the math you know by using Webers bias calculator for class A/B or class A by switching his chart to class A !
I use a plate voltage probe and a grid bias probe to set the Webers recommended value at slightly lower value because voltage in my location varies - 123VAC to 116VAC at home !
You put your tube in the probe and plug the socket of the probe into the amp , first the plate voltage probe then bias using the grid probe setting my DMM to the probe instructions which is critical !
Whatever works for you, IJS. It sounds like you have a good procedure.
Excellent video. I have one question. You calculated plate (+ screen grids) dissipation by multiplying the B+ Voltage with the current (calculated using Ohm's law). The cathode resistor however raises the cathode Voltage above ground so it has a positive Voltage. Shouldn't you substract the Voltage that falls over the cathode resistor from the B+ Voltage before you multiply by the current. E.g. when B+ is 400 Volts and the Voltage over the cathode resistor is 30 Volts, the Voltage the tube "sees" is 400 - 30 = 370 Volts. Or am I missing something?
You are correct, HPV. The plate dissipation calculation would be more accurate if the net plate voltage (to cathode, not to ground) had been used. This was a very basic video. More complex and accurate methods of PD measurement are covered in subsequent videos. Thanks for your inquiry.
@@UncleDoug Thanks for your reply. I highly appreciate your videos and the fact you are sharing your knowledge with others! Keep up the good work es 73!
@@bugabootang You're welcome, HPV. Will do :)
excellent doug,,this is the best i have seen,,i worked in electronics production,but i forgot all this stuff,,now im ready to get in there and test my amp,,i want a warmer sound and mabey this amp is set different,,thankx again do more old school electronics,,no one knows much about this stuff in the computer age we live in.
You're welcome, Noah. Good luck.