1940s GUITAR AMP Conversion with SPOOKY MERCURY VAPOR Rectifier!
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- Опубликовано: 23 сен 2019
- SEE PART 2 HERE: • EPIC Guitar Amp Conver...
In this vid, we'll check out a 1940s Knight 35MDX PA Amplifier and begin the conversion process into a guitar amp. This amp is remarkable in that it uses a 6F6 tube into a transformer as a phase inverter, a 6SA7 HEPTODE tube in the preamp, and also uses an unusual Type 83 tube rectifier which contains mercury vapor. Other tubes are 6SJ7 and 6SC7 octals.
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I found it listed in the 1941 Allied radio on page. 86-87. The remote control “allows volume changes at distances up to 1000 feet”. Still curious about how it works. www.alliedcatalogs.com/html/1941/hr086.html
I'm guessing it was a remote rheostat in series with the 6SA7 cathode resistor. Probably operated by varying the bias of that tube, limiting the current flow through it. Not sure why it would need a voltage supply though.
Glad you're back
Another theory... They're using a rheostat to introduce the positive DC voltage from the power supply to the cathode. Looks like it simultaneously introduced an out of phase AC signal to the extra grids on the 6SA7. Never seen anything like this circuit that I recall.
Brad, I watch your videos because I'm extremely impressed with your knowledge of electronics; specifically amplification and it's very educational for me. I think that your nickname is befitting, "The Guitologist", and so I will continue watching with eagerness and excitement ! Thank you !
@@canigetachannel I appreciate that. Thanks for watching!
that schematic was discovered hidden underneath the 80-year-old kitchen wallpaper of a house that is still haunted by the restless spirit of a forgotten amp tech.
he still wanders the neighborhood at night, crying out, "time to bias the tubes..."
above the forgotten lost Michelangelo masterpiece that turned out being worth 7.5 million
Hi Brad. My vote is to keep the design of the amp as it is. The coolness factor is too high to change it!
Some notes:
I think you already know about the mercury rectifier. They really last a long time and you shouldn't need to replace it (just don't break the tube!). The only drawback is the slow warmup time.
The devices on the inputs of the amp marked "B.C." are bias cells. They were used in old radio circuits to put negative bias on the input grids of tubes. They are very stable and noiseless. Because they don't really have to provide much current, these cells can last may years. They should read about 1.5 volts with a high impedance voltmeter. If they go bad, you can sometimes (but not always) rejuvenate them by soaking them in distilled water for a few days. The water will seep in and reactivate the lead-acid inside. You can't really use a normal carbon-zinc battery as a replacement, as they have a different internal resistance.
The 6SA7 is actually a radio tube that contains a pentode and diode used for the detector circuit in AM radios. It has very low noise and high gain and was readily available at the time. The 6SJ7 and 6SA7 stages are used to provide a lot of voltage gain, which is needed by the 6F6. The 6SC7 has both elements tied in parallel to deal with the impedance of the next stage, which includes the tone section.
The 6F6 is used to drive the interstage transformer, which gives you the phase inversion for the push-pull outputs.
As it stands, I bet this amp was very quiet and had really great tone. A guitar played clean through it would have a very warm sound. Changing the circuit would change the whole character of the amp. If you re-cap it and replace the out-of-tolerance resistors, I bet the amp would work just like when it was new. I think the people that designed this amp really knew what they were doing!
Hope this helps. Can't wait to see part 2!
Thanks, as always for sharing!
xraytonyb the 6sa7 is a pentagrid tube, think of a pentode with added control grid and 2nd screen grid. Grid 1 is a sharp cutoff and is used in the local oscillator with the cathode as a hartley Osc. Grid 3 is a remote cutoff and was connected to the antenna tunning circuit. The tube mixed the the two signals to produce the IF output. The tube could also be used to in a compressor circuit by applying a negitive bias to Grid 3 to reduce the gain of the tube.
Thrilled! I love these conversions.
I love vintage guitars covered in detail as well. These are your sweet spots!
Thank you for this incredible content.
"I thought of quittin' baby, but my heart wouldn't buy it" -The Guitologist
We'll see.
The 6SA7 was a weird choice for a driver. I used to see this tube in the RF mixer/local oscillator stage of "Amercan Five" radio chassis. This use of a power tube and transformer for the paraphase suggests this was top-of-the-line stuff. Really glad to see another repair/conversion video. As much as I like your rants and SPFs vids, these projects are what brings the boys to your yard...
Maybe you could use the heptode stage to make a tremolo circuit...
Thanks for watching.
I have no electronics knowledge and generally couldn't care about the workings in these amps but you make it interesting in these videos and I enjoy watching them
YOU HAD ME AT MERCURY VAPOR and I don't even know what the means. Has anyone ever built a nuclear powered amp? Seriously. I have a nuclear battery in my chest.
I just love process and I learn so much stuff from guys like you. Not being mechanical or very handy it is fun to watch those that are. I have a great love for these things.
There was a time, not that long ago, when every streetlight in every town used a mercury vapor bulb.
Adding a comment to thank you for uploading one of these videos, love 'em! Hope your dad's doing better, Brad!
I can't count the number of times I busted out a tube amp schematic and discussed its peculiarities with my friends at a party !! laugh !! ;)
I do hate to interrupt a good poetry reading, but sometimes it's necessary.
@@TheGuitologist Of, course to some of us, schematics are poetry in motion (electron 'motion' that is). And just like poems, some schematics are pretty straight forward but others - well lets just say that for me, some might as well be written in Greek! ;) I like the ones that rhyme - both poems and schems :) .....BTW, Brad thanks so much for your videos, for your knowledge and espec for your time!!!
Yep I remember well Donald. Party prep. Shower. Get dressed. Check for folded up copy of milspec tube amp & half a dozen frangers in back pocket!
Your news and commentary, etc. is always cool with me (I'd speak up if I thought sumpin wuz bogus) but your amp videos (?) well that's what I appreciate the most... being privy to your analysis in what seems like 1st hand, is really great, and most instructive... Thanx so very much... can't wait for part two. Even with the unfiltered 60/120 noise you may have a real tone jewel here... Go Brad ...and thanx for another great vid.
A heptode is a frog with a fedora and a soul patch.
Jeff J, a Pen-Toad is one who writes to his girlfriend from a prison cell!
The 83 mercury vapor rectifier needs a long warm up time and should be used in the vertical position. The long warm up time is so the mercury puddle in the bottom of the tube can turn to vapor. Then the HV will get stable and conduct reliably. Be careful they are dangerous.
I suggest converting it to a 5U4 for safety and similar performance.
The type 83 Mercury vapor rectifier tube is standard for the main B+ in a Hickok tube tester, which includes the military testers such as the TV3 and TV 7 that all use Hickok circuits. The rectifier tube is mounted horizontally and not vertically in those units, which also have a 5Y3 rectifier as well. The reason they used the Mercury vapor rectifier is because it has a constant and consistent voltage drop regardless of how much current you pull through the tube , making it a good stable reference voltage for test equipment. The #83 is reliable and long-lived, and the only proviso is that you should ideally let it warm up for 5 to 10 minutes to vaporize the mercury before playing the amp and pulling any current through the rectifier tube. Therefore, the only reason for Brad to change that #83 tube to a standard rectifier tube (5U4 etc) is if he wants something with a little more voltage drop to deliver some sag and compression when the amp is being pushed hard.
i missed your amp videos! glad to see a new one back on the channel :D
Great to see you back doing what drew me to your channel Brad, fixing and modding amps
Grate! Glad you're back with tube amp repairs and mods. Great news you're not giving up. Cheers!
Great job Brad ! Stay strong man love your stuff!
Thanks for getting back to the amps. It's when you're at your best. There's enough SP for any day for the week all over the interweb. This video is the type that won't exist unless you and Tio Doug are doing them. Please keep it up. No one else will. Props to you.
Can't wait for you to work your magic on this amp, love your channel man.
Thanks Brad, I don't really work on amps as much as I used to (changing a fuse) - maybe the odd guitar or two but I never forget your advice on all sorts of things and stuff. Cheers, Good Week to you from England.
goddamn i don't understand much of the technical aspect of this but I enjoy watching.
The bluegreen rectifier tube in that 1940 amp just clearly blistered Brad. It left him speachless...and in the ozone. I know very little about electronics...thats why i leave such things to the experts...and i also as a stupid teenager...stuck a butter knife into an unplugged color tv set and touched the flyback unit...thought i was gonna die.It held on to me untill the voltage bled off enough to give me relief and mercy. I felt bad for two weeks afterwards...so electronics scare me to death.
Thanks for watching! Good to have you.
@@TheGuitologist Thanks for posting videos! Seeing you describe the circuit diagrams always reminds me of ElectroBOOM channel but you've only set something on fire once :) Keep up the good work Brad.
Killer part 1 video Brad! What a cool and bizarre amp! Can't wait for part 2!
I must be on the wrong channel, fixing valve amp ? Huh ! Good to see again finally :)
So glad to see back into sumthin. Pre Leo taboot, this one is awesome so far. Keep up the quest and bring some gain if ya can.👍
Ahhh yess! Back into glowing tubes of loveliness! That's definitely a prime example of out-of-the-box thinking from the days before HiFi and high powered guitar amps...designed by radio engineers from that aspect....COOL!..Looking forward to part 2! Thanks Brad!
At about 19:30, re Western Electric Corporation: min the 30s and 40s, the best movie theaters had Western Electric audio systems, and a bit later, Altec speakers which were made by a manufacturing division of WECO (later Altec-Lansing). Western Electric never actually sold that equipment to the theaters, they leased it; and when monopolies were being busted up post-WWII and WECO was getting out of the theater sound business, they never bothered to reclaim most of that gear, and much of it was dumpsterized as the theaters periodically upgraded their systems. BTW, if you look up pictures of early WECO equipment you'll see horns so big you could almost park a VW Beetle inside of them!
Could not have picked a Better Project to Mount Back Up With. Its Been a Rollercoaster of a Month on all Counts for You & Yours. Great to See you Back in Action Brad. Be Well!
Good to see you back with the amps! Best wishes to your Dad.
Pretty cool! Thanks Brad! I can't wait till part two. I have never seen those Beaver caps before either.
That’s a cool video! Love watching you work on amps. I don’t understand everything I find it compelling to watch lol
Nice to see you back to analyzing amps.
Nice to see you back on this type of content.. Might be useful for you to know that it appeared in my Google news feed too
Great to see you in an amp video again Brad! Looking forward to the next parts. My first attempt at a PA conversion was a 50's Stromberg where I cascaded the two 6SJ7's that then go into 6SL7's, and a pair of 6L6GB's. I need to revisit it and revise things now that I've learned a bit more, you can make it squeal if you turn everything up too high haha
Can't wait for part 2!!!
Whatever you do, keep the mercury arc rectifier! They are super cool and unique.
Hi Pman! Aren't they just! I thought it was gonna blow up & was yelling at the screen for Brad to turn the flopping thing off! But if they're supposed to glow that colour then he's gotta keep it in there absolutely!
Thank you Brad for this one! Uncle Doug will be proud of you, as if that matters. More and more we find out how the engineers used the components of the day. It seems the use of a interstage transformer for phase invention was preferred in high end amps of this vintage, and later. The Mercury Vapor rectifier used here is the 1st. I've seen. Even in 1940, that tube probably cost a lot more than say a 5Y3 pr 5U4. They were going for quality here. I think when you get done with this beauty, the original designers, god rest their souls, would be proud! Can't wait for the next installment! It will be interesting to see what you do to convert this guy to a 2019 guitar amp. New parts, new design, new purpose! It did not end up in a land fill! Thanxz
They probably didn't have 5U4's back then and they needed extra current capability for the output stage. Mercury vapor rectifiers were in common use back then.
No matter what you do, you must keep that meter and get it in working condition, it’s the crowning jewel to this beast.
I love seeing this ancient gear!
Prayers for your pops!
Cool project! I have a 60's Knight KN3050HD that someone converted to a sort of JTM45 clone. It's a great conversation starter, and it sounds cool.
That Mercury Vapor tube is really really cool!!!
This made my day! Thanks!
105 to 107 volts used to be pretty much the "standard" house-current voltage in the 1940s, but by the 50s most homes were running, or supposed to be running, at about 115 to 117VAC (the higher voltages being more efficient for running motors and large appliances such as refrigerators). Now that AC voltage is often running at 122 to 124 volts, it definitely places a strain on vintage tube amps which typically have about a 1:4 ratio ( or slightly higher) of input AC to main B+ internal voltage. In other words, 122VAC input will often equal at least 488 VDC on the filter caps; and the tube filaments rated for 6.3 volts might actually end up running at 6.7 to 6.8 volts or thereabouts..
83 Mercury vapor rectifier tubes-haven't seen those in a very long time.Used to have a Philco radio that used one.Got lost in a flood.Sad to see it go.Excellent set-TRF.The pentagrid convertor tube in that amp is a mystery.These tubes usually used as convertor tubes in radios.IE RF input to one grid,IF out on the plate.The "triode" section of the tube was the local oscillator stage.Tuned by one of the air variable cap tune gangs.The 6F6 driver is OVERKILL for PP 6L6.This circuit used is like what was used with triode power tubes that had less gain and needed more drive power.I have encounted mercury vapor tubes of many different sizes and power ratings in broadcast transmitters.In AM and TV transmitters(analog)the mercury tubes would flicker to the audio modulation and glow brighter during black picture on TV transmitters.Have a few in my collection.The biggest is an 872 if my memory is correct-this GIANT glass mercury "firebottle" was used as a power rectifier in 50Kw AM broadcast transmitters in the 3Ph HV rectifier stages and same with 50Kw VHF B tv transmitters.An even larger metal case version was used in larger supplies-UHF TV and very large HF amd MW AM transmitters.Those could be for 15Kv @60+Amps!!!At the VOA site here we used to have transmitters that used those very large rectifier tubes-converted to solid state in the late 60's.Then those transmitters scrapped in the early 2018.A mercury vapor tube can "talk" to you from its glow-no load-very faint or no glow-Average load blue glow.Heavy or overload-very BRIGHT blue-almost white glow like a mercury vapor lamp bulb.Chrome and glass broadcast transmitters that used mercury tubes were indeed a BEAUTIFUL sight!!!So miss it!!
That blue glow was a trip, looking forward to the second installment, :)
I used to play chess & checkers with a guy who had at least one like that ... He was a Ham Radio Operator that had vintage tube compressors and eq's in his collection of gear ... he often talked to people around the world through an old broadcast ribbon mic ... his radio room had shelves full of radios, amps and other gear ...
Very nice, now I want one
That's an 83 Mercury vapor rectifier. Most good old tube testers used them. The flakes are little pieces of mercury, it vaporizes when the filaments heat it up. It will glow blue/purple down the cathode stacks.
Thanks for sharing Brad. Best wishes for your dad.
20:42 We had to wait for the humour to creep into the video! LOL
APPRVD. RLLY BEAUTIFUL AMP.
Very interesting amp for sure.
Holy shit, amp work!
I’ve got a 1950s Knight PA amp with 50 watts output (massive output back then).
It has bass and treble two line inputs, crystal phono and two balanced three prong mike inputs.
As a young bass player I used this as a bass amp. Because of impedance mismatch the mike inputs wouldn’t work with my guitar cable, so I chained into the line input a tube preamp that I got from a tape recorder.
In between the bass and preamp I put an Mxr ten band eqaulizer.
The amplifier had a jumper wire at the back to change the speaker impedance in options, 16, 8, or 4 ohms.
I noticed that if one selected 4 Ohms with a 8 ohm load, treble response was increased, so that’s what I did.
The amp had a boutique HiFi quality with extended flat frequency response.
It kind of sounds like a Ampeg with the warmth of a fender bassman.
Unfortunately, with only 50 watts output, it hasn’t the headroom for a bass amp.
Bass players who like tube amps enjoy the transparency of the sound, but better realize tube amps are only as good as their tubes, and they wear out quick.
Cool amp!!!
Hi Brad. Those input resistors were an early low-noise resistor. The most common resistors in use back then were carbon composition resistors which were quite noisy in the inputs of amplifiers. When exposed to the thermal emissions of the tubes plus the heat from the higher wattage resistors of old, and with the high voltages applied to them, these were more stable and less likely to change resistance, which is often seen in the tube amps of that era, although there would not be higher voltages found on the inputs of the preamps.
Cool old amp. Should be an interesting project.
Permanent magnet speakers were a rarity in the '40s, especially in permanent installations where a PA would have been used. They were mainly being employed in portable devices like early guitar amps, etc. Many speakers at the time that PA was being used had field coils that would have higher values of impedance. Some were like 1500 ohms, in which you could put 3 in parallel to get 500 ohms. They also had 500:8 (and I assume others) impedance matching transformers that could be used with older PA systems that were built for the field coil speakers to be used with the permanent magnet speakers. If you look at amp schematics from the '20s, '30 and even into the '40s you will see output transformers with only a 500 ohm output impedance. There are also some with 1000 ohm, 3000 ohm, 8000 ohm etc. Again they were all made with a particular field coil speaker or speaker configuration in mind.
However, this amp doesn't supply the field current. The advertising brochure says it's for PM speakers only. I'm thinking the 500 ohm speakers were early attempts to solve problems that later got fixed by 70V systems. By using a transformer at the speaker, you could have thinner speaker leads.
@@russellhltn1396 Yes. It was common to see impedance matching transformers in speakers of that era.
The way that tube lit up was awesome
Brad's word for the day is Heptoad !
Wow, we are in kneedeep, kneedeep, kneedeep.
@@therugburnz , in The World According To Garp, the constant but underlying refrain was "Watch out for the Under-Toad"! Great book, not so great a movie....
@@goodun2974 keep passing the open windows
@@therugburnz , lucky my names' not Frank! But that was from Hotel New Hampshire, of course, a place where "Sorrow floats". And you're all right with me, 'cause obviously you're a reader! . My favorite John Irving was A Prayer for Owen Meaney. Boy, did that one turn into a trainwreck when they tried to make a movie out of it!
Tube Amps are so fucking cool.
I got an old radio from the 40s one of the RF tubes glows green and looks like a hologram when trying to get the station in, it's really cool, I can get Moscow and Mexico and other foreign stations. I rewired the whole thing, I couldn't believe it was working the way they had some of the wires ran, the only bad thing is the feild coil speaker but it's in good shape so I left it alone.
Magic eye tube. Good stuff.
@@russellhltn1396 , yeah, probably a 6U5 or 6E5. The miniature 9 pin tuning-bar tubes came out later, in the 1950s.
I'm digging on that tube glow too.
Finally a repair... Rob
that PA could be something special if perhaps a reverb unit was incorporated, even cooler would be to have the reverb unit stand alone and plug into the two 1/4" plugs you were thinking of re purposing. Classic looks, very cool vibe that would be right at home in a boutique amp shop.
Wild to see the resistor values fully written out in Ohms.
Those "capacitors" in the input line are bias cells, actually little batteries, that set the bias for the input pentodes. They still might be good, but check them.
😆hes back!
83s are cool stuff. At least one early Bassman had one and most Hickok tube testers.
LOL@20:45 "Thanks I just had it stuffed."
Super cool amp, and a transformer as a PI, yeah, that's uncommon. First time I saw one was in a '67 Gibson GA-5T, driven by a 6C4. The PI was missing from the amp, so finding a suitable replacement took a few minutes.
Almost all prewar amps had transformer PIs.
Thats cool they could " cue up" or fade mic to record player or commitee to speaker
Am I the only one who thinks the coke-bottle tubes are just a thing of beauty...?
That heptode probably can be used to make a compressor type circuit, as it has 2 control grids and both of them control current throught the tube, so you can use one to set the gain, while the other is an regular input!
I think someone else might have mentioned, those flakes are perfectly normal. It is a Mercury rectifier and when the Mercury cools it tends to Splatter all over the tube. You probably would be best to bring it up slow and then let it cook for a good long time and that should kind of go back to where it's supposed to be. Also if you keep it in the vertical position all the time that splattering doesn't really happen. And it doesn't matter if the splatter only whines up on the glass and external things the problem is if any of it settles on the inside and shorts out a cathode or something that would be a problem. Amazingly my tube testers all use the same rectifier and they use it sideways how it was never meant to be used and they're still working all these years later. There is an 83v which is a later version of it that is a regular vacuum rectifier. Also there was a bigger version of the 80 called the 5Z3 I believe which may be big enough for what you need. The 5z3 is a 4 pin 5u4. I have several if you need one. I have over 5000 tubes and would not mind sending a few to your channel. Keep up the videos.
I will add to the small chorus of people here suggesting you get that mercury tube out of your house. A tube hobbyist got mercury poisoning from these tubes and unsuccessfully tried to sue several tube suppliers for selling him those tubes. At the TCA we made a comprehensive list of mercury vapor vacuum tubes so we knew which tubes could have huge liability to sell. Seriously, if that tube broke after heating up and the government found out, the cleanup might involve tearing down part of your house. There is plenty of reading online about mercury poisoning and what is involved to clean up.
All glory to the heptode! Could only imagine how much this thing must have cost back in the 40s.
Allied Knight equipment could be ordered as kits or completed form. Judging by some of the connections, this may have been kit built.
@@TheGuitologist That would make sense. Looks like this thing would be seriously expensive in 1940s but as a kit, it might actually have been somewhat affordable.
Cash price was $89
Retail was $175
That’s like $1600 & $3k in today’s dollars!
@@ColeWheeler4Lyfe Dollartimes calculator says $1,513.63 for the catalog price.
Hey Brad, I heard recently from Uncle Doug, that the screen grid has a capacitor to cathode to ground to keep the negative voltage even on screen and cathode, when using Pentodes in pre amp. I believe that's how it goes, look at schematics, Uncle Doug. All the best, C.
Vellllllly intelllesting ! ;-)
Damn this is cool
Hey my medication wore off I just saw a frog
Bro its soo weird, but I love looking at your hands in the head/ Indiana Jones Maps 😉
I would wear a Rad Suit before even TOUCHING that! ;-)
I was going to suggest the remote control was for a volume rheostat, but you figured that out. Note there is no overall volume on schematic. Maybe use it with a pedal for swells etc.
"...could bust out the schematic.and discuss it with his friends," = party killer. ;-) Excellent video though, thx.
Beaver caps are from Cornell Dublier. I have a 73 amp that came with a few of them in it.... although different color.
Yeah, I saw that after cam went off. Can't recall having seen them before.
On another matter. My Hughes and Kettner Grandmeister 40 Deluxe has an output which can be used to connect to another amp.
Can this be used for headphones? Hughes and Kettner themselves could not answer the question. How can the output be used as a headphone jack?
Could the remote be like a momentary switch to key a mic ?
Be careful touching that crystalline residue. You may end up on a 9 hour LSD trip...
This it would be cool to see it as a 2 channel with a clean and high gain
I am definitely curious to see what the hell you do with that thing. That mercury tube is just scary
WRT that remote jack.... Is this the first channel switching amp? 🤣
Seriously, though, I’m really curious as to what was supposed to be plugged in there. Maybe it replicates that knob for cross fading the phono inputs?
Also, that mercury vapor tube looks so cool! To bad they didn’t set it up so that the glow backlit a logo or something.
Back to amps!!
A 6F6 driving an interstage transformer was also utilized in this device:
ruclips.net/video/LColZ8MOtd0/видео.html
Tube light like that when there is too much current draw. I bet the cathode resistors or the capacitors on one or both output tubes tubes is shorted and this causes an over bias condition that causes too much consumption.
Hang on to the old paper caps, especially .022. Whether they're leaking or not. Great for Les Pauls or other Gibsons.
Man the 190 volt suicide jack and the mercury poisoning valve are options you can live without.
Hi Paul! Yeah, they're interesting, unusual, cool & dangerous! Aw can we keep them? Hehe! He's gotta keep the meter though!
How about modding this amp and add Heptoad tube rectifier fuzz?
Hi Brad those flakes are the mercury in solid form, that tube should be heated until it turns to vapor and rises to the top of the bottle or it can lead to short tube life.
I've read all sorts of contradictory things on these. Some say the flakes are harmless, some say they can cause a failure (which was my initial assumption as well), some say to only operate in upright position and to even only transport and store in upright position, not to shake, all sorts of things. The datasheet makes no mention of any such restrictions.
@@TheGuitologist , Hickok Tube testers use the type 83 tube, mounted horizontally , not vertically ( and FWIW, those should NOT be changed to a different tube or to solid-state diodes or you'll never get the tube tester calibration right!). There was also a short lived Fender Bassman 4 by 10 version that used the type 83 rectifier as well. I repaired a transitional Bassman years ago that mostly matched the schematic for the type 83 version except a few parts values were more like the next version, and the rectifier was a 5U4 with factory-installed octal socket instead of the 4-pin socket required for a # 83 tube.
@@TheGuitologist All my tube testers they are mounted horizontal. Only time they can be dangerous is if it is broken while the mercury is in a vapor state. Great tubes with nothing to replace them that work the way they do. I've built the solid state kits for tube testers and they never calibrate right. One B+K tester I have the SS replacement worked pretty good, Hickoks don't like the SS rectifier at all.
Good to see you back and more to your previous self.
Is the schematic available for anyone to download? I'd love to analyze the circuit but its hard to catch it all on RUclips's viewer window. If not, I'll just screenshot and stitch it together. Its just a really interesting circuit to me with all they are doing with what really amounts to a minimal amount of parts.
Best wishes to you and your family.
I'll upload it to a community post for you.
ruclips.net/channel/UC2DDLacv42cfgT7Urv9n3dgcommunity?lb=Ugw4uAtUo9VWMiUI17h4AaABCQ
Eh, nevermind. RUclips compressed it to lower res. Email me for a PDF and I'll send it to you. bradlinzy at gmail
@@TheGuitologist Thanks! Look for a Yahoo dot com mail. Can't miss the tag line.
@@TheGuitologist , schematics for many Knight amps and PA'S were published in Sam's, I'll have to look to see if I have a cleaner copy.
Beaver caps were only around for a short while. I only ever ran into them in PA and other quasi-industrial gear. The special resistors were (IIRC) high accuracy (for the time) pieces. You mostly ran into them in test gear made back then. They were around 1% of the marked value, which was pretty spectacular for the day. Common parts of the day had 20% tolerances. 10% was considered special back then and 5% or lower was "laboratory grade."
I was thinking low noise audio signal grade, but your theory works too. Could be they were selected to be close tolerance, but I wonder what the purpose of that would be considering the forgiveness of tube stages.
@@TheGuitologist That puzzles me too. There must have been SOME idea behind it but it eludes me. My best guess mirrors yours - noise. Those early high-gain circuits often had a lot of noise so this may have been a way of cutting noise from open jacks.
@@TheGuitologist It could be something simple like getting a good buy on overstock parts.
Those components are bias cells, tiny batteries that provide negative bias for the input pentodes. They may still be good, since there is no current drawn from them, but check them.
@@quantumleap359 Wow, I never ran into those before, except in really early radios where they were used to bias certain tubes. They were "C" batteries and many kept being used even after AC adapters came in. Thought they were long gone by the time this was made. Mercury cells?