• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Help with tube amp design, resources appreciated

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Hi guys,

This is the first time I've posted on this site but I have lurked around the forums for a while now.

I have undertaken the project of designing my own tube amplifier circuit. This is my first time trying out anything this big, I do a lot of rewiring guitars/basses and adding little mods here and there, fixing the lamp cord when a prong breaks off etc, but this is new to me and has me googling non-stop for every hour that I'm home, and I still don't have a great understanding of it all.

I have a Stromberg-Carlson AU-36 chassis with original power transformer and output transformer, tube sockets, and a lot of wiring that looks VERY old. (which I guess it is). I want to use this for its chassis and transformers, but take out all of the wiring and caps/resistors/sockets and redo it myself, for one helluva learning experience.

Though there is still a lot that I don't know, I'm having the most difficulty with maybe a broad topic. I have the PT and OT already, but how do I match those to the valves I want to put in? How do I figure out if the voltages from PT will work with the valves and how the impedence of the OT plays into all of that (I get heater voltages being supplied directly from PT, but what about plate voltages?)

PT data from an old schematic I found:
Primary: 117vac @ 2.2A
Sec. 1: 760vct, 0.380ADC (does this mean mA?)
Sec. 2 5vac @ 6A (for rectifier 5u4gs?)
Sec. 3 6.3vac @ 0.45a (this would seem to be for 12ax7/6sc7/6sn7 pre valves)

Sec. 4 6.3vac @ 4.7A (I assume for power tubes 6l6)

OT data:
Impedence: primary-2.7kohm CT
DC Res.: 30ohm CT

The valves on the AU-36 are:
6sj7 as preamp, 6sc7 as phase inverter, 6sn7 as what I think would be a driver for the two inverter signals, 4-6l6gb for power section, 2-5u4g rectifiers.

Using the pentode 6sc7 as a preamp seems more complicated than I want so I thought I'd use a 12ax7 there instead. So I know the transformers should work with all of the other tubes, but will it work to switch those two and why/how?

I guess I think I'm just confused about plate voltages and how they get supplied, are they controlled specifically through biasing with different resistor values or are they set by the transformers?

Any resources you guys have for general amp circuit design I would love. My most helpful resource by far has been Rob Robinette's website. All comments and suggestions deeply appreciated!
I read that thread and looked through what was recommended before i posted this. The poster was looking more for information about preamp circuits and why use certain values for resistors/capacitors. I don't mean to flood the forum, just looking for information on a different topic, what do I need to do to insure my power transformer, output transformer and valves work together? If this is too basic of a question I apologize.
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This may not be what you want to hear, but your questions about things like how to convert 0.38A to milliamperes (mA) and whether or not a 5V AC 6A winding is meant for the 5U4GB rectifier tubes suggests you don't have a firm grounding in basic electronics theory.

Working with a big 760VCT power transformer can be really, really dangerous. You do need to have a keen instinct for not touching the wrong things the wrong way. A transformer like that can deliver a lethal combination of voltage and current. Yes, lethal.

The project you're undertaking is pretty advanced. That amplifier has high voltage at fairly high currents (four 6L6GC means something like 450VDC plate supply at somewhere around 0.2A current draw, which can pack quite a whallop).

Have you built a pre-existing tube amp design from scratch successfully? If not, it might be best to start there.

Perhaps you could start by studying Merlin Blencowe's Valve Wizard website. It's a primer on the basics of tube amplifiers. I think it would be a great place to start.

Also, there are some good books that could help you a lot. I'd recommend Merlin Blencowe's "Designing High-Fidelity Tube Preamps", even though it doesn't cover push-pull power amplifiers. It does cover a lot of the basic design concepts with some example preamps you can build.

Another good book is Morgan Jones' "Valve Amplifiers - Third Edition", which has a chapter on a nice push-pull EL84 amplifier design with a very complete explanation of design choices and calculations.

(Valve Amplifiers - Fourth Edition does not have that PP EL84 amp design, so I think it's not quite as useful in this particular instance.)

There are several amplifier designs contributed here on diyAudio that would make good introductions to building an amp from scratch. One is Eli Duttman's El Cheapo, which you can make with 6V6GT tubes.

Once you've gone through building one of these, I'm pretty sure a lot of your questions would be answered. I'd start with a known-good and not too complex amp design to work from. Build one, get it working well, and go on from there.

Anyway, that's what I'd recommend.
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Much appreciated. Sadly maybe I'm still at that stage. I am drawing a schematic for the whole thing and won't be doing anything involving electricity until that is completely done so there's no risk, and I understand what should be done to keep an amp safe.

I don't want to build it from a kit because kits are like $500... and it seems like you're mostly paying for chassis and iron which I already have. I certainly don't have a proper grasp on the theory and I think that is the problem, I have always just put this wire there and chose this value cap because someone told me to, so trying to avoid that. The other thread recommended Merlin Blencowe's book but I have the best grasp on the preamplifier section over anything else in the design so far.
Might end up double posting not sure...

Thanks for the comment. I agree I don't think my theory is solid enough for this project yet, I am drawing a schematic for the entire thing and want to understand every piece before I do any wiring, and I know the precautions to take when working on tube amps. Thanks for the noob link, already found some good stuff.
Nobody is born knowing everything about everything. Valve amplifiers are a great thing to spend time learning because they're knowable yet deep, and the knowledge gives a good perspective on other disciplines later. And, they can play music!

If you can manage to keep one hand in a pocket, it's reasonably safe to poke around in an amplifier, and you seem like a thoughtful person. Maybe a good way to start is with some grunt work - draw out a schematic of your existing amplifier. If you already have a schematic pretend that you don't (some cheating allowed). Book learnin' is essential, but so is a hands-on feel for real parts (however ancient).

This must seem a foolish task, and maybe is, so please feel free to ignore it. I might well have ignored it myself. Just a thought.

All good fortune,
hey there. Without completely putting you off, here are some facts to help you make a decision on your next move.

Within the bounds of reason, the typical human can cope with a current limited 110v AC shock. Its usually unpleasant, but not often lethal. At 230v AC (the stuff we have) things become less certain - you may, you may not, all depends on your health at the time and the path the current takes, whether you are holding the conductor or just brushing past it...

At 500v AC, even with dry skin and hand to hand the current across your body (and worse, across your chest and your heart) is going to be iover the 100mA. Physical harm is certain and many people will experience heart disruptions from temporary to permanent (!) stoppage.

At 1kv AC, death is near certain.

Also to be considered - at 110V, you have to directly touch something to get bitten. At 230V, brushing against a conductor is a startling experience that requires a calming whiskey and three days consideration. At 500V or thereabouts a near pass is likely to give you an unpleasant surprise - you didn't even touch it and still you had consequence.

At 750V and higher, humidity and a tailwind depending, the current is going to come looking for you and will jump a considerable gap to close the distance between conductor and flesh.

Your PTX is capable of around 1100V AC open circuit.

Make your own decisions.


Joined 2003
Paid Member
...I have the PT and OT already, but how do I match those to the valves I want to put in? How do I figure out if the voltages from PT will work with the valves and how the impedence of the OT plays into all of that....
4-6l6gb for power section, 2-5u4g rectifiers....

This beast WORKED, no-fooling, with this iron and the six big tubes.

At the moment I do not think you are anywhere near any major change in the Power section. I would even (at your level) keep the 5U4s.

To some degree, tubes is tubes. You can't send a small 6V6 to do the work which was expected of a 6L6, no. But the EL34 is very nearly as big as the 6L6. Only slight bias change needed. (However the AU-36 appears to be grounded cathode, but in fact is self back-biased with the 90 Ohm resistor way over at the PT.) (And the real glitch: EL34 uses a lot more heater power than 6L6. My AU-33-like S/C ran HOT with stock 6L6, and EL34 might be too much.)

Keep the 90 Ohms and the 6L6 (or 5881 or KT66). Fix the rotten wire, replace the old caps and check the old resistors. I see nothing offensive about this design as a guitar amp. The driver is more complex than a Twin, less complex than an SVT, and Known Good. The "Guitar Amp" flavor comes in the preamp stages which you can steal from other designs.

You really do learn more by working with a good design and evolving step-by-step than you can by rip-out and staring at a blank canvas. The Dead Guys did know their stuff.


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Read this: http://sportsbil.com/other/Basic Electronics, Volumes 1-5, (1955).pdf

Then you will have a much better understanding. The US military used these books. There are illustrations.

I just stumbled across this same link on another reply of yours from another thread, and I'm actually printing it out as we speak. It is an absolutely fantastic resource, with lots of great explanation and illustration- a good "period" look at how things were done back then too. Highly recommended. I'm keeping an eye out for actual copies to put on the shelf now. Most of my books are very technical, enough that they aren't an easy pickup to just grab and brush up- these are a nice way to get someone into the basics much easier. When my kids are old enough I may have them get into it as well.
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