DIY Tube Power Amp

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DIY Tube Power Amp

Post by ckeene » Wed Dec 05, 2007 3:06 pm

So I scored a tube amp from an old Conn organ. It's the kind with a separate amp and power supply. I guess what I want to do is grab the power xformer and roll it into a home stereo tube amp. I'd like to do something with pairs of 6L6s in push-pull with a 7025 inverter

So first, are there any designs that I might want to look at? Would a traditional guitar amp power section like an AA864/AB165 do the trick, provided I pay attention to bias and what I buy for output xformers?

Second, the power xformer I have from the aforementioned scrounge has 6.3V filament taps, a weird one at 50-odd volts and the primaries measure at like 890V. Could anyone here help me with how I might go about calculating appropriate plate, grid and bias voltages? I'd be using a SS rectifier.

Thanks, any help is appreciated.

-Chris

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Post by A-Barr » Wed Dec 05, 2007 3:26 pm

A good rule of thumb is to use that transformer to power a similar set of tubes, You don't know the specific current capacity, but you can assume that if the donor amp ran a pair of 6L6's, that that power transformer would work best driving a pair of 6L6's or similar, certainly not two pairs of 6L6's, but you can have a stereo single ended amp, much simpler to build because you don't need a phase inverter and it would yield around 11 watts per channel (which is plenty!)

What tube compliment did the donor run?

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Post by ??????? » Wed Dec 05, 2007 3:28 pm

Unless you find another one and make mono block amps, you're not going to have a "home stereo" amp. You're going to have a "home mono" amp. :D

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Post by dtguitar04 » Wed Dec 05, 2007 3:38 pm

Well if I could venture a guess, I'd say the 50v secondary would've been meant as a bias feed.

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Post by ??????? » Wed Dec 05, 2007 3:51 pm

also FYI, the primary is probably either 120 or 220v or something in that range. The 890V high voltage taps are the B+ secondary. If you wire that up as the primary then you will have it hooked up all backwards and it will be a huge step-down transformer.

Perhaps you have your terminology (primary versus secondary) confused. In a power transformer, the primary is the side that the wall power starts on, and the secondary(ies) are the sides where you get your transformed (desired) voltage.

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Post by ckeene » Wed Dec 05, 2007 4:03 pm

??????? wrote:also FYI, the primary is probably either 120 or 220v or something in that range. The 890V high voltage taps are the B+ secondary. If you wire that up as the primary then you will have it hooked up all backwards and it will be a huge step-down transformer.

Perhaps you have your terminology (primary versus secondary) confused. In a power transformer, the primary is the side that the wall power starts on, and the secondary(ies) are the sides where you get your transformed (desired) voltage.
You are right, I meant the 890V secondary. I meant primary as, "the tap with the highest voltage" which of course isn't quite right.

Also, I can use a single power supply to supply both left and right channels, right?

The original amp had 3 pairs of push-pull tubes, i think they were 6V6, but I'll need to check.

The

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Post by ??????? » Wed Dec 05, 2007 4:12 pm

ckeene wrote: Also, I can use a single power supply to supply both left and right channels, right?
That depends on how much current the transformer was designed to handle. Once you've handled a few of them you can make educated guesses about how robust they are from the size/weight, but there's not always a large margin for error. For example, the 6L6 draws more filament current than a 6V6. I don't remember the exact numbers offhand, but 4 6L6s just might draw more filament current (or every bit as much) as 6 6V6s. And that's just that one winding. Then you've gotta think about plate current. What type of current was the B+ winding being asked to deliver in the original design, and what will you be asking it to deliver in the new design? You see what I'm saying. It requires careful thought. The best bet is to use a power transformer whose original application was similar to the one you're trying to replicate. For example, if I was building an integrated stereo hi fi amp with 4 7591 tubes, I could pull a good PT from a broken integrated amp that used 4 7591s (or 4 6L6s, or...) and be reasonably sure that it would be able to deal.

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Post by ckeene » Wed Dec 05, 2007 4:21 pm

ok, good points, Brad.

I just took a look at the unit and it was in fact running a pair of 6L6s plus two pairs of 6V6s, plus a few other smallertubes, so it would seem reasonable the existing PT can handle current demands. But I get what you're saying.

Basically, I'm trying to be thrifty here and figure out how to drop voltages appropriately and learn a little about P/S design.

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Post by ??????? » Wed Dec 05, 2007 4:28 pm

not tryin to rain on your parade. :D

Just trying to save you the sorrow of melty smells and sparks flying.

I bet that with a few modifications, that amp could be converted to be a FINE guitar amp. From the way you describe it, it would probably require a lot of work to make a stereo power amp.

Alternatively, with that tube complement I bet that transformer would handle a quad of 7591s just fine. I tend to like 7591 power tubes for hi fi.

If you want a stereo amp for home hi fi, look into getting a used Eico ST-70 or ST-40. Those are sleeper amps that go for not much money (a bill or two) and with a few small modifications compare favorably with MUCH more expensive amps. There are a couple well-documented design flaws that are easily corrected, but the fundamental design of the amp is very good. The mods are all over the internet.

That way you get to get your soldering iron hot, have the DIY satisfaction, and probably end up spending less than you would spend on this project. Just a suggestion.

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Post by A-Barr » Wed Dec 05, 2007 4:31 pm

890 volts will yield about 560 volts rectified, which may be ok for a 6L6 but is really high for a 6V6. There are some tricks to compensate though - what kind of rectifier was the amp running? An old coke bottle 5U4 will drop your plate voltage by 60 volts or so (but of course introduce sag) and cathode biasing the tubes will lower the effective plate voltage by about 25 volts (with more sag). That gets you to 475, which is just about the very most I would ever consider applying to a 6V6, not much room for error!

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Post by ckeene » Thu Dec 06, 2007 6:41 am

Even if I use a SS rectifier and a fixed bias supply, can't I just bring voltage down to whatever I need with a resistor? Isn't that how most tube amp power supplies work: to first supply the highest voltages to the power tube plates and grid, then progressively step it down with resistors for the lower voltage tubes?

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Post by A-Barr » Thu Dec 06, 2007 6:59 am

ckeene wrote:Even if I use a SS rectifier and a fixed bias supply, can't I just bring voltage down to whatever I need with a resistor? Isn't that how most tube amp power supplies work: to first supply the highest voltages to the power tube plates and grid, then progressively step it down with resistors for the lower voltage tubes?
Wellllll...

Sort of.

Generally you have your power tube plate voltage coming right off the rectifier and into the power tubes, with resistors controlling the voltage to the other tubes as you described. You just don't want to hit your power tubes with more voltage than they can handle. 6V6's usually don't want anything over 400 volts, if you are using JJ's or a good set of NOS tubes, you can hit them with up to 450 somewhat safely.

You can use a resistor to drop the plate voltage but it will induce sag just like a tube rectifier (well, almost) and you will need a really hefty resistor. Say you are running two pairs of 6V6's idling at 25ma each and you want to drop 100 volts. That resistor is dissipating 10 watts, and to be safe, you probably want to use a 20 watt resistor, which will get pretty hot, although there's no real reason it can't be done, I guess. Another option is to use reverse-biased zener diodes in place of a resistor. You can string together 5 5 watt, 20 volt zener diodes and get the same effect without the sag, I believe.

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Post by ckeene » Thu Dec 06, 2007 8:28 am

Since the target is a hi-fi amp, i think sag would be largely undesirable. In theory, I could mayb use Zeners for each voltage stage of the power supply and end up with something pretty solid?

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Post by ckeene » Thu Dec 06, 2007 8:30 am

I guess this topic is getting a little theoretical at this point, but anyone's help regarding power-supply design is really helpful. Thanks!

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Post by A-Barr » Thu Dec 06, 2007 8:47 am

ckeene wrote:Since the target is a hi-fi amp, i think sag would be largely undesirable. In theory, I could mayb use Zeners for each voltage stage of the power supply and end up with something pretty solid?
Not necessary in my opinion....

The plates of the power tubes draw a lot of current, much more so when "asked" to produce a loud, low note. Voltage = Current x Resistance, so if you are dropping 50 volts across a resistor at 50 milliamps (idle), you will then drop 100 volts across that same resistor at 100 ma (full blast) this voltage drop to the plates is what creates sag. Tube rectifiers have internal resistance that works the same way, although maybe with a slightly less linear curve.

But, super-audiophile theory aside, the plates of the power tubes are the only parts of the amp that really draw enough current to initate this kind of sag. Preamp tubes usually run at a couple milliamps, so it's not relly anything to worry about at any other stage, imo.

Even if you use a solid state rectifier and no voltage-dropping devices, the power transformer itself has some internal resistance and will also generate a bit of sag. Ideally you would avoid it as much as possible in an ultra-hifi amp, but when you're building something to just sound good for recreational listening, a little sag and compression is not such a terrible thing, at least for my tastes.

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