small push-pull amp to preamp

[adason]

Hi all tube amp experts!

I have a couple of restored old push-pull amplifiers, most of them have very small output transformers, so their application is limited to driving midrange or tweeter. I think I have seen somewhere, how to use it as preamp. All it took was two capacitors, if I am not mistaken. Picture attached.
Please correct me if I am wrong, before I do something stupid.
If the idea is correct, what would be the value of the capacitors?
Thanks,
Ed

[bigwill]

I think you would get much better results with re-wiring the circuit as something more preamp-like

[DougL]

As Drawn, The 2 capacitors are an AC short across the plates and would not work..

I have seen 2 capacitors used to drive electrostatic headphones.

The major problem with what you propose is that the voltage gain is excessive. Assuming 2 V rms ( 6 v p-p) will drive the power amp to full output, 6 volts in will result in over 400 V peak at the plates of an EL84. Av of about 66. If you have an application that this is good for, by all means, go for it.

Other than the ESL thing or a Cathode Follower 6550 driver, the application escapes me.

HTH

Doug

[adason]

Thanks guys for input.
I am sure I have seen such schematics somewhere. Are you sure it would not work? I know gain is excesive, but there can be pot on the output.
I was thinking of using it to drive solid state classA amp with unity gain which requires high input.
But if you think caps short AC plates and it would not work, I will not try it.
Thanks,
ed

[DougL]

Are you sure it would not work?

As drawn, the circuit will have a very small, very distorted signal. The 2 EL84 plates have signals that are equal and opposite. The 2 capacitors Are an AC voltage divider and form a virtual ground.
If the amp was perfect, there would be no signal of any kins at your "output"

I was thinking of using it to drive solid state class A amp with unity gain which requires high input.

if its a single ended design, delete either of your added capacitors and you would have a signal. If it's differential, use both caps, just do not tie them together.
Its a compromise, but it will work.

A para-fed "inter-stage" 4:1 or 8:1 step-down might bring the gain into a better place, lower the output Z, lower the distortion, etc.
Using an Edcor you could do it for $12 per channel.

HTH

Doug

[Poindexter]

I think you guys are getting too elaborate.  Just take the preamp out off the existing output tranny; use either the 4Ω or 8Ω tap, as suits the voltage requirements of your following component.  If it sounds too frisky'n'crispy due to the light load, parallel it with a nice Mills 10Ω noninductive wirewound resistor to calm it down.

I do this in every power amp I make; I put a pair of (volume controlled, thass why) line out RCA jacks in the back of the chassis, and just hook them up to the 4Ω taps.  Don't need no paralleling resistor, since there's already a pair of speakers on the 8Ω taps.

Aloha,

Poinz

[DougL]

Good Point.

Doug

[adason]

The only problem with taking signal on secondary of the trafo is that it has bass severely rolled off.
That's why I wanted to take it from primary.
I would normaly use some resistors instead on speakers to keep the secondary closed.
I do not want to replace output trafo for better one. I have done that successfuly with few amps and they are being used to drive speakers. I do not need more amps.

My understanding was that those two caps would sum the oposite phases of original signal. I guess I was wrong.

As far I can see, the easiest way to use it as preamp is just to take signal after first tube.
Thanks for the discussion.
Ed

[adason]

It's me again. I realize that I do not want to take signal from first preamp tube, that would be too easy. Plus that is not the reason for this thread.

As you said DougL, having just one capacitor should provide the signal. I think I would like to try that. So back to the original question. What would be the capacity? How do I calculate (approximately) to have roll off few Hz to tens of Hz? Or should I just simply try some capacity and measure fr response?

[Poindexter]

The usual suspect for shy bass in a small commercial power amp is lack of inductance in the output transformer, so you're right there.  A way to ameliorate this would be to hook your driven component up to, say, the 8Ω tap, and then put a resistor across that output of about 22 - 33Ω, stabilizing and lightnening the load so as to give a lower bandpass.

I bet it would work.  You could also try a resistor across the primary, of about three times the nominal primary load, and maybe that would work even better.

Easy enough to try.

Aloha,

Poinz