Need tips on using interstage transformer - phase inverter

[Roe]

In my next amp I want to try an interstage transformer for the phase inverter. Any suggestions on transformer and implementation?

It will be a 100w guitar amp (quad of 6l6 or el34, 1k75 primary, 395-473V plate voltage) and I prefer to start with 12a*7 tubes. I am considering using one triode as an ordinary gainstage and the second triode as a cathode follower driving the interstage transformer.

I am considering the musicmaster transformer made by classictone. The nominal primary DC resistance 1785 Ohms and the secondary is 3140 Ohms. The turns ratio is 1:1.35.

[martin manning]

Are there some existing circuits you are looking at for the power stage? Fender did one (PS160) as I recall, others?

[Roe]

Not really, there is the PS160, the musicmaster and the weber ab200 bass amp (as well as old gibson and magnatone amps if I remember correctly).

[bb5000]

Fender Musicmaster Bass Amp (Silverface), Fender PS300, Fender PS400 + lot of old radios from 30-50's.. Plus the ones mentioned in the previous post. I've built the Weber AB200 myself and used it for 8 years now.

[Roe]

Yes, I am looking for a phase inverter that can drive a quad of el34s or 6l6s into full breakup. I want an inverter that is better balanced than the typical long tail pair phase inverter (which tends to be unbalanced if you crank the amp). Still, this is only a 100w guitar amp, so even a 12bh7 driver is perhaps too much. a 12au7 might suffice

[Rockwell666]

an old version of the Garnet BTO uses an interstage P.I Transformer and its a beast of an amp (4xel34)

[pompeiisneaks]

If you're worried about PI balance, why not install a PI balance pot? Seems more straightforward than the interstage transformer, and I think it's generally less expensive.

~Phil

[Roe]

If you're worried about PI balance, why not install a PI balance pot? Seems more straightforward than the interstage transformer, and I think it's generally less expensive.

~Phil

this works quite well actually. But an interstage transformer is supposedly even better under hard drive

[pompeiisneaks]

If you're worried about PI balance, why not install a PI balance pot? Seems more straightforward than the interstage transformer, and I think it's generally less expensive.

~Phil

this works quite well actually. But an interstage transformer is supposedly even better under hard drive

I don't know that I understand how hard drive would impact a well balanced PI tube? the balancing is to just get their base conductivity and performance balanced so they perform very nearly identically, or are you indicating you have evidence or information that shows that the two triodes in the tube would respond balanced at some measurement points but not at other frequencies or current ratings?

~Phil

[martin manning]

The balance will shift with the drive level.

[pompeiisneaks]

ahh interesting, but with a transformer it doesn't?

Sounds like it's a HiFi approach then, but for guitar amps, isn't that counter to the desired overtones added like 2nd harmonic etc?

Just curious.

~Phil

[Roe]

The balance will shift with the drive level.

yes, at least in part because of negative feedback

[dorrisant]

Gibson Minuteman has an interstage TX... I have one on the shelf. This is very intriguing.

[Stevem]

The Fender ps400 is a class AB2 output stage is that also what you are shooting for?
About 12 years ago there was a build posted somewhere that used a small choke in place of the cathode resistor in a PI circuit to provide more drive current somewhat like a class AB2 stage.

I think I have it and I will dig it out tonight and post it up tomorrow for those who might have interest.

[martin manning]

About 12 years ago there was a build posted somewhere that used a small choke in place of the cathode resistor in a PI circuit to provide more drive current somewhat like a class AB2 stage.

That would be more about providing a constant current source for the PI to improve its balance. That can be done with a MOSFET too, I believe.