Phase Inverter Plate-To-Plate Snubber Capacitors

[idsnowdog]

I saw a modification on Rob Robinette's site where a small value capacitor is placed between the plates of the phase inverter.  Is this modification specific to a certain style of phase inverter or can it be used on all to reduce high-frequency oscillation?

https://robrobinette.com/Voicing_an_Amp.htm#Phase_Inverter_Plate-to-Plate_Snubber

[sluckey]

I've only seen that cap on LTP PI. Several Fender amps and many Marshall amps use that snubber cap. I'm sure you will find it used in other manufacturers' amps as well.

[idsnowdog]

I have only seen it with LTP too.  Generally is there any reason why it wouldn't work with other types?

[sluckey]

Try and see.

[SnickSound]

I have only seen it with LTP too.  Generally is there any reason why it wouldn't work with other types?

Well, a cathodyne doesn't have 2 plates. A paraphase technically does, and yes the cap would work in theory.

Any capacitor that shunts high frequencies (to ground or between reverse polarity lines). Putting one plate to plate is just simple and easy.

On a cathodyne you could have one plate to cathode, or just have it between the two signal lines to the power amps. The exact value required may vary.

The Cut control on a Vox is the same thing, just works at a lower frequency and is variable.

[brewdude]

I have also tried using the same cap across one of the PI plate resistors instead of between the plates.

[pdf64]

I have also tried using the same cap across one of the PI plate resistors instead of between the plates.

I think the cap value would need to be doubled for the same corner frequency.