Purpose of the "fizzy" (PI) cap?..

[BluePat]

and by altering the value, what does that do?

Pat

 

[RussB]

It's there to supress high frequency oscillations

The higher the value, the more high frequecies will be diminished. In Marshall circuits, I have gone as high as 180pf and couldn't hear a difference from the stock 47pf cap. I could hear a 250pf cap

 

[BluePat]

Thanks Russ :BEER

 

[cap'n'crunch]

Whatz you guys talking about? My 2204 is fizzy at low volumes. Is this a remedy for that?

 

[donnyjaguar]

The capacitor reduces the gain of the amplifier at frequencies above its normal operation due to phase reversal through the output transformer. To my knowledge its not a "tone shaping" component and RussB's experiments bear this out.