Cap across the PI plates

[JJman]

I was messing with my ’71 SFDR today and figured I’d try to tame the highs a tad. (Bright cap already gone.) I wanted to reduce only the highest highs rather than all the highs the way the treble control would. I tried a cap across the PI plates since I’ve often seen this on schematics. I tried 47pf, 250pf, and 500pf. None noticeably reduced any of the highs at all during repeated tests.

Does one need to go up more to get results? Are those particular caps on schematics always for merely preventing ultra-sonic nasties? I know I could drop a cap to ground somewhere else, or bypass a plate resistor but I’m surprised this approach didn’t work. Anyone ever experiment in this area?

 

[TweeDLX]

Run the cap from Plate to cathode on V1 or V2. Depends on which channel you use.

Mike

 

[jay42]

Are those particular caps on schematics always for merely preventing ultra-sonic nasties?

Pretty much.

 

[donnyjaguar]

Because the plate-to-plate cap is inside the NFB loop changing it won't have the same effect on the tone as if it were not. It does however reduce gain of ultrasonics which generally stabilizes the amplifier.