bypassing the plate-load

[memphissound]

I was looking at a couple of older guitar amp schematics recently and noticed that in a couple of instances there was a bypass cap on the plate load resistor. Found it in the pre-amp section on some 12AX7's.

Why is this done? How is it's value determined?

Thanks.

peace,
memphissound <><

[Joel]

It's rolling off the gain of the amplifier at high frequencies, probably to control instability due to a feedback loop.

Joel

[memphissound]

I suppose that's in parallel to the load resistor at a value of

1
------------------
2*pi*freq*C

peace,
memphissound <><

[EC8010]

No, you have to find it by experiment (testing with a 10kHz square wave for minimum ringing but maximum sharpness). It's determined by the output transformer and the various HF time constants in the driver circuitry.