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Old 11th December 2006, 08:20 PM   #1
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Default bypassing the plate-load

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.

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Old 11th December 2006, 10:32 PM   #2
Joel is offline Joel  United States
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It's rolling off the gain of the amplifier at high frequencies, probably to control instability due to a feedback loop.

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Old 12th December 2006, 12:57 PM   #3
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I suppose that's in parallel to the load resistor at a value of

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

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Old 12th December 2006, 01:46 PM   #4
EC8010 is offline EC8010  United Kingdom
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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.
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