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I have a Gibson GA 8 with a squeal/oscillation dependent on volume and tone settings. Squeals at just about any setting but can be manipulated by the volume and tone controls. I've reversed the output tranny primary wires with no change. Power supply voltages are within 10 volts of schematic. When played at low volume, the squeal is not always there but the output is very distorted. Sounds like the signal is being "squashed" if that makes any sense to anyone. I've installed a full set of new tubes with no affect, subbed another speaker to be sure the speaker was ok. The amp was missing one input jack and the remaining jack was wired incorrectly. I installed a new jack and wired the input to match my GA9. I don't have a lot of experience tracking down oscillations so any help at all is appreciated.
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Problem solved. I had chosen not to replace the filter caps until I determined what else might be wrong with the amp. I started subbing caps and cured the squealing by subbing the last 10uF/500 volt cap. No more squeal and new caps on the way! -
That cap is a decoupling cap.
If you could see that part of the power rail on a scope, you would see the signal actually riding on it.
A good cap will 'decouple' the signal from the power rail.
Glad that you got it.Comment
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