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squeal in amplifier

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I have a little Philmore stereo amp that uses one 6bm8 per channel. I have been using it for some time now and recently it has developed a squeal in one of the channels. I have replaced all caps and tubes even the preamp tube. I can vary the pitch of the squeal by turning up or down the vol. I have used my oscilliscope to trace the squeal all the way back to the tone controls and volumne controls and there it stops. I have noticed that turning the vol up and down that there are spots that it disappears. Also at very loud vol's it sounds like arcing. Will a dirty pot do this? Also I have noticed that I get a very weak signal off of the chasis. Where should I look next?

One last note, I have also noticed that during lower vol levels if I place my fingers around the tube without shaking it the squeal goes away.
 
Could be a microphonic tube, especailly if its only in one channel and moves when you swap the tube.

Otherwise, I would take a look at the grounding- sometimes a stray signal from the output gets coupled into the input and you get feedback. You might have some bad ground connections, especailly the cases of the pots or input circuitry. Try implementing a star ground.
 
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Hi n0plb,
Are you sure you have replaced all the caps? I am thinking a supply cap is going open since it used to work well. If you replaced the caps to improve the amp, the position of those parts (or size) may be your problem.

-Chris
 
n0plb said:
Will a dirty pot do this?

Not that I'm an expert (I'm pretty helpless, actually), but I have had a squeal problem in a tube amp that was related to a badly-grounded pot. It may not be your answer, but make sure that the pot is appropriately grounded.

I figured this out when I touched the stem of the pot (heh, heh-- pot stem) with the plastic knob off (the bare stem) and the squealing stopped.

Just my $.02 which, given the devaluation of the US dollar, is worth considerably less than ever.

Kofi
 
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