I have a 1987 JCM 800 100 watt amp head and when I turn the mater volume up to 6 or more it screeches and I cant figure out what makes it do that. this is with the pre amp gain tuned all the way. This amp is very loud and I hardly ever turn it up full volume but im wondering why it screeches like that and how can i fix it?
Are you near it with a guitar with single coils? That could make it screech a lot. Also, when was the last time the tubes were replaced? Could be a bad tube.
are you playing to close to the amp??????
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Is it feedback?

Stand away from the amp does it still screech?

What kind of guitar/pick ups do you have?


You could turn down the amp. (not likely)
Get a noise gate
stand away from your speakers
check your cables are they good quality?
No,its not feed back, I know what feed back is and this is def not feedback. Its when the amp starts to get at a loud volume it begins to screech and if you turn the guitar volume down to 0 itll still slightly do it,its def an electrical issue but i wouldnt know what it is
haha i love how the first three posts all said the exact same thing...

are you running any pedals? if you got your gain cranked and crank the amp this can cause this in some situtation especially depending on the wiring around ur surroundings, although that part usually causes buzz....

but if your using a booster/distortion/ OD pedal in front too, it might just be to much for the amp
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No,this is plugged directly to the amp and no why should I turn my gain down? shouldnt this amp be able to take the gain at 10 as well as the volume at 10 without screeeching?
few amps can take everything to 10 without screeeching or at least making some sort of noise. so turn down the gain some, there shouldnt be any reason to have both your pre and power amps cranked.