#1
So i got my first tube amp about 3 months ago. Last night when I was playing through it started making this high pitched noise when the volume got between 2 and 5 o'clock on it. Also, I noticed that the amp itself does not get as hot as it used to when I am playing. The high pitched squeal can be heard over the guitar when playing. Took it to a shop they said it was normal, but how would they know when I've been playing the amp for 3 months and just now noticed this. What could it be?
#2
The cable line by any chance? Remove the line and if it still squeals the high pitched nose, then it isn't it (damaged cable lines produce either no sound or a lot of noise). I doubt its the tubes unless you bought it used? Reverb tank? Maybe it broke or something?

I'm no expert but these are the first things to look for.
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#3
Probably just feedback, stand further away from the amp, turn the gain down or turn the volume down.

Could be a microphonic tube, try tapping on the tube gently with an eraser of a pencil, you should hear some noise no matter what, but if the tapping is clearly audible then you have a bad tube.

It could also be your pickups going microphonic.
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#4
well the thing is it doesn't happen at most volume levels just about 1/2 -3/4 of the way. turn it to full and no more high pitched sound.

Also this happens when nothing is plugged into the amp no cables or guitar
#5
I'm betting on a bad tube then. You could try plugging the amp into a different socket and see if that helps.
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#6
you think bad preamp or bad tube. I have an epiphone valve standard. it has (3) 12AX7 Preamp ; Power (2) EL84. Noth EL84 are glowing the same intensity. Is there anyway to determine which it is thats causing the problem
#7
It's probably a preamp tube if anything, but you can try tapping them all. Turn it up to a volume that doesn't cause the squealing, the louder the better, then try the tapping. Be very gentle, if it's a microphonic a light tap should come through pretty clearly.
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#8
Check yer toobs. Seriously, your pre-amp tubes are the little ones, you can generally see if they are burnt or blown. Same goes for the Drive tubes. What kind of amp is it? Was it new or used?