JVM Problem.

[Si.]

Just back from rehearsal and I'm having problems with my JVM. I'm getting a lot of feedback through the amp when on the overdrive channels. Seems ok on the clean, also stops when I turn the guitar volume down to 0 or mute with a volume pedal. I've tried it without my board and plugged direct into the amp and getting the same problem.

I'm assuming it's a valve but any ideas which one? I noticed last night when gigging that I was getting a feedback/whistle through the amp when using the neck pickup on my LP custom, so assumed it was the guitar as the bridge pickup is fine, but I'm not sure now as tonight I was using a different guitar when the above happened.

Any ideas?

[rockmonster]

Try swapping V1 pre amp valve with one you know works (the one under the can)

[Si.]

Just got round to changing the valve this morning and still getting the same problem..

[ICBM]

If it stops when you turn the guitar volume down, the problem is the guitar.

It's either microphonic pickups, or just possibly (if you're very close to the amp) direct electromagnetic coupling between the amp's output transformer and the pickups.

It may do it with more than one guitar simply because most pickups are at least a tiny bit microphonic and the JVM is very high gain, but it also might indicate it's electromagnetic, which would probably do it with any guitar.

[Si.]

OK thanks, guess I need to investigate more. At home it was just going it with 1 guitar (My Les Paul) but at Rehearsal it was also doing it with my EMG loaded Jim Root Tele which is what made me point the finger at the amp.

[ICBM]

Try turning it up to rehearsal volume at home - you don't have to play anything, if that would be a problem regarding noise, but you do need the guitar volume up (just keep the strings muted) - and see if you can get it to squeal. If so, then move the guitar away from the amp and/or change its orientation and see if the squeal changes or stops. If so it's likely to be electromagnetic coupling.

I'm inclined to think it is, since if I remember correctly EMGs are completely resin-potted and so shouldn't be microphonic.