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.

[Sorbz62]

Hi mate,

I had a JVM and sold it because of exactly these problems - mega feedback on the OD channels. I changed valves and took it to a tech etc - no deal, still feedbacking. What did help a little was working with levels in the loop. When you have FX in the loop, the channel vol works as an FX send volume, which can push a little too much volume out of the loop. But I was fighting it!

Cheers,
Jim

[Si.]

Just tried that. I'm getting a little feedback on my EMG guitars but I put that down to the volume in the small room i'm in, if I mute all the strings carefully I can get it to go away..

On the other hand, the Neck PU of my Les Paul is terrible... so I assume that's the problem (which is what I thought originally).. I'll have to get it looked it. Not sure why I had feedback problems at the rehearsal a few weeks ago but I'll discount that for no until I can re-test it properly with the band.

Thanks for the help.

[Si.]

Hi mate,

I had a JVM and sold it because of exactly these problems - mega feedback on the OD channels. I changed valves and took it to a tech etc - no deal, still feedbacking. What did help a little was working with levels in the loop. When you have FX in the loop, the channel vol works as an FX send volume, which can push a little too much volume out of the loop. But I was fighting it!

Cheers,
Jim

I've been running my JVM fine for a few months, gigged it several times as well, the problem Im getting isn't a general JVM issue, it's a specific problem that appeared a few weeks ago.

I run the gain quite low on all my channels (usually 2-3) with a high channel volume. Running it that that I generally don't get any feedback issues. I also run a Decimator out front and if needed my G-System has a gate as well (although I rarely use it)

[Roaster]

Hello mate,

I had a JVM combo which did exactly the same on the high gain channels. I found that when I was running cables through the loop, and wart powered effects it mad things worse. Bearing in mind I was using quality planet waves cables, when I got the "feedback" ( I say feedback, it's more a microphonic whistle/ screech) I moved the loop cables to the left side of the amp and by moving the cables I could alter the tone of the whistle almost as if the cables were microphonic. Try having no wall warts etc see if ny noise in th esystem I being picked up and re amplified. There's several threads on this on jvmforum.com. Hope this helps