Amp making weird noise!

[TomW]

My jcm800 has just started making a really weird noise. I've attached a recording of the sound its making, it seems to be humming really loud and has a sort of scratchy noise all the time even with the volume all the way down. I've tried different guitar leads, guitars and with no guitar at all and the noise just won't go away. I've also tried moving the amp away from electrical objects but still no luck.

Any ideas as to what it could be?

Cheers

[hamfist]

It's very likely to be a tube. You'll need a known good 12ax7 to swap in for each of the pre-amp tubes in term, to find out if it's one of them that's faulty. Otherwise, you'll need to try another set of EL34's - if it's just for a minute or so, you won't need to bias them, so you can just borrow a "mate's" set if you want to try them out - just keep an eye out for any red-plating if you're quickly trying any out without re-biasing.

[TomW]

Cheers hamfist, I've got a couple of spare EL34's so may give that a go if I'm feeling brave, but i'll have to send it to an amp tech anyway to get it rebiased properly, so might just get them to check it out to be safe.

[QS-Nalle]

Cheers hamfist, I've got a couple of spare EL34's so may give that a go if I'm feeling brave, but i'll have to send it to an amp tech anyway to get it rebiased properly, so might just get them to check it out to be safe.

What it could also be is a bad solder joint or a cap going bad, but start with the tube. 99% is tube related. :o

[Gary]

Would this be true even though it's still happening with the volume all the way down?

[hamfist]

Would this be true even though it's still happening with the volume all the way down?

Good point Gary.  As it's a JCM800 (pre-PI MV), if it's a tube problem that narrows it down to the PI (V3, the 12AX7 nearest the power tubes), or the EL34's themselves.

Just a hunch, but it sounds similar to dodgy power amp tubes I've had in the past.