Valve Amp Troubles

It wasn't me!

Playing through it the other night, it just kinda fizzled out, still makes a tiny, fizzy noise when strings are struck, but that's all. I drive it full out, so it gets some heavy abuse when it's played, if that makes a difference. Anyone any ideas?

david

Grounding? Are the valves glowing right?? Have a look in the back to see if any are dead.

Gaspode

Havent had a valve amp in donkeys years, but it sounds like a valve/tube is gone. Should be easy enough to replace with another if thas all it is.

It wasn't me!

Power tubes glowing very brightly, but I'm assuming that's because they're on full tilt. Didn't check preamp tubes, but if one goes shouldn't all sound stop?

Eoin Madsen

Valves don't "go" in any predictable way, unfortunately. Failing valves can have any number of effects. Given that you're experiencing a total loss of sound, it's unlikely to be the power valves, since they operate in push pull. As the first and easiest thing to do, I'd suggest you swap out the preamp valves temporarily and see if that helps.

But to be honest, there's a lot of things it could be, and I wouldn't put valves at the top of the list. Does the amp have an effects loop? If so, that can be pretty helpful for diagnosing the problem.

It wasn't me!

I think it does, but I don't really use it or know how to use it to test the problem, so will need help. It's not a complete loss of sound, it's a dramatic loss, but there's a fizzy, crappy-sounding thing left.

Eoin Madsen

It wasn't me! wrote:

It's not a complete loss of sound, it's a dramatic loss, but there's a fizzy, crappy-sounding thing left.

Sorry yeah, I know what you mean. In a push pull stage loss of one valve should still leave the amp with a respectable volume. Your problem is more likely to be occuring in series with the signal path, either before the phase inverter or in the output transformer. The easiest way to check if the OT is okay is to plug something into your effects return (like the effects send of another amp, or a line out from a mixer).

If you get a clean signal then your OT is okay and the problem is part of the preamp. In which case I'll suggest other things.

It wasn't me!

Not really sure what you're saying, I have neither of those anwyay.

Eoin Madsen

Neither of what?

It wasn't me!

another amp with an effects loop or a mixer...

Eoin Madsen

Oh right. Eh, change the valves. If that doesn't work, take it to a tech. There's nothing else you can do.

If it does somehow turn out to be the power valves, don't forget to rebias the amp after you change them.

Eoin Madsen

I dunno, maybe you could plug an active guitar or something into the effects return, you might get enough signal to tell if the fizziness problem is still there or not.

It wasn't me!

Preamp valves only?

Eoin Madsen

That's where I'd start. You only really need one working spare to find out if one of them is the problem by process of elimination. But it could easily be the output valves too - there's really no way that I could be sure, even if I'd heard the sound the amp is making.

Valves are the only thing you can really do without opening the amp, which I wouldn't advise unless you have a lot of time and patience on your hands... or you want to kill yourself.