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I have a Peavey VTM, the other day at practice I plugged it into my cab plugged in my guitar and the volume was considerably quieter than before (maybe 15-20% as loud). I looked at the tubes and they were glowing blueish so I figured okay it's time to change tubes.

Fast forward two weeks, have new tubes, 6l6's the amp is self biasing therefor all I need to do is switch with 6L6s and it's good to go, I have done this before and it worked fine as.

So I plugged them in turned it on and it still has the same problem, it's very quiet, and the tubes are glowing a blueish/orange.

Any advice on what the problem could be?

Probably Gremlins.

have you changed the preamp tubes?

I haven't, I assumed it was the power tubes because I thought they affected volume and preamp tubes affected tone?

no, no, no...preamp tubes affect the entire signal. it's called a "preamplifier" because it amplifies the original signal so it's high enough to work with the power amp stage...think of it as a little amp with a jumper to a big amp. a bad preamp tube will fuck up everything.

there are other things that could be problematic, but if you've already changed power tubes once in the past and never changed the preamp tubes they're probably due, and i'd try that first.

Ah, that could be it then, I'll try it out and give feedback of how it went, thanks!

when changing the preamp tubes, do them one at a time. swap one - test. swap another - test. it most likely isn't all of them...
also, blue glow on the power tubes is ok and light orange is ok - red or excessively bright orange is bad.

Did you change your impedance setting on the amp (or cab if it does that) or change your cab load? Can affect volume and make life harder on the output tubes.

I've only noticed drastic volume drops with bad preamp tubes.

As for power tubes, I've only had 'em go fast and furious. Volume was present in spades (along with other unpleasant noises) until the amp blew a fuse and no more power section.

Thanks everyone, is there a way to tell if a pre amp tube is bad by looking at it?

Not usually. You can gently tap them with a pencil eraser and listen for any microphonics. Best to test a new tube, one at a time, in each preamp position until you narrow it down.

i like to use a guitar pick to tap, because i''m usually in back of the amp and i can better hear the higher-pitched click the pick makes on a microphonic tube. n.b. just because a preamp tube isn't microphonic doesn't mean it's still good.

dubkitty wrote:n.b. just because a preamp tube isn't microphonic doesn't mean it's still good.

Very true.

In this case though, with the volume loss, the "tap test" won't even be necessary. Once you start swapping, it'll be very apparent which one's to blame.

Hey everyone, switched out both power and pre-amp tubes, still the volume is at a minimal turned up to 8 and it's barely making a sound. Any idea what this could be?

It's probably a burnt out resistor or cap somewhere on the circuit board then. Check the big caps for any sort of burn marks, bubbles or burst seams.