Combo valve amp question

Si Pie · Jan 25, 2009

Symptoms:

Volume output very low.
Turning up gain and volume does increase output but not by much.
Turning up controls causes the amp to hum very loudly.

Does this sound like one of the power valves have gone? I guess I could change out all the valves but I don't really wany to change all of them out for the sake of it.

The guitar is working fine, I've tested through my Zoom multi-effects pedal and headphones.

Thanks in advance, I'm a complete 'noob' to this sort of thing.

Mick51 · Jan 25, 2009

No mention of amp, style, etc. Not a whole lot to go on here.

My first suspicion would be a bad preamp tube. I would try swapping them out one at a time, with a new - known good - tube, from the first in the chain to the last. Typically, the first preamp tube has the greatest impact on the quality of the sound, and so on down the line. (if it's not a tube amp, my regrets..)

That's where I would start. Best of luck.

Si Pie · Jan 25, 2009

Thanks for the reply. It's a laney LC15R, I did put that when I originally typed the message but after mass editing I must have accidentally omitted that detail.

I'll give your suggestion a whirl.

Si Pie · Jan 25, 2009

Ok I swapped around a couple of the preamp valves with each other and the hum disappeared, I also have more output but not as much as when the amp was running well - obviously the bad valve is still in there and so this is the cause.

The main thing is I have diagnosed the problem and can sort it out, so thank you for your help.

Mick51 · Jan 25, 2009

Good job!
Along the signal chain in the preamp section, the biggest contribution to sound color - and signal amplification - is provided by the first tube, then the second, etc. So at the least, you want your better/best tunes at the beginning of the chain.

You've identified the fact that you have a "bad" tube; and, in fact, you know which one it is because of the swap you made. You're getting more power and less noise because the "bad" tube is now farther down along the chain.

I keep a drawer full of preamp and power tubes around, just in case I run into a problem. The preamp tubes - mine are 12AX7 (ECC83) - are cheap enough that you can probably afford to have a couple spares for just such a purpose.

I'll leave the more serious stuff to the good amp techs. Enjoy that amp!

Combo valve amp question

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Combo valve amp question

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