Calling All Amp Doctors...

[veatch]

I modded an amp, and all went well. I was doing a final check on the bias to get it ready to put back in its cabinet. When i turned off the standby (to be clear - went from standby to play mode...) i heard a noise that sounded like a dirty pot. Then, the amp sounded anemic - very thin. The master volume had no effect - wouldn't adjust the volume at all.

I thought it may have been the PI tube giving up the ghost, so i popped a new one in, and all worked well. Fine. Then i put the original (suspect bad) one back in, and it still worked. :/

Should i replace the tube anyways, or should i be looking elsewhere? I have a gig on Saturday, and hate going in with a questionable amp. (i bring two anyways, but would prefer to avoid the drama...)

The sound the amp gave did sound like a failing tube - at least like one i've heard fail in the past. But the master vol thing has me puzzled. The master feeds into the PI. I've triple checked the pot and connections. All looks good, but then again, it does work now...

Thoughts?

Tnx.

[Hfan]

I fixed other equipment for years and have played around with some amp circuits (know enough to be truly dangerous), I'm no expert for sure but several things come to mind.

Maybe the tube in question has an intermittent fault. Have you got another amp to put that tube in? Maybe the problem will follow the tube.

Is the tube mounted on a circuit board? I've had to reflow the solder joints on the tube pin contact points on my Blues Jr which has a circuit board mount (never again will I buy a pcb amp.although it is pretty cool and I have modded the hell out of it, sounds ok). Maybe just reseating the tube had some effect.

Or, you mentioned doing mods. Maybe reflow any soldering you did to accomplish the mods..or just review everything you did. It was the last thing you did to an amp with no issues right?

I was working on a Deluxe Reverb build once and had an odd issue crop up, turned out one of the connections that run under the turret board had come disconnected..maybe something out of the line of sight?

I've gotten good assistance on the Music Electronics Forum..lots of knowledgeable amp guys there happy to answer questions. You'll probably get better input there then mine. Good luck.

http://music-electronics-forum.com/f10/

Edit: or maybe the tube pins and or socket have oxidized..have you got De-Oxit or similar? Ideally you would get the problem to occur on a constant basis..much easier to track down. You mentioned the bias..maybe it's a power tube issue..sorry I'm reaching.

[Armitage]

9 out of 10 times it's the send return jacks.... with the power off, spray a little contact cleaner on them and plug and unplug a cable a few times.

[veatch]

Tnx, guys - actually, that was the mod - to remove the effects loop and add a solo boost.

The problem ended up being the master vol pot being intermittent. I sprayed it out, and the more i worked it, the worse it got. Soooo i now have a non-master volume Rebel 20. It will get me through the gig. I'm having a buggar of a time finding a direct replacement - these are 12mm Alphas, so the footprint is uber-small and there's not much room to use a "normal" size pot with solder lugs.

I love these amps, but man, they're a bitch to work on, and replacement parts are a few odd-balls.

[gorch]

You could open the pot and clean it. Often that helps.

The taper might have collected some dirt rather than been worn of.

[veatch]

Yeah - i tried - it's gone.

Found a place that carries the little buggars - Studio Sound Electronics. Small Bear has the slug termination ones, too.

I think i'm set. I'm glad i kept poking around - with my luck, the amp would have worked fine until 5 minutes before the gig.

Thanks again.