Ugly distortion only at higher gain levels - reverb transformer?

[mbetter]

The amp in question is a Bruno Underground 30. Sounds fantastic clean to semi-dirty but quickly develops an UGLY distortion as you crank it past there. By the time it's dimed, all you get is an ugly, distorted buzz (not a constant buzz, only when there is a signal present).

Swapped all preamp tubes, swapped reverb send (I'm guessing - a lone EL84 apart from the power tubes) tube. I'm going to swap power tubes around tomorrow, I need to get some known good EL84s.

I decided to swap the reverb send because the reverb Mix knob directly affects the buzz. When the Mix knob is at 0 (dry), the buzz is at it's strongest, when the Mix knob is at 10 (wet), there's barely any buzz at all. When wet, however, there seems to be less gain so I'm not sure that this behavior isn't just a side effect of the circuit design (buzz directly correlated with gain - less gain, less buzz and the reverb has nothing to do with it).

So a quick search turns up this thread: http://www.thegearpage.net/board/sho...30#post3097130

Those symptoms seem to be very similar to the ones I'm experiencing. Is there a way that I can safely test this diagnosis? I've read that you shouldn't unplug the reverb tank. Can I pull the EL84 reverb send (?) tube and run the amp without it safely? Would this even be a useful diagnostic method?

I might add that the amp is run through a THD Hotplate. I would never have discovered this problem without it as I can't get the amp up to those levels in my apartment without it. Attenuator and speakers are known good, my Clubman copy runs flawlessly through the same rig.

Any help would be greatly appreciated. I've got no problem taking this to a tech but I would like to do as much as possible myself.

[Blue Strat]

I didn't read that thread but, yes, you can pull the reverb driver tube and see if the situation improves.

[mbetter]

It's not the power tubes or the rectifier. I pulled the reverb driver, same symptoms just no reverb.

This is going to Tim Schroeder next week. I'm still interested in a possible diagnosis, though.

[Blue Strat]

No one can diagnose an amp better than the guy the amp is sitting in front of. All the rest of us can do is guess.

[mbetter]

Can anyone guess, then? Maybe give me things to look for?

[Blue Strat]

Originally Posted by mbetter

Can anyone guess, then? Maybe give me things to look for?

The list is too long and it's pointless. You have to take readings, use a scope and signal generator, etc.