Mesa boogie dual rectifier wierd noise

[metal666]

Hey,
I have an older, possibly 80s or 90s dual rectifier. i bought it used in 1999 or something so I am not sure. It has had all sorts of trouble, and I have had it fixed a few times for things like faulty pots and stuff like that. A long time ago, it would lose all balls on the main modern distortion channel, and just be like a wierd cleanish crap sound, got that fixed blah blah.

The new problem. All channels and modes work, vintage, clean modern. except, when I push the modern channel above like 15 % volume, and do a "chug" the amps makes this wierd high pitched squeak squeal. its like a vibratory feedback. sounds like a speaker is tearing itself apart. I have retubed the power tubes and checked all preamp tubes for microphonic-ness.

I have also tested this on separate cabs, and i have other heads that i pushed these cabs to the brink of what they can do, and no wierd shit goin on there.

If this post gets any attention, or if there are any amp gurus out there, I could record this noise. It's unlike anything I have heard before. And it's strange because the amp sounds awesome, and has had none of its old troubles.

Really want this amp to work and be reliable. Just today I bought a peavey 5150 ii just so that I could have a workhorse high gain amp that won't shit on itself when i want to push the volume past bedroom levels.

Thanks in advance to anyone that has any insight into this.

[soundmasterg]

A lot of times the problems in the Dual Rectifiers are caused by the channel switching optoisolators. I can't say that is the case with yours, but it could be. I have a 1993 Revision F Dual Rectifier but it works perfectly so I haven't delved into it. The schematic is really complicated on these amps and it takes quite a bit of studying to figure out how it works...and I haven't had the time myself since I work full time and go to school too. Maybe someone else knows what the fix might be or how to test for it....

Greg

[metal666]

The wierdest thing about this problem, is that the tone of the amp is great, and everything is working until you push the volume to loud. It's not the power tubes, because I switched them out and the problem remained. You would think it was power though, because when you push it it starts to shit. The sound it makes, sounds more like a speaker problem than an amp problem, but it makes the same sound on both my Mesa and Marshall cabs. Burns my ass. Probably have to take it somewhere... Balls.

[soundguruman]

The wierdest thing about this problem, is that the tone of the amp is great, and everything is working until you push the volume to loud. It's not the power tubes, because I switched them out and the problem remained. You would think it was power though, because when you push it it starts to shit. The sound it makes, sounds more like a speaker problem than an amp problem, but it makes the same sound on both my Mesa and Marshall cabs. Burns my ass. Probably have to take it somewhere... Balls.

A carbon trace through the fiberglass. It arcs HV when you push it hard.

[Enzo]

Parasitic oscillations.

[metal666]

A carbon trace through the fiberglass. It arcs HV when you push it hard.

Soundguruman, I'm a not as tech savvy as any of you dudes, I'm a player mostly, so I apologize for my newb-ness. Are you saying that there is a carbon trace in my fiberglass? Or I should do a carbon trace. It's ok to laugh at me, I can take it.

I am not even quite sure where my fiberglass is. So I guess, is this a terminal problem? What steps can I take to fix this. Or is there a way to test your theory?

And Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out? If you dudes can help me figure this out I will be the happiest dude around. Desperately want this amp to work again.

[exclamationmark]

You could try putting the head on the floor and plug it into the cab - this should tell us whether it is related to microphonics. The vactrols in these things do commonly go out of spec, but I don't think they would cause it in this case, as the problem seems to lie past the master volume control - replacing them is still a good idea though. Parasitic oscillation can potentially occur in PCB amplifiers, but the idea behind a PCB is consistency - the only reason this would occur is if mods were attempted or the wire runs to and from the PCB are routed poorly, or components severely drifted (vactrols). I don't think anything would be arcing as it should be a problem on all channels if this were the case. Arcing usually causes instant fuse blowing failures - however, it can sometimes be insidious, and lower the impedance of the circuit board enough to cause some strange problems.