Mesa noise?

[drewl]

Another odd one.
I have a Dual Rectifier, the owner described what sounded like a ghost noting problem.
When I play short quick single notes above the 12th fret, I hear like a "thump" noise.
On the scope it looks like a 60 or 120 hz wave when I turn the volume down quickly with a signal generator into the amp.
With dc coupling on the scope, I noticed the trace jumping up a bit, and sure enough it looks like a dc voltage swing when quickly turning the volume down.I checked all the filter caps, the DC supplies look nice and clean, think the filter caps are causing it?

[Chuck H]

I don't think it's the filter caps. Something has to be allowing DC into the AC circuit. There are many amps that bump and jump up and down in voltage when played. They don't thump. I have to think that something is allowing a spike of DC from the caps charging to get into the AC path. I'd be looking for a leaky coupling cap. Mesa uses a lot of little transistors for stuff like reverbs, effects loops, etc. It's possible I suppose that if one of those is running in an improper state it may cause this because I've noticed that the caps which are used to couple these transistor circuits are often of meager spec. It's also possible that you have a loose or corroded tube pin somewhere and playing is consistently cutting contact. I've seen a lot of loose tube sockets on Mesa products. If a tube very temporarily loses bias it could place DC on the grid. Try banging the amp and see if you get the same spike on your test. It could also be a bad ground. If the amp has both AC and DC grounded in the same place (and most Mesas do) and that ground has more resistance than it should the DC could be momentarily jumping onto the signal path during current spikes.

This is a tough one.

Chuck