Question now with repairing Mesa Boogie...

You guys are such saviors around here :razz: . I am working on a Mesa Boogie Studio 22+. It is oscilating when the preamp level is high and the bass knob is turned all the way up. I am assuming that this is a new problem and that the amp used to work. What would anyone suggest is the thing I should be looking at? Should I trace this and see where this is happening, or would that help?
Thanks a million!!!
:guinness:

I know this is the dumb and quickie answer but I don't have a schem handy. Try the easy thing first and change the tubes! I just had my Marshall 18Watt oscilating the same way and the new tubes fixed it.

cheers,
kent

Try swapping the first pre-amp tube for another. It may be microphonic which causes the problems you describe.

You could swap the first and second preamp valves to see if this helps in order to prove that the first valve is microphonic.

Mesa preamp circuits run at such high gain that a microphonic tube can spell disaster. I agree with the others: check this first.

Sorry for the delay in replying-I did switch out tubes with no change. After frustration, I called Mesa to ask what they thought. They said basically "oh yeah, those amps were very unstable-they at extreme settings they just will oscilate." I've heard of some boutique amps doing that because they didn't want to put any extra caps in the circuit, but I didn't know that mesa subscribed to that as well. This is a mid 80's amp. They probably don't do that sort of thing anymore.
Joel

My totally subjective opinion, gleaned from years of working on guitar amps (which used to be my full-time job): Mesas are [descriptive term deleted]. The answer you received from them does not surprise me.

If you have a scope, find out what stage is oscillating. Then you can look for what would be causing instability. If it is a tube stage with passive tone controls, the oscillation may be caused by lead dress, lack of plate to grid feedback cap, bad cathode resistor,etc. compare this stage to a Fender or other similar guitar amp schematic to see what they do to get around this problem. Judging from some of the other replies, it sounds like poor design to me.

I find irony in this statement from the Mesa website:

From the first, Mesa/Boogies have used a combination of point-to-point and printed circuit board methods in order to ensure absolute consistent placement of critical parts and conductors. It is easy to demonstrate how moving some parts or lead wires as little as 1/4" can make a huge difference in the top end "transparency" of the sound--exactly where a lot of the magic lives--or dies.

Click to expand...

And then the fact that they didn't fix an oscilation problem with their pcb's... I already gave back the amp. Now I've got several Marshall valvestate heads. I have had to deal with those more than anything else...What pieces of %^$&