Mesa Boogie Dual Rectifier head BIG Problem!!!

[Poukie]

Hi everyone! Have a really big problem with my Mes Boogie Rectifier Solo Head 2 channels 1997.
The clean channel = work's really good.
The dirt channel red =great sound but after few minutes the sound begin to cut like a tremolo pedal.... Clean sound, dirt sound, clean sound, dirt sound..... And after few minutes just have a bad clean sound and sometimes have a distorted peak ...

After I turn of the amp for 30 minutes. Start the amp and the both channels works great ... And the Problem start again after few minutes of playing...
HELP ME please....

[Gingertube]

Hopefully someone with experience with this particular amp will respond BUT FWIW here is something to start with.

I take it that the amp was made in 1997. That means it is nearly 20 years old and so the electrolytic capacitors are probably in need of replacing.
The fault description is typical of what happens when the power supply caps start to dry out and go high impedance.

I don't think this is the 100% correct schematic but it will be close.
http://bmamps.com/Schematics/Mesa_Bo...Schematics.pdf

See the last page of the schematic.
The electrolytic caps, 2 off 200uF/300V, 1 off 30uF/500V and 4 off 10uF/450V to points B,C,D,E and F are the ones to find and replace. From your fault description I would assume that one or more of the 10uF/450V is the culprit but with an amp of this age I would just replace the lot.

EDIT: Sometimes the culprit cap can be identified by visual inspection, any sign of a bulging end is bad news. Make sure you get the replacements in the right way around, the leads will be clearly marked , usually with a black arrow on the side pointing to the negative (-ve) lead.

Cheers,
Ian

[The Dude]

I would first start with tube substitution with known good tube(s). Eliminate the easy stuff before ripping the amp apart. A tube might be failing after it warms up.

[Steve A.]

If it was power supply caps wouldn't you get similar results with the clean channel dimed? I would first try replacing V3 and if that made no difference then V2.

Steve Ahola

[Gingertube]

If it was power supply caps wouldn't you get similar results with the clean channel dimed?

Steve,
Not necessarily. If power supply bypass caps start to go higher in impedance (as they dry out) then the signal current will start to cause a signal dependent voltage on the supply from voltage drop across that supply impedance. That signal on the supply appears as positive feedback to earlier stages WITH THE SAME PHASE and problems (motorboating and oscillations) will show up first in the high gain channel. If left long enough and the problem worstens it may eventually show up in the clean channel too. The high gain channel is more susceptible simply because it has higher gain (to form the oscillator) and because there are more gain stages.
You rarely have troubles with 2 cascaded gain stages (because they have opposite phase) but with 3 stages the first and last have same phase and problems are more likely. With 4 stages you double the likely hood of problems again as 1st and 3rd have same phase as do 2nd and 4th. Hence the old golden rule to NEVER run more than 2 stages from one power supply node (Someone should have told the Fender Supersonic designer this, it has 3 stages from a single supply node in its BURN channel and is VERY suceptible to breaking into oscillation).
Not claiming that this is the problem with this amp but was to indicate what to check. I would certainly try the Dudes suggestions above before ripping into the amp.

Cheers,
Ian

[Enzo]

I'd also look at the supply to the switching elements.

[Poukie]

Hi guys! Really thanks for all of you for your suggestions! Replaced V2 and V3... Same problem. Check out my amp on my vid post... https://m.youtube.com/watch?v=YdMwkajWBus

[Poukie]

Hi . Really thanks your suggestions! Replaced V2 and V3... Same problem. Check out my amp on my vid post... https://m.youtube.com/watch?v=YdMwkajWBus

[Poukie]

Hi Steve. Really thanks your suggestions! Replaced V2 and V3... Same problem. Check out my amp on my vid post... https://m.youtube.com/watch?v=YdMwkajWBus