Farty/mushy distortion in Mesa MKIIB

[beasleybodyshop]

Hello everyone, i have been working on a friends Mesa Boogie MK2B for some time now.

Originally, it was brought to me because one of the 30uf 500v electrolytic caps had ruptured. I went and replaced all four of the ones in the power supply. I also installed a fresh set of new, working 6L6GCs in the power section ( I had tested all the preamp tubes, they tested working with no shorts) Upon firing the amp up, I get this wimpy, farty distortion tone in the lead channel - its not a low output condition (oh its loud alright) but just farty distortion I put in another set of know working 6l6GCs - still does it. I swap out known working 12AX7s, no joy. I plug in another speaker cabinet - that doesn't fix it. I look through the schematic and check plate voltages on V1-v4. All of them are approximately 20-30 volts higher than the spec listed on the schematic (see attached) With one exception. Pins 1 and 6 of the phase inverter are showing like 435 volts - exactly 100 volts higher than listed (335). So i trace the plates backwards to the power supply - signal traces back through the reverb transformer, then through the big 10K plate resistor - that resistor does look bad, and voltage drops when measured across it. Tracing it back even more leads me to voltage supply "B" (one of the caps I replaced). Im getting 465vDC at that capacitor - which measures about 20V more than spec, but I think thats within tolerance? Im confused. Is it possible that there is a typo in the schematic? even so I wouldnt think there should be 435 volts on the PI plates - or am I looking in the wrong direction? Everything I have learned is because of this place, and for that I am very grateful, but here I am stumped. Any ideas guys?

[Leo_Gnardo]

Mesa's often factory biased cold cold cold. And that would also help explain lousy tone as well as higher than expected B+. So, you can 1) measure the bias current - I use the highly hazardous shunt method but use any method you want, and/or B) let's see the output waveform, sine wave into the FX return, driving a 4 or 8 ohm load. Betcha the "top don't meet the bottom" IOW deep notch distortion.

FWIW I often bias these & similar amps @ 30 milliamps per output tube 6L6GC. A little more if the B+ is tending toward 400V and a little less if B+ is closer to 500V.

[beasleybodyshop]

Mesa's often factory biased cold cold cold. And that would also help explain lousy tone as well as higher than expected B+. So, you can 1) measure the bias current - I use the highly hazardous shunt method but use any method you want, and/or B) let's see the output waveform, sine wave into the FX return, driving a 4 or 8 ohm load. Betcha the "top don't meet the bottom" IOW deep notch distortion.

FWIW I often bias these & similar amps @ 30 milliamps per output tube 6L6GC. A little more if the B+ is tending toward 400V and a little less if B+ is closer to 500V.

Leo, as always you are the man. I will check the bias when I get back to my bench.

These amps don't have a bias control - I'm assuming that its worth it to go ahead and install one?

[beasleybodyshop]

Even better yet, couldnt I just replace that 15K resistor in the bias circuit (the one going to "E" in the schematic) to change the bias (once I've determined how many milliamps they are drawing)? The bias circuit at that point was measuring exactly -52V as per factory spec - so its safe to conclude that this amp is still pretty much set at factory bias settings?

[Stevem]

Yes, 52 is about the factory setting dependant on your wall outlet voltage, and its a cold setting as was posted.
A idle current point that I shoot for is .032 to .036 amp with the .036 letting the output stage add its grind into the mix sooner!
Stiff in a 15 to 25k pot in place of the 15k resistor and see what idle current range that gives ya, then you can choose to leave it in and BLACK silicone it in place, or measure the pot and make up a set of 1/2 watt resistors to = the pots value for the most part.
While your in the be sure to bend up the chassis grounding finger on the top of the chassis and make sure its contack points are clean and shinny, also clean and lubeall the pots as it never fails that 2 days latter one or more of the pots are noisy and your pulling the whole thing out of the cab again!!

[Leo_Gnardo]

Even better yet, couldnt I just replace that 15K resistor in the bias circuit (the one going to "E" in the schematic) to change the bias (once I've determined how many milliamps they are drawing)? The bias circuit at that point was measuring exactly -52V as per factory spec - so its safe to conclude that this amp is still pretty much set at factory bias settings?

Often I find an extra resistor in parallel - in an attempt to approach a useful bias point. If you're feeling dexterous, a pot-resistor combination will give you adjustment flexibility but won't let you (or some other well meaning tech) dial bias all the way to zero. (And I've seen that done.) FWIW I use the Bourns mini-cube trim pots but anything competent will work.

Would be good to have an actual bias current reading. If you have some 1 ohm resistors now's a good time to put 'em in. That or train your multimeter to the "shunt method."

Oh, and don't tell Randall Smith, boss of Mesa. He publishes a 3-page "reasons I don't put in a trim pot" paper & includes it with every amp sold. And he left out the real reason: to cover his own ass. Early Boogies DID have a bias pot and guess what, people dialed it all the way up to the red-plate zone, maximum bias current, then demanded new output tubes. Randy got tired of that game, and ever since, no bias control and bias set cold as last week's supper.

[surfsup]

Did you verify all Ck and CCs in the signal change to see if anything was changed prior to your receiving the amp? Perhaps there's too much bass being passed through...