New Member Rehabilitating a 50 Caliber Plus

[Fred2202]

Hello everyone. I am new to the forum. I have lurked here at times just to see what people had to say about Mesa amps in general. Always wanted a Boogie Amp and I finally picked up a semi functioning 50 Caliber + last week. Amp makes sound but has multiple issues. I have scratch built and fixed quite a few amps over the the last 25+ years and thought this one would make an interesting project. The price was right so I went for it. I am in the process of tracking down all the problems and trying to get the amp working as it should. Someone has been inside this amp before and really hacked the thing up. Already replaced three missing resistors and removed some jumper wires placed on the board that made no sense at all. A couple of good gutshots of the circuit board I found on this forum really helped in that respect. Aside from some dead preamp tubes, the Power tube bias and phase inverter bias is drifting and the power tubes are redplating soon after turning it on. I think I have a bad cap in the bias supply so I had to order a few new caps for that section of the circuit. I'll just rebuild the bias section completely once the new caps arrive.
Did have a few questions/comments if anyone would be kind enough to respond. I am in the process of tracing this circuit out and the hand drawn schematics floating around for this amp do not look to be completely correct?? Also the recommended bias voltage (-47v) at the 435-450 volt plate volts looks to be on the cold side for 6L6GCs?? Should I adjust the bias to the cool side to get the amp the sound like it should( may add a bias pot so I can get a little flexibility with the bias voltage)? The voltages for the LDRs and Slider EQ section of the circuit look pretty low although most of the resistors in that section of the circuit seem to be still near original values. I have not been able to really check out function of the EQ sliders and channel switching until I get the bias problem fixed.

Fred

 

[Fred2202]

I got this 50 Caliber+ head all straightened out. Tore out the strange jumper wires and changed several resistors back to stock values. Replaced some of the smaller electrolytic caps and replaced some carbon comp power resistors that had drifted out of spec. May replace the bigger caps somewhere down the line but they seem Ok at the moment. The voltages to various points in the amp look OK. Added a bias pot for adjusting the negative voltage on the power tubes. The way the bias circuit is set up is odd with the tail end of the bias string hooked up to the phase inverter cathode but it works. Replaced the power tubes and phase inverter with some MESA branded tubes and set the bias. Amp sounds very good with nice cleans on the low gain channel and some nice punchy distortion on the lead channel. The lead channel can get a little raw sounding if you don't dial it in right but overall I am a happy camper with the way it turned out.