Music Man 100B
This came into the shop today and at first I didn’t pay much attention, thinking it was a Fender. But upon sitting down with it for a chat, realized it was a 100B Music Man. Sort of fenderish.
Customer said it had no output, having all of a sudden lost it’s voice. Only 2 valves, 6L6Cs but the plate said 100 watts. Umm. Anyway with the chassis eventually relieved of its cabinet (all the Rexine and tin foil catching everywhere), I saw that things were not “standard”. A quick butchers in the Fender diagram draw and luckily found a circuit diagram. Now the thing has a gaggle of tone controls on the front, all being driven by dual ops amps. The output valves have driven cathodes, with a fixed bias arrangement on the grids. The HT fuse was open circuit. Checked the output transformer and silly solid state components around the drive stage. Introduced the 6L6s to the AVO valve tester, which showed they were a little tired and didn’t seem to have any mental health issues.
So, Dim bulb current limiter in place powered up. We had an output, but low and with considerable crossover distortion. Now, a word of warning for those who have not seen these amps before. Most similar looking amps have a HT voltage around 400VDC. That’s not good if you are caught by it. This little treasure runs with a massive angry pixy count of 700VDC which is far far less gooder for you, assisting you in meeting your heavenly extended family. Be warned.
After some messing about, we found that the 22VDC bias was low at 12V. The Zener diode responsible for the bias was showing more of its knee that it should. So that was replaced along with the filter capacitor. With the cathode balance pot adjusted quite a bit, we now had a good symmetrical output.
I am not a great fan of multicultural amplifiers (transistors and ICs driving valves). One little flash over and all hell breaks loose. But, if put on a lie detector, I have to day this amp sounded very good, almost HiFi. Don’t tell anyone I admitted to this.