This amp came to me for restoration in pretty bad state. Missing output transformer and no knowledge what else was missing/broken. I got to work and here is some info what had to be done, I hope this is useful for someone with a similar task… :)
Missing output transformer. You can get new ones from TAD in Germany. Great quality. If your amp squeals after installing it, reverse the plate wires :)
Added some missing parts like fuses, caps, output jacks etc.
Zener diodes on the preamp board. Original ones are tiny and I’ve seen Music Mans with cooked zeners. I used 1N5353 which is a 5W 16V zener, perfect for the job.
Output driver transistors were broken. Original ones are JE1962 which is an obsolete part. Music Man service bulletin recommends a 2N6488 transistor as a replacement. I ordered mine matched from eBay and the HFE’s were almost identical. After installing they need to be biased and here’s how: Turn amp on and switch standby to HI. No signal. Measure voltage across 3.9ohm emitter resistors on both transistors and adjust bias trimmer until voltage reads 25mV DC. If the voltages are different between the two transistors set the lower one to 25mv, higher can’t exceed 55mV.
New electrolytic caps. For preamp and driver board I used Panasonic FC and FM series caps because they are nice size and could be found in correct values (for example 150uF). For psu filters I used F&T 500V caps…the old ones were dimpled and totally shot.
Flyback diodes on power tube sockets (from pin 3 to ground) were replaced. I used three 1N4007 diodes in series. While doing this I also replaced the 0.001uF 2kV caps on tube sockets with fresh ones, these sometimes short out.
When standby is on “HI” position there is way over 700 volts on the plates. That is some serious voltage for modern tubes. I recommend 6CA7 or JJ EL34L tubes for this amp and when starting up use LOW position first to not shock the tubes too hard. Or never use the HI position :) Tube sockets also need to be cleaned thoroughly to avoid arcing.
This is actually a really good sounding amp considering it has a solid state preamp!
Too bad I didn’t take good “before” pictures of the amp guts but here are some taken after the restoration.