Music Man Guitar Amplifier
Musical instrument amps are one of our few exceptions to the “no consumer equipment” rule. Most of the well known names are still designed to be serviced instead of thrown away.
Music Man was a joint venture between Leo Fender and a few others in the mid 70’s to early 80’s. They are a hybrid design, solid state right up until the final output tubes. In this case a pair of EL34’s
Here is the rear view. The output transformer is nearly as large as the power transformer. And this is a heavy amplifier.
Simple filtering for the High Voltage supply for the tube plates. ESR has crept up on these, they’ll all be replaced.
JJ Electronics is a Slovakian company that manufactures tubes and Capacitors that are common in musical instrument amplifiers, particularly older tube designs. They appear to be of decent quality.
Solid state portion of the amp. Op amps driving a matched pair of transistors, which drive the final tubes. Previous repairs are evident, as are expected on older tube gear. We’ll remove the silicone glue and replace it with proper mounting hardware, clean up the leftover rosin and clean up residue from previous failed parts
There is just one adjustment to make- idle bias. Measuring the voltage across the emitter resistors, adjust so the lowest tube is about 25mV . These transistors matched up pretty well and the bias currents were nearly identical.
Reassembled, tested and back to the customer for another few decades of use.