Musicman HD-130 (Reverb/Trem)

The Music Man instrument comapny was started in the 1970’s by Leo Fender after he sold Fender to CBS in 1968. He was head of the company in every way except on paper as he signed a multi-year Non-Compete deal with CBS. Leo Fender and team designed these amps to be cutting edge and to continue his pursuit of the cleanest and highest-volume/highest-wattage amplifiers possible. These amps can sound a lot like Fenders but the main differences are: They are made with vintage style PCB’s and some hand-wiring, they are hybrid amps that use all analog solid-state preamps and effects circuits, the power amps use 6CA7 power tubes (later models use 6L6GC’s) with a mix of Transistor cathode bias and fixed biasing on the grids, and they use extremely high plate voltages (700+ VDC!!) on the power tubes resulting in even more pristine clean tones that are suitable for guitar, lap/pedal steel, and bass. This particular model claims a massive 130 Watts, has the earlier circuit design incorporating a 12AX7 preamp tube for the Phase Inverter (later models use a solid-state Op-Amp), and has really cool sounding solid-state driven Reverb and Tremolo.

I have made it clear in earlier articles that I really like Music Man amps, and I think this circuit with it’s “extra” 12AX7 preamp tube makes this HD-130 sound even better. It came in with leaking filter caps, heat-damaged resistors, dirty/corroded switches and sockets, and bad tubes. The amp needed a full restoration.

All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated F+T/CE brand electrolytic’s. New 2 Watt Metal Oxide/5 Watt Cement power dropping resistors were installed for better reliability and lower noise floor - also more practically, to replace any that were heat-damaged. The cathode ballast resistors for the power tubes were replaced with 2 Watt Metal Oxide type units for the same reasons as above - this was also done with the 10 Watt Cement Screen Resistors which were replaced with the correct type of make. The MR250 Flyback Protection Diodes were replaced with upgraded GP02-35-E3/75 3.5KV 250mA Diodes as one of the two diodes were actually missing from the circuit, they also commonly fail so the upgraded units will help with reliability.

The original tubes were 20 years old and did not test good. The tubes were replaced: V1 = EHX 12AX7, V2-V5 = JJ 6CA7 Matched Pair. These were picked for best tone and reliability. The 6CA7's Fixed Bias was set to 60% Class AB Plate Dissipation with a B+ 706VDC. All tube sockets were cleaned with De-Oxit and re-tensioned. The pots were sprayed out with De-Oxit and the amp was cleaned inside and out. A new 3 prong AC cord was installed to replace the worn out original and the Death Cap was deleted rendering the ground switch deactivated. Amp now functions well.