Interesting tube guitar amp. Leo's last hurrah. The music man is a hybrid. Running OP-amps for the preamp rather than 12AX7's. There is one 12AX7 which serves as the Inverter driving 4-6CA7 tubes. This one arrived dead. Immediately blew the fuse when AC was applied. I knew the problem must exist in the power supply. Filter caps looked nasty, but the primary failure was a shorted diode on the rectifier board. So, I replaced the diode and re-capped the amp (electrolytics only). One of the negative bias caps was open. That more than likely started the sequence of excessive current and finally shorting out the power supply section. After installation of the filter caps, a fresh set of tubes were installed and the amp took off running again. On all of these vintage amps, it is good practice to install new filter caps. There is no need to test them. Replacing a few and crossing your fingers will only prolong the next failure. Save yourself time & expense. Note: The high voltage in these amps hovers around 725vdc! Use caution when inside. Also, to prolong the life of the tubes, lower the idle current. The manual states .5VDC at bias test point. Lowering that will increase tube life.
16 окт 2024
