Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: matt mincer on February 28, 2018, 09:03:48 pm
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Hey All,
Guy brought me a Musicman amp to fix. The main problem was a bad 12AX7. I replace that and now get sound out of the main channel but not the bass channel. I do know that the volume pot for that channel is flakey but I don't think that's what's causing the problem. Anyone have any ideas? Also, does anyone know where I can get a decent legible schematic and or manual? The ones I have found are a pain to read.
Thanks
Matt
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pretty readable schematic can be found here:
http://el34world.com/charts/Schematics/files/musicman/Musicman_Schematics.htm (http://el34world.com/charts/Schematics/files/musicman/Musicman_Schematics.htm)
Not very familiar with theses hybrid designs but maybe check if you have similar voltages on main and bass channel opamps.
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> a Musicman
Best narrow that down. Musicman made a LOT of different circuits.
NO none nada zero sound? Signal-trace or/and signal-inject until you don't/do get sound. In the 15V chip sections you can inject with your finger. Also in the small tube stages if you are VERY careful to avoid high DC voltages.
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I'll post a picture later but it certainly doesn't look exactly like either of the schematics here. Obviously, it closer to the 12AX phase inverter schematic though. No guitar or finger sound on the bass channel but it sounds okay on the normal. I have connected the bass channel input to a 1khz signal and it looks like it's getting lost at the 2nd opamp (via an o-scope). I thought I saw where those things were reliable but maybe not this one. I'll try swapping it with one from the normal channel and see if that's the problem.
Thanks
Matt