Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Harpman Blue on July 26, 2016, 10:46:30 am
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Recently bought a 1990 Fender Bassman Reissue that has been modified for harp. Problem is the amp owner recently passed away and no one knows what was done. There is some handwritten info but no real detail, just how to set other tone controls, presence, and volume for best tone. The mid pot and bright volume pot were removed. Anybody know why this was done or is this a common mod? They are included in a bag of parts that came with it. Should I put them back? Tube lineup is just as it came from factory, all Groove Tubes. Adding an adjustable bias so I can do some tube swapping and get them in their sweetest tone for harp. Just never seen one with those pots removed and not sure what else was done. Any help appreciated.
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I would guess that if they removed the pots, they've done one of two things: 1. hard wired in the resistor for a specific sound or 2. removed the caps and resistors and routed directly past that area, so they're not used. Either way, it's just a guess, you'd need a schematic to know for sure. You could look at the schematic and/or layout and compare expected values and routing. If you're not aware of how to do that, you could post some pictures of the interior and we can give it a try. You may be able to just replace the parts removed as well, but again, you'd need the schematic to ensure you're restoring it to stock. If you don't know how that all works, I'd suggest taking it to an amp tech.
As for a bias mod, it's usually not too bad, I just learned how to do one on my Fender Blues Deluxe, and it just required desoldering one resistor, adding a variable resistor and another lesser resistor in series and you're pretty much done, but you'd again need the schematic. I found this service manual, and it is probably the right one, but I'm not 100% sure. In this case R41 would be replaced with a 25k pot and a 27k resistor in series so that it has the ability to go up or down a bit but not get to 0 bias voltage (bad news). That schematic also should help you see the area that is related to the pots that were removed. The Bright Volume area is R11 C4 and the mid pot is R20 and C7. I'd check that those caps are still there and in circuit or not or if you see some jumper wires around there. There's a layout diagram there too. Just make sure it does match your amp.
~Phil
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Thanks Pompeiisneaks. My thoughts exactly. I have the schematic and just have to sit down and trace it all out and see what was added and/or removed. Bias pot addition should be simple. Just have never in any forum ever heard of that kind of mod (pot removals) for a harp specific Bassman amp. Especially with an amp where with a few tube swaps you can get just what you desire. Do think I'll put the two pots back in so if I ever want I can go with the original tube lineup and play a guitar thru it if needed. Thanks again. If I get in a jam I'll post some pix.