Ampeg SB12 volume pot hum... but only when turned down?

[cryptozoo]

I've been meaning to post this here for a bit -- I have a vintage Ampeg B12 Bass amp, one of the old flip tops. It'd been messed with pretty hard when I got it, The OT had been replaced and the new one oscillated like crazy, part of the board had a hole burned in it, bad caps, etc. Slowly but surely I've gotten it back together, but the last hurdle has been an issue with the volume pot. When set to 2-10 on the dial, it's fine, very little hum, volume taper works as expected. But when I turn it down all the way and get to 1 or so, it makes a sound and hums like a bastard, almost as if I'm engaging some sort of hum switch at the end of the pot taper. The pot is a replacement, and the old pot did the same thing. Any idears? There are a few different versions of this amp out there but this schematic is pretty close to the one I've got:
http://www.freeinfosociety.com/elect.../ampegsb12.gif

[Jazz P Bass]

Whoa!
Forbidden link.
Can you simply post a copy that you have?

[Enzo]

Well, there is this one:
http://www.webphix.com/schematic%20h..._portaflex.pdf

Much as I don't like all these Joe P drawings, they are better than nothing.


My suspect is not in the schematic anyway. I bet your pot has its ground leg grounded somewher other than the same place the cathode resistors of V1 are grounded. Like to the panel maybe?

[madkatb]

Take your pick.

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