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This is an old thread, but thought I would add my 2c to the discussion. I am also currently working on a Kalamazoo bass 30. A few weeks ago, I had the pleasure...or I should say, displeasure of playing my bass through this amp in a jam situation. The hum really irritated me. And, as mentioned, the hum is mostly related to the volume control setting, implying the source of the hum is on the input side of the circuitry. As a test, I disconnected all the AC wires that were routed to the control panel. I simply clipped them off at the terminal strip next to the power transformer. I then temporarily alligator clipped the AC to energize the amp. This removed absolutely all AC wiring in the control panel, as well as the bundled wire train going to the panel and chassis connector. This removed 95% of the hum to a very tolerable level! It was mentioned above that this is of questionable design. I strongly agree with that. It is never a good idea to route AC wiring around sensitive input circuitry. Yes, there is a coolness factor in having this flip out panel....but, at the expense of producing hum? This sort of design work would prevent me from buying such an amp. So, I have approval from the customer to make this modification permanent. I will be mounting the fuse holder and power switch to the metal chassis in such a way that they will be accessible through the power cord storage hole in the back. The indicator light I have not yet decided what to do there. The front grille is somewhat light transparent, so I may mount the indicator lamp behind the grill, and see if it is visible through the grille.
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