Donovan Banks

I was playing bass on a Fender Bassman 100 recently. It had a constant annoying hum, but when the lead was unplugged or muted it went away. I asked the owner if he knew what the problem was and he said it had been like that since he got it.

It was being used in a church. On the stage each instrument is connected straight into a DI box, the signal is then split. One to the desk, and one to the amp for monitoring.

what happens with some DI boxes is that the desk supplies phantom power. It sends 48V up the cable to the DI box. In this case the phantom power was passing through the DI box and being fed into the amp, thereby creating the hum.
when I flicked the ground switch, the power was no longer passing through the DI but was being used up instead so the humming went.


I had a Fender Bassman 50 previously and it too had an annoying hum. although I never used a DI box the ground switch on that amp too was in the wrong position. If you have a hum, check that first before you start replacing costly parts. It could save you some good time and money in the long run.