Vibro Champ hum

[Randall]

Late 70's Vibro Champ came in for a cap job and a slight hum. I figured the cap job would cure the hum, but it didn't. It had the filaments wired on side to ground, which I have read can be the source of hum, so I re-wired it with the 100 ohm resistors to ground on each side, but it didn't fix it. What else could it be? I tried moving the resistors from ground to the cathode of the power tube, but I think it made it a little worse. I have a 120Hz at 20mV p-p hum.

[Jazz P Bass]

As far as I know, from the factory that is the way they are.

Try this read: http://music-electronics-forum.com/t31980/

[Leo_Gnardo]

Since the Champ's power amp is a single ended circuit, there's no push-pull effect to cancel power supply hum. If it's important to knock the hum down, say for recording studio use, you can add a pre-filter to the hi voltage power supply. Cheap way involves a 22 uF or so filter cap and a power resistor say 100 to 470 ohms. More expensive and more effective, I've made pre filters with the cap plus a choke - same choke as you find in a Twin Reverb supply. Knocks the hum down to acceptable levels.