SWR Redhead Humm

[jim66miami]

Working on an SWR Redhead Bass amp (240W), MFG in 1997. The amp is producing a hum. Signal goes through pre-amp and power amp, sounds good except for the hum. The hum is present with no input and master volume and gain low. Replaced some caps that had some visible bulging on the power supply (4700uf, 63v), no change. Any suggestions on where to look?

 

[Cortez]

Pre-amp is a built in one?
No other device is connected to the amp?
It's only plugged in the outlet and the hum is already present?
Try to short the input as well.
The hum was always there or it was okay for a time and suddenly appeared?