I have a frontman 15, and its not bad for a practice amp, but something strange is happening with it. It was making a low frequency hum, so I opened it up to check for loose connection or missing shielding, but I didn't find any. I tried it out open, and the hum was gone. I put the back on again, and the hum came back. As soon as I took off the back, the hum went away again. What could make it hum but only with the back on?

I would appreciate any advice you can provide.

Did you buy the amp new? Is the back panel touching any wires?

The only thing touching the back is the main power cable, which is shielded and insulated coaxial cable. I got the amp new, but that was 5 years ago. Thanks for replying.

most hum comes from ground problems.. Did you ever find out the problem?