Why does my amp just hum when turned on?

I plugged my 120V amp into 220V outlet direct by mistake. (Forgot the Voltage converter). Amp would not power up. I opened it up, found the in-line fuse and replaced it. Now the amp powers up but just hums. Volume and pitch of hum do not vary with amp controls. I know I damaged something else. Any advice on where to start troubleshooting?