5e3 6bm8 440 ac30 amp ampeg amplifier amp modification bass bass amp behringer beta lead bias blackstar boss broken buzz cabs camplifier carvin coil crate design distortion eden emg fender gibson guitar head hillary clinton thesis hiss hum humbucker impedance jcm900 jmi kustom kustom electronics "the bag" m-audio mackie marshall mcaire mod mods mxr noise peavey pedal pickups power power supply preamp problem repair reverb roland schematic sound sovtek switch tone transformer traynor troubleshooting troublseshooting tube tube amp tweed twin upgrade valvestate vintage volume vox wiring yamaha