After playing a while, my JCM 602 fades to black. No audio, no hum, absolutely nothing. When this happens, none of the preamp tubes are glowing. The power tubes are glowing and the power button is still lit. I am suspecting the transformer. This is intermittent, as some times the audio comes back. Lately though, I have to shut it off for a while and fire it back up to make it work. Any suggestions would be greatly appreciated.