Bit of a strange one. The amp worked fine for a whole tour and then died mid set. When I got it home I discovered the HT fuse to have blown. Replaced it and it worked fine. Then when I was messing with various sounds, while chugging the low strings, I turned up the volume (this is a JCM 800 4211 so it has TWO Volume controls - one on the channel after the Gain and one in the Master section to control the overall output - the one causing the crunch is the channel Volume), there was a crunch (as in a dirty pot) and the volume dropped considerably. So I turned the pot back down to zero and turned it up again. At this point there was a huge crunch followed by silence. Having seen my dad 'fix' the tv on numerous occasions by thumping the top I did the same. Bingo! The amp worked fine again. Messed with the turned up the channel volume - CRUNCH!! - total silence. And this time it didn't come back on. Checked the HT fuse and sure enough it had blown! So I replaced it again. And as I had noticed during part of my questing that one of the output valves was making a fluttery, crunching sound if I tapped it, I replaced that as well. It seemed to be working fine and the valve noise was no longer there when I tapped it. But as soon as I started chugging and gently turning up the channel Volume when it got to around 8 - CLUNK! - it cut out again. I suspect that POSSIBLY when the amp gets to past a certain volume the sheer vibration from the speakers (it's a combo) may be causing a dry joint to rattle and break contact. Or crap in the pot once it gets to about 8... Does that make any sense or does anyone have any other ideas of what may be that is causing this? Would it be possible that a dirty pot could blow the output fuse? Or a dry solder joint? At no point have the Mains or T1A fuses blown. Any ideas or suggestions will be gratefully received and tried out.