Sound fading out

Hello everyone. I have come across an interesting problem with my F-50 head. The sound fades out when being played at bedroom volume (8 o'clock). It started about two weeks ago. The sound fades out with a crackling sound within about 15mins of using the amp. It comes back again but then fades out again and will keep doing this. I have still been using the amp at gigs (taking a big risk) and it does not happen at all, all night and thats with the volume at half way or just over. Its not specific to either channel.

Anyway i changed the output valves with normal mesa 6L6's and the problem is still there. I am gonna have to try preamp tubes next cause i cant think of anything else to do. I tried tapping the preamp tubes with a pencil to see if they went microphonic but they seemed fine.

Can anyone help with this please????????

Actually, this is not uncommon. I would bet that it is poor contact in the master pot when it is at the low end of travel.

You can try new preamp tubes, but I would also work the knob back and forth a bunch of times to try to work out any oxide in the contacts. If the pot is not sealed, you can spray some tuner cleaner into it as well. You will get better results for low volume control if you put a volume control in the FX loop and keep the master knobs up higher.

I don't hear a THING with the master at less than 9 o'clock. I thought it was normal.

Agreed. Though around 9:00 I get some scratchiness and the volume can go up and down a bit erratically. Above 9:30 all is well.