I am running into a weird issue with my tube amp. On occasion it will get real quiet for 5 10 seconds and then come back. I jiggle the tubes while it is doing it and there isnt any noise or microphonics.
Any idea where to start troubleshooting?
After you've ruled out cables, pedals, and guitars, clean the input/speaker jacks and especially the fx loop jacks...spray some contact cleaner in each one and plug a patch cable in and out rapidly to clean out anything....it only takes a little debris (not visible) to make for some weird issues, especially in the loop jacks. Also the cabinet jack while you're at it.
Preamp tube(s) would be the next likely culprit