Spider Valve MkII HD100 - Extreme sound loss

[MetalGod83]

So I got my amp last March and it's been doing it since it was brand new. I'll turn it on and let it warm up a bit like ya do, then start playing. For starters, the tones are sick. Lots of options. The first time I had this issue, it was about 30 minutes into my jam session that the sound almost completely cut out. It reminded me of when your instrument cable isn't fully pushed into the guitar, you can still hear it but just barely. Very far away. Slightly disappointing since it was quite an expensive item. I've tried multiple guitars and instrument cables. Same issue. The time now varies though. It still starts off strong but will start cutting out anywhere from 2-20 minutes in or sometimes longer. I can't imagine that all 17 of my guitars have the same defect, brand new Monster cable doesn't work. 

Bear in mind, this was my first "tube" purchase. I'm not all that familiar with what does what in there. I just know that it starts out working perfectly and then after a while of playing it just won't quit cutting out. Need this for performances and would like to see if there's anything I can do without having to get it serviced. Please help.

[napynap]

It sounds like either a preamp tube is dying (easy to replace yourself), or the volume pedal on a FBV is getting moved (I always disable the volume setting and just use it for wah).

[MetalGod83]

I'd guess the preamp tube makes sense. Don't have a pedal for this thing yet. Just disappointing because it started the very first time I turned it on. 

[Stormtrooper12]

The master volume knobs on these are notorious for being flakey. Maybe swapping that out might help. Although your specific time thing does sound like a tube/heating type problem.