Thirty combo fading sound

Start by investigating the sockets for the Fx loop.  Check for this by putting a short patch lead or a spare guitar one into the loop sockets to bypass their switches.  I mean plug straight into the Send and directly across into the Return.  It is a common problem on most amps that when the switch wafers get dirty or they corrode simply through lack of use the sound fades in and out, even cuts out completely.  These switch wafers are used to carry the signal from the Send across to the Return socket when nothing is plugged into them.  You will need a proper contact cleaning spray and you simply spray it for about 2 seconds directly into the mouth of the socket from the outside then plug a jack in and out a few times to wipe the switch contacts in the socket.  Remember, this is not the contacts that the jack plug makes, it is the switching contacts that open up when it is inserted.  That may cure it.  Make sure the amp is off and unplugged when you do this, (fire hazard), and leave it for about 10 mins afterwards for the fluid to evaporate before turning it back on.  The fluid will not harm anything inside the amp, that's why I say use special contact cleaner.  DO NOT USE WD40 OR ANYTHING LIKE IT!!!

That said this could be a valve problem but check out the loop with a patch lead first to eliminate that as a potential problem and we'll take it from there.

Incidentally, I've moved your thread into the Legacy Amps forum which is really where it should be.  It may get more views there than in the Anything Else That Needs To Be Talked About section which is more for general non-amplifier chat.