If it looks like a flash upon startup this is normal with some European tubes.
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I have this amp also, and I recently lost a 12ax7--fortunately, they aren't that expensive. You always should have some spares around, so if you hear a funny sound (some fuzz from one channel), you can just pop a new tube in and see if that's it. It almost always is.
If it's an internal problem, you probably won't get any sound at all--or you will get a lot of hum.
Another thing to have around is some spare fuses. I think that this one uses a 3 amp slo-blow. A few pennies at Radio Shack.
BTW, what speakers are you running?