I've got a Bugera 1990 that I've had since December (so, 7 and a half months or so?) and today I was playing it and the sound decrescendo'd (faded out) until it was nothing.

Is it the tubes? If not, what else would it be?
No, if it was the fuse, the lights would have gone out, wouldn't they? It sounds like the tubes to me, I just wanted a second opinion before I took it to the local store.