2:90 making no sound!! ARGHHHHH!

hi guys, this is my first thread here and I need help!
I bought a second hand Simul 2:90 about 6 months ago, i haven't changed the tubes and as far as I am aware the previous owner hadn't changed them in over a year. Today I turned it on and left it on standby for five minutes. When i flicked the switch, nothing happened! no sound, nothing. The amp still turns on so I assume that it is not a blown fuse but a bad tube. Has anyone had an experience like this? I don't have any spare tubes to test it with, but i will buy some. any help would be really appreciated.
Jesse

Ok, so I tried the poweramp again, now it works! When I switched it off standby my signal was coming through normally. I played for about ten minutes, then the amp started to smell, sort of like the smell after you pop a christmas bonbon. I totally assume this is bad, and im not sure if its a tube problem anymore. I have heard of this happening before but dont know how it was resolved.

Do the tubes glow when there is no sound?