ok, i've had my fender twin reverb for about 2 months, and today i moved it to my friends house where my band has reharsals. up until now i've never played it louder than 3 on the volume knob, because i live in an apartment and i can't really play it loud.

so i took this as an occasion to really test my amp out. i cranked it up to 5 first and played it for a while and then i set it to 7. after like 15 minutes, i felt a burning smell, and when i looked at the tubes, the two left ones were glowing red, compared to the other two which appeared to be normal. the whole corner of the amp in which the tubes are located was pretty hot too.

i don't have a lot of experience with tube amps, so i don't know, did it happen because i was playing it too loud? could something be wrong with the amp or with the tubes?
Redplating is usually a sign of a bad set of tubes. I'd take it to a tech and have them install a new set of power tubes and bias them. They'll let you know if there's a bigger problem, but it sounds like it just needs new tubes and a bias.
thanks for the answers. i took out the 2 bad tubes, and put the other two in the right way. i'm running half power and it seems to be alright now, so i guess those tubes were shot.