Two questions: 1-is your amp getting enough fresh air? 2-when you say "longer", do you mean 5 minutes or 2 hours of hard playing? Your only real worry should be if one of the tubes glows very brightly-your bias may be off.
Tubes that Glow Different
DOes it mean anything when tubes on my amp sometimes glow less bright than other tubes that are of the same "batch"? I have some 6SN7s where one of them isn't glowing as brightly. Sometimes, my output tubes also differ in how they glow... the longer I leave the amp on, the more the same their glow becomes...what's up with that?