Tubes that Glow Different

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.

Sorry, just reread my post, and realized it didn't make sense.

The longer I leave the amp on, the more the tubes glow the same brightness...

it's just this one 6SN7 that isn't glowing as brightly. The 300bs start off at different glowing brightnesses, and then become more alike as the amp stays powered on...

It's a VAC 70/70, and it is self biasing...

Tubes are pretty variable in their internal construction from one to the next. Sometimes all that this will mean is that one tube reveals more of the heater filament to the eye than does another. If one tube shows a noticably different color than the rest (usually either much whiter or bluer), or its intensity goes up or down periodically as it is being run, then you need to check it out. But this is one reason why I might hesitate to buy an auto-bias amp - you don't have an easy way of checking whether a tube is operating in its optimum range, or how far out it might be from the other tubes. Even 'pre-tested' tubes can arrive out-of-spec.

I'm with Jab. Give Kevin a call. He's the best. He's given me tons of advice for my VAC PA160 mkIIs.