• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6bx7gt envelope temperature

That seems pretty high. Most tubes are rated for 200 degrees C max envelope temp. You're somewhat above that- is the tube well-ventilated? Power ratings assume that the tube isn't stuck in the middle of a lot of other warm stuff and has free flow of air. If that's not the case, even run within power ratings, the tube can destruct.

Get some air on that puppy before it bakes to death.
 
6BX7 is hot!

I done a SE simple 6BX7 amplifier.
I tested lot of brands like GE, Sylvania, Amperex, Rayteon etc and all the tube get really hot in operation, also within the dissipation limits that is 13W for the sum of two section (not 10W each section if both section are used).

The hottest part is the plastic socket, and can lead to early degradation of the socket itself, a good ventilation will not be a bad idea.

Electraprint use this tube in overdissipation, ( http://www.electra-print.com/docs/6bx7se0001.pdf ) but I was told that this tube is really a robust one.