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Chuck Harris

wrote:

Several years ago a friend gave me several 304TLs. He didnt want them
because they were gassy(glowed blue). This was about 1971. Two of the
tubes still work fine used in a stereo amp, a couple more I gave to a
friend building a modulator the others still sit on the shelf in case
i ever need spares.

Jimmie

A nice cobalt blue glow isn't gas, but rather is caused by electrons
hitting a slightly fluorescent glass. Some tubes have areas in the
plate that can leak out electrons from the cathode. They will glow
blue from the very start.

Usually, a gassy tube will glow pinkish, and the glow will be situated
inside of the volume surrounded by the plate.

-Chuck