From what I've read, it's energy being released by the electrons when they hit the glass, slow, and release energy. It's definitely *not* from gas within the tube, if that were the case, the gas itself would be luminous.
The gas is luminous - where it is being struck by the electrons at the correct speed for it to become electroluminesent(sp?)
The electrons are waaaaay too small to see. Even a million of them.
It is energy given off by the gas that we see, excited by the electrons. The cathode electrons are never visible.
I guess it's really just semantics. Physics wasn't my bag, anyhow.
I wonder if the glass on the Sovteks is coated with some chemical that gives off the glow, like a fluoresent light.....
A ploy to get you to replace them early ?
Bottom line is don't get overly excited by new tubes showing blue, but NOS tubes might be glowing blue due to approaching failure.
Either way, it ain't over 'til it's over. Play the tubes until you hear the low output or distortion.
Or better yet, run them through a tester, if you can find one.......
And to think that I threw out at least three commercial testers back in the 80s.