Blue glow in tubes

[Firedome]

In some pictures of amps and tubes taken in dark conditions I've notice some kinds of tubs glow a very prominent blue. My amps over the years have had 6BQ5, 6L6GC, 5881, 6CA7, 6V6GT, and probably others but all glowed a faint red (but were not red-plating). I've seen Mercury vapor rectifiers that glowed purple, and Thyratrons seem to be colorful. Do certain types of power output tubes glow blue like these pics? The second one looks like a KT88.

[Motorola minion]

I am guessing here but it looks like the cathode emission is "literally shining" onto the glass envelope, through the gaps in the plate.

6L6 - does it best I have seen it myself: https://www.bing.com/videos/search?q=yo ... ORM%3DVDRE

[Firedome]

Great video - blue glow varying with the guitar, and upside down too!
I wonder if they do that along with input from a cd or tt too?
Are certain tube types more prone to do this?
Thanks for that Dave!

[electra225]

The 6F6 output tubes in the Grunow 1191 I have to that to some degree, although not as much as those.

[William]

I have seen red plate but never blue glow. Interesting!

Bill

[19&41]

Higher voltage discharges within a vacuum will have a blue glow. If a fluorescent tube had no phosphor coating inside the envelope, you would see a similar glow. The glow can emit UV radiation at high levels, as with the old ozone bulbs, like they used in the old Puritron air purifiers.

[Firedome]

Mercury Vapor high voltage rectifier tubes emit their own cool glow:

[Firedome]

purple EL156

[19&41]

The phenomenon has been known since at least the mid-19th century. In high school, among our small lot of demonstrators, we had a Geissler tube that would contain a glow when high voltage AC was applied to it.

https://en.wikipedia.org/wiki/Geissler_tube