Classic Hifi Care

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.

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

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!

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

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

Bill

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.

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

purple EL156

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