The following web page shows several photos of vacuum tubes that glow
because they’re either gassy or induce florescence in the their glass
bulbs. Although the English is a little choppy, the narrative and
pictures are informative:

http://www.jacmusic.com/html/article...w/blueglow.htm

The author claims he’s revived gassy tubes by re-heating the getters.
Has anyone tried this?

-Dave Drumheller, K3WQ