Just for your viewing pleasure… very cool
Top image is a pair of MV “Mercury Vapor Rectifiers”
Middle Image glow is caused by electron bombardment on the inside glass surface causing a fluorescent glow it has no effect on the operational performance of the tube. The tube is not “gassy” gas in a tube has many other effects and usually shows as a pinkish glow
Bottom image is a pair of high power transmitter RF tubes under full output load (they run really hot!)
“Tubeology” coming soon!