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!