Bottlehead crack 1.1 power tube glowing blue

[Hmmmmm]

I have a tube that has started glowing blue, has a white powder all over the inside the top is still shiny and silver, probably lost vacuum just wanted to know if anyone knew what it was


Jacob

[Doc B.]

Typically a deep violet blue glow in the glass itself is just fine. It's just elements in the glass envelope flourescing. However if the glow is like a nebula inside the tube and more pinkish it is a bad tube. The white powder is usually gettering (the silver coating) that has absorbed gas. If the tube is gassy you may find that the getter has continued to turn white since you posted. Sounds like a bad tube with a slow leak.