A tube is a light bulb with some extra wires in it. In most tubes the filiament glow can be seen. It glows red rather than white (like a light bulb) because it's purpose is to make heat, not light.
If there is a true vacuum in the tube it will not glow. If it does it means that a trace of air is leaking into the tube, and it is not going to last much longer.
I understand that some tubes have some gas (not air) in them by design. This gas may be excited by the electron flow, and glow.
Also, it's been mentioned that some glass has trace elements in it which glow when the tube is operating. This may be deliberate on the part of the tube manufacturer, because it looks sexy. I way about to say "it certainly doesn't affect the sound", but on this website...?
If there is a true vacuum in the tube it will not glow. If it does it means that a trace of air is leaking into the tube, and it is not going to last much longer.
I understand that some tubes have some gas (not air) in them by design. This gas may be excited by the electron flow, and glow.
Also, it's been mentioned that some glass has trace elements in it which glow when the tube is operating. This may be deliberate on the part of the tube manufacturer, because it looks sexy. I way about to say "it certainly doesn't affect the sound", but on this website...?