Blue glow with KT66 tubes

[BrownSound1]

I would imagine that tubes have argon or nitrogen in them as an inert gas, because like someone pointed out, oxygen will cause the internal components to oxidize and fail...especially with the presence of heat.

What one should consider is that there is no way to absolutely get rid of all oxygen from a tube...unless you are in outer space. Basically what happens is the tube will be evacuated, then backfilled, and repeated until the oxygen content is within an acceptable range.

Convection heating may or may not be a factor with tubes, as it is dependant on the amount of pressure within the tube. If the tube is at a relatively low pressure, say 20 Torr (millimeters of mercury) then convection is out the window, and you only have radiation happening. If a tube is at 500 Torr, then convection would take place. FYI 760 Torr is atmosphere at sea level.

Now, I'm not an expert on tube manufacturing, but I do work around a lot of vacuum related stuff. These are general things that effect all vacuum based systems.

Something else to think about is leak up rate. Glass cannot keep oxygen out of a vacuum forever. Oxygen will eventually get in and start the oxidation process, even though it might take years to do so. When you say you see a blue glow, that almost makes me thing that some oxygen is present. Usually metals either turn black or blue after being extremely hot and oxidizing.

[BrownSound1]

Here's some good info I found...

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

I knew most of the other, but I wasn't aware that some tubes get pulled to a vacuum of 1 microTorr. That is a VERY low vacuum. I work with some equipment that pulls down below 100 milliTorr (.100 Torr). Hell that's a tenth of a millimeter of mercury, and a microTorr would be .000001 millimeters of mercury.