Does this mean my 21AXP tube has gone to AIR???

[rca2000]

I spoke about this on another thread--but i am not sure anyone saw it.

I picked up that CTC 4 set in Wheaton Ill, On Sunday. Me and Doug Harland huffed and gruffed it out of a basement, up some difficult stairs. I hoped the tube was ok, because the owner says the last time it was on--it DID have a good pix, then lost vertical. BUT..that was about 40 YEARS or so ago, he thought. Still--I had high hopes, since my last 2 sets with AXP tubes have both had excellent tubes.

SO--I tested it yesterday on my CR-70. This tester works VERY well, and has NO trouble testing color roundie tubes. but on THIS tube...the fils. will NOT light...AND it seems to be OVERLOADING the fil supply on the tester--because the meter will NOT go above 4 volts, even WHEN I turn the fil knob to 8 volts !! Also--the tester is making a MUCH louder whine than usual. The neck IS getting pretty warm after about 3 minutes--but it will NOT glow at ALL.

SO--does this mean the tube is FULL of air? I take it to mean the fils can't warm up enough--for the current draw from them to drop to normal. But I do not really understand WHY the fils will not light-...if i air---even WITH full--or even excessive--voltage on them ?? I would not think a bit of air would cool them THAT much--to keep them from glowing--or WOULD it ?