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

[Tubejunke]

You mentioned having another tube which the tester worked fine on. If that means you have another set, just position them butt to butt and hook up the socket from another set. See what happens.

B.T.W. I am fairly certain that Mr. Thomas Edison had some of the same questions with the early incandescent lamp. A quick read on that would answer questions about why heaters burn out in open air. That aside, I don't get why a tube without open heaters and gone to air would do much of anything more than give you a momentary light show before pretty quickly opening up presenting NO load to the test instrument.

Perhaps these old things are more robust than I give credit for. Phil Nelson's story contradicts my thoughts all together. I don't think anyone mentioned the number of times these tests were done; or duration. Seems like it would be a one time thing....