hi Ed. .... this question keeps coming up and needs a 'library card' for checkout ... a tube tends to lose vacuum with time passing ... either air seeps in around seals or is ungettered from metals in the tube .... so a long storage time tube needs to be tested under controlled condx to determine state of internal vacuum.
this test can be done in an amplifier if a means is made to control available power .... how many hams have plugged in a 'good' tube, hit the switch and heard bang and smelled smoke ?
so either make a tube tester or adapt means (ballasts, variacs, etc) In the power supplies to keep it in check