Back then the Military had a great need and relied heavily on ruggedness as well as quality. Can't remember where I read it, but the article stated that they spend much less time creating the vacuum in modern tubes. Back then, an hour on the vacuum pump, now 20 minutes. I never had a nos tube that was dated before 1940's and they were great. If quality was waning then, hard to imagine how good the tubes from the 20's and 30's sounded.
