As Jmcgrogan2 mentioned, when military electronics, including radios, guidance systems and computers, were run on tubes, their reliability and endurance was paramount. Therefore, the tolerances, consistency and reliability of the tubes than ran the equipment were driven to the extreme. Hence, vintage tubes of the 40s, 50s, 60s and even 70s (particularly Eastern Bloc) were of excellent quality that cannot be matched today simply because modern military electronics don't require it, and therefore there's no large scale market to drive tube quality.