Part of the reason for their long life was derating - notice that all the "special red" tubes have lower voltage and power ratings than the base tubes. They probably were selected for close tolerance, possibly had some different materials for longer life.
It is not really derating, but that the operating point is tightly specified. So, if you follow the recommended operating conditions then the 10K hour lifespan can be expected.Was going to start a thread on this same subject, I just ordered a set of NOS RCA 5693's on Ebay for my MP301. Might as well revive this old thread.
I know that the Bendix tubes even show a chart with expected lifespan at certain operating conditions. With the expected longest use at the conservative conditions and lesser hours as the voltage and current increased. The filament voltage is also specified to be within 3 or 5 % of nominal.I've had my MP301 since 2014 and still using stock preamp 6j8p tubes so i'm wondering if i'll be able to tell a difference in sound. I always order extra tubes so I guess i'll be good for 20,000 hrs. I will be using KT88's and EL34's so this will be new for me.
I know I have been on the look out for awhile and bought 4 as I only need 2.5693 tubes were ultra ruggedized build to withstand outrageous conditions and still work. If you can find this type, hang on to it. Nobody will ever make anything like it ever again.
The tolerances are tighter and the physical structures are more rigid and better supported than its non military counterpart. This may result in closer matching and lower microphonics. Ymmv. And did I mention that these will never ever be produced again at any price...