Tim said : I have looked through my personal library of vacuum-tube era technical books, and have not been able to find any quantitative discussion of the lifetime of heaters (in indirectly-heated cathodes) powered by DC or AC.
me too, I might add almost : ALL real sensitive front stages run at DC and 50-60 years later, those tubes work just as good as any AC driven heaters,
it is really need to have ONLY dc into the first stages of low frequency range equipment, look at RIIA and such as a very good example.