Michael A. Terrell wrote:

There are a lot of things that are involved in MTBF.

Yes, indeed.
But I guess that we are interested in those which we can influence.
Like e.g. (?)
- heating before applying anode voltage,
- avoiding long periods without anode current,

Useless on low power vacuum tubes that are not used in some bizarre
circuit. Its a different matter on high power transmitter tubes.

Your kind posting shares a property with many usenet postings:
It provokes the most valuable question: "Why?".

Regards,
H.