Burroughs long-life tubes (5092, 6091): Will they last forever ?

[greg...@hotmail.com]

Going thru my growing collection of tubes, I have not come across any of the long-lifer's that are showing signs of failure or aging. The original Burroughs datasheet extrapolated lifetimes of up to 500,000 hours, which really makes me wonder how long these things will last. I've probably got 60-70 of these guys, most of them used with no idea of their history (other than their date-codes from 1963-1967), and they glow perfectly. Has anyone experienced one of these actually failing after longtime usage ?

I have a few 5031 tubes (also from Burroughs, apparently an earlier version of their long-life tubes) showing signs of cathode poisoning (dark areas, glowing interconnects), and I've had some amount of success 'cleansing' them (even 'dead ones with high current.... they get pretty warm....). But I suspect from the lack of purplish/pinkish hue the 5031's have little or no mercury. If someone has a detailed explanation why mercury improves lifetime, please post.