I strayed over into the audio threads...

[Pete Deksnis]

and came across a cool thread about heat. Dear to my heart it is, and so I paste my response here fer u'all:

Intriguing thread idea...

Although my high-power tube audio days are over (Citation IV and V), and have shifted to vintage color TV (36 tubes in a CT-100; 410 watts at 115 Vac) I do get concerned about stress on the old sets anytime I run one for more than a few minutes, summer or winter. Never worried about the 12AX7's in my Citation preamp, but like the Citation V then, I still gauge equipment stress indirectly by the warmth of the power transformer: if it's quite hot to the touch, it's been on too long in that given environment. It's a subjective assessment to be sure, but one I've used over the years to coddle important tube-based electronics. My philosophy here is steeped in troubleshooting high reliability tube-based military electronic systems in my youth; you could grab any power transformer that was in 24/7 service and just have it warm to the touch. Two basic reasons: it was designed (over-designed) to run that way and there was forced-air cooling everywhere (even refrigerated when necessary).

HK Citation amps and preamps were very well designed and built, as they used late fifties-style military terminal-board construction and IIRC the PA stages ran in AB, so the load on the power transformer and supply components was not constant. Nevertheless, in the heat of summer or a hot apartment or home, I did and still do the transformer-heat touch test. And finally, the RCA CT-100 was not designed to mil-spec by a long shot, and so the touch test is more useful today than it was for my Citations.

Pete