The heat of tubes...

Although I've never been bothered by the heat of my tube amps in the summer - the AC's usually on anyway here in DC*, where it's almost always hot and humid during July and August - I would be wary of keeping my amps in a closet; I'd much rather that they make me a little hot, than for me to allow them to run in an environment even hotter still, which will rapidly degrade their operating lifetime. If the amp(s) can raise the temp of a whole listening room a few degrees, think how much higher they'll raise it in a small confined closet. Ventilate, ventilate...

*[Good grief, I can't believe it took me two long years to arrive at that inadvertant bad pun of abbreviations for air conditioning, The District of Columbia, and alternating- and direct-current!}

Peter, it's not necessarily the tubes that suffer most for being run in a hotter ambient environment, though it's good to keep them cooler as well - it's also many of the other electronic parts inside an amplifier, especially capacitors, which will have their rated operational life exponentially shortened by running them in a marginally hotter environment. Similar laws do apply to the tubes (and actually transistors as well) - that's why there are tube-cooler tweaks for sale - but at least tubes can be easily replaced (well, maybe not some NOS...).