Does higher elevation equal running hotter?

[super&plexi]

Does anyone know of or have experience with an amp running hotter at a higher elevation. I'm usually around sea level but presently at around 6500 feet and noticed my rectifier tube gz34 glowing bright yellow on the plates and I don't think it usually does, at least that much. After playing about an hour the amp seemed abnormally hot, although the room temperature was a little warm to begin with. Wall AC was 121 and it's usually 119 -122. I haven't played the amp a lot in the past couple years, and do usually play it through a variac at about a hundred and ten, but definitely have played  it without the variac in that time and don't remember it being this hot.

[PRR]

Bright yellow on the plates is very wrong.

Thin air does mean less cooling, but not so you would notice (any altitude you can function at).