How hot is hot when moving from class a/b to to a or tubes?

My last pure class A speaker amplifier was a 1987 Classe Audio DR3 VHC. It was designed by David Reich (hence the DR prefix). The VHC stood for very high current.

The amplifier was a work of art that weighed in at just over 100 pounds. It cost close to $5000 in 1987 dollars and ran extremely hot. You could fry an egg on its dual transformer case. It also sounded great. However, forget about using it during the summertime though, as the heat put out by the DR3 VHC could drive you out of your listening room. I also have a pure class A Schiit Asgard 2 that I use to drive my planar magnetic headphones. While it’s built to a much less expensive standard, it also runs quite hot. It tends to be a finicky amplifier yet, with my inexpensive planars (Fostex T60RP and Hifiman H4XX) it sounds quite good. I do believe that pure class A (as opposed to sliding Class A) does sound better than class AB amplification. Just be aware of the heat put out by these amplifiers as well as their inefficiency. 😀

A true Class A amp will always run hottest with no signal. When playing music the current/voltage goes to the speaker and is dissipated there. The true Class A amp then runs increasingly cooler! A little-known fact that most audiophiles are unaware of! My Bedini 25/25 fulfilled this design aspect as a true Class A amp. In use it always started out extremely hot and ran progressively cooler as output level was increased. Stopping playing music it again got hotter until it reached its bias limit.

There are very few true Class A transistor amps. Most so-called Class A amps switch over to Class B after a few watts. A true Class A amp of considerable power will always run extremely hot under zero load and then progressively cooler as output level increases!

All amplifiers can reach their normal operating temperature, but class A amps in general may become warmer internally as if you are driving speakers for up to an hour. If speaker impedance dips, the amplifier will be tasked at supporting them with more current. 

Just make sure you use a Class A amplifier in a dry environment and leave vents unobstructed; so heat loss can occur normally. Otherwise, nothing to worry about...

Regardless of what tube amp you get, I highly recommend if you have kids and/ or pets to get one that has some type of cage around all the tubes! Better safe than sorry for a bad burn or blown amp. 
 

I have a Rogers High Fidelity RHF 200 Mark II w/ two KT - 150 power tubes per side that generate 100+ watts / channel in full class A. Great amp & love it but it gets real hit. I bought the very well made vented lexan cover because I have two very curious cats who gravitate towards warm things. You can still see the beautiful amp & it’s outstanding workmanship but it’s safe!