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

@mpoll1 My main concerns were: potential burns to the kids (probably their friends.

Two added points of information have surfaced here. And, if you plan to keep your nice L509x amp and rotate in a tube amp once in a while, going a new/lower power direction will be key for less heat and you can also look at integrated tube amps with an enclosure. Do you want to stare at tubes or keep them enclosed, separates or integrated all-in-one unit?

While some will push for the fleawatt single-ended-triode tube amps down in the 2-3 watt range, there are also some very nice EL84 based tube amps in the 20w range that can be nice too. Thinking if you want something [enclosed], we can also make a plug for this one, makes it nice for those starting out with tubes. Here is another to give ideas. Some used ones out there right now too. Another option, fwiw.

@jasonbourne52 

You are quite right about TRUE Class A. It does run hottest on no/low signals.

Some here have mentioned modern Krells, by which I mean post around 1990.  These do not run particularly hot on no/low signals.  How can this be?  it is because they are not TRUE Class A, whatever Krell may claim.  They run in Class B and have so-called anticipator circuits which sense an increased signal coming in and re-bias the amp to Class A and allow it to morph back to Class B when the volume has died down.

I have never discovered any scientific basis for the claim that this re-biasing can occur so quickly after detecting the increased signal that it can power the sound of that signal.  It seems impossible since the cable runs in the amp are relatively short and the signal is travelling in them at close to the speed of light.  A myth put about by Krell so they could play in Green Park while still claiming to produce Class A amps.  Yes they will exhibit patents but anyone can get a patent.  The issue is: is  the move to Class A completed before the detected incoming signal is amplified?  The patent makes no claim that it is.

No guys.  A TRUE PURE Class A amp like my pair of late 1980s Krell KRS200s, uprated by Krell to 400w per channel runs HOT.  And as Jason says, there aren't many of them.

I can feel the heat from my First Watt class A amp from about a foot away. It runs much hotter than my 300b tube amp. I can tell the difference in room temperature when the FW has been on for a while. 

My Mark Levinson 20.6 class A mono blocks pull 400 watts at idle and run 10x hotter than my Marantz 5 tube mono blocks.  I would swear there is a fan in the amps blowing heat.  I also noticed my electric bill goes up  $100 a month to run the 20.6's.  They are the best SS class A amps that I have ever heard.

Heat definitely varies from person to person and I think room size and location in the world play a huge role in understanding anecdotal information . I have an upstairs south facing room that gets sun for the entire day, I'm located in California, and is only 9x13 and is on the opposite side of the house as my AC thermostat. For me, a class A tube amp and a class A/B BHK250 got my room way too hot to be comfortable during the summer time. I'll be trying out class D next. My font end is all tube and surely didn't help either amp. We will see.