You might try making the same post at http://www.otlamp.com/ which is the website for Atma Sphere owners. I am sure someone there will give you the appropriate feedback on your question.
Depending on which amp, i.e. the larger number of output tubes used, the hotter the amp runs. I have the MA-2's which consists of twenty 12AS7's per mono-block. Hot, at times, is an understatement. I live in Wisconsin, so a good portion of the year temperature is a non-factor. The only other A-S amps I have heard are the MA-1's. Fewer tubes, less power, less heat.
Regarding tube replacement, the drive tubes, 6SN7's have a reputation as long running, consistent sounding and reliable; especially NOS. The outputs....not so long, but not all that expensive.
My amps, nor the other two I have personal experience with have ever had reliability issues. Do take Scifi's suggestion and research either ASOG or threads on Audio Asylum.
The amps are very impressive. Well worth investigating.
I have M60 II.2's and use only half of the output tubes since 30 Wpc is okay for me. Not much heat--no problem with the amps sitting behind the speakers a fair distance away from the listening area. Of course there would be more heat with all eight output tubes. You have to balance them and adjust bias once in a while, and need a multimeter ideally since the built-in meter is said to be poor, and that's probably beyond the capabilities, and also not to the liking, of a nonaudiophile. But hey, that isn't you, is it? After all, you posted this query on Audiogon.
The Atma-Sphere amplifiers are totally stable and absolutely reliable. The output tubes are very rugged tubes and will run for a long, long time. When replacement time does come, they are not terribly expensive and they do not have to be matched. A-S sells them at very reasonable price.
The bias on the amps remains very stable, in my experience. Over the first 30-days, you will need to monitor/adjust it every few days, but once it settles in, it will stay very stable.
All of the Atma-Sphere amps operate in Class A. Any output device run in Class A, solid-state or tube, throws off a fair amount of heat. How much heat depends on which model amp you're talking about due to the varying number of output tubes. The M-60s are modest heat producers compared to the MA-2s. The MA-2s, which I own, will definitely warm up a room.
I agree with the recommendation to visit the Atma-Sphere Owner's Group web site, www.otlamp.com The folks who post there would be happy to answer other questions you may have.
The Atma-Sphere amps do generate more heat than most amps, comparable to any Class A amp of a similar power rating. The big MA-2's dissipate 800 watts per chassis (idle; a bit less when playing loud), so a pair of them are like having a 1600 watt space heater running wide open. Down in my homeland (Louisiana), that's really too much heat to be practical.
As you can see from Tom's post, you can use less than a full complement of output tubes (assuming that works with your speakers). And of course the lower wattage models don't put out the heat that the big ones do.
Occasionally a tube goes bad, but in my experience no moreso than in a push-pull tube amp. The only time I've ever had a tube failure is when I made a mistake and biased the tubes too high. Ralph warns that setting the bias too high will shorten tube life, and might I add, spectularly so! Flash/pop tinkle. But there was no damage to the amplifier; I popped in new tubes, corrected the bias, and haven't had any problems since.
What speakers were you considering driving with Atma-Spheres, and which model amp were you looking at?
Best of luck to you,
I also run the AtmaSphere M60 monoblocks. I have always run them with a full complement of tubes.
As far as heat goes, I am not sure if any amps exceed their capabilities to heat a room. Yes, they run hot, hot, hot. As Rush pointed out, they are pure Class A. That means heat and a lot of power draw.
The only other negative I can think of is that they do not grab hold of a woofer's voice coil and shake it to the ground. Although the bass they do put out is nicely extended, and exceptionally tight.
Also, some may find the sound a bit too clean and pure(I have a friend who feels that way).
On to the positives.
You ask if they are stable. Well, how do you feel about the anecdotal stories about Ralph Karsten shorting the amps with a quarter when asked this question at a show, only to have the amp not harmed in any way? I can tell you that while I have never witnessed Ralph do this, I believe it to be 100% accurate. I am embarrassed to say that more than once, while moving the loudspeakers around to get that last bit of performance, I have discovered no sound coming from a speaker. Only to realize the reason was that I have now shorted the stiff loudspeaker cables on the speaker binding posts. As soon as I correct the situation, all is fine, and the music resumes. Are there many other amplifiers, OTL, tube, or solid state that are that bulletproof to the point of not even blowing a fuse???
Next to an autobias amplifier, this is as easy as things get. I can literally bias both of my amps in about 2 minutes.
Ralph's mantra is the "20 Year Rule". What he builds should be around for at least twenty years. Every element in his thought process follows that rule. Hence, his choice of the 6AS7G output tube. This tube has earned tremendous respect in the field by the Eastern Bloc countries who have used it in their military applications. I know that from a Russian person I work with. They don't have much in the way of a more rugged tube. As has been mentioned, in the AtmaSphere amps, they can go for a long, long time.
The tubes are also very low cost. I have routinely seen dealers sell them for between $6 and $15 new. Although, the more official price of the Sovteks are about $16-$18 and $22 - $26 for the Svetlanas. Please note also that there is no difference whatsoever in the two brands, they are manufactured in the same plant. Some feel that the Svetlanas are a bit better due to going through more QA/QC, but no one knows if this is really true. Supposedly, people who have A/B them in every way can see or hear no difference at all.
In summary, I am not sure one can find a more rugged tube amplifier than AtmaSphere.