more reliable amp: tube or solid state class A

If you want an idea of what one renowned amp designer/builder thinks about Class A, the heat it generates, the effect on the amp's circuitry, etc, I would suggest a read of an owner's manual of an early Pass Labs Aleph amplifier (available, for example, here). I would never have thought an owner's manual entertaining but it is. One telling excerpt...

The amplifier does not require any maintenance. While the design is conservative, this is a hard running amplifier, as single ended Class A operation is the least efficient operating mode. In fifteen years the electrolytic power supply capacitors will get old. Depending on usage, you will begin to have semiconductor and other failures between 10 and 50 years after date of manufacture. Later, the sun will cool to a white dwarf, and after that the universe will experience heat death.

Amp reliability is determined by the level of conservative design, and this applies to tube and SS. The reason that SS class A amps tend to be more reliable than tube class A amps is that a SS amp will blow up immediately if it is not designed with adequate margin. With tubes, you can get away with marginal design for a few months before failure. Designers of SS amps HAVE to build in more safety margin. Class A tubes amps can be built to very high reliability levels, but they have to be designed very conservatively. I do not know of any class A tube amps in this category. For example, a 6L6 tube is rated at a plate dissipation of around 30-35 watts. They are typically run at this level. If I build a class A amp out of these tubes, I will run them at a dissipation of no more than 10-12 watts. They will run many years under these conditions. The amp manufacturer needs to use six of these tubes (per channel) to make a 30-35 watt amp reliable in a tetrode mode transformer or ZOTL coupled amplifier. More are required in an ultralinear or triode mode amplifier. Many more would be needed for a traditional OTL amp, assuming that these tubes were appropriate for that type of amplifier (they are not).

On the other hand, it is of questionable merit to demand a class A tube amp, as tubes behave much better than SS devices in a class AB application. SS devices have much more abrupt cutoff characteristics than tubes, and as a result, they inject high-order harmonics in the crossover region. Tubes have a very soft turn-off and as a result, they don't inject these harmonics to the same degree. The gain in going from class AB to class A in the SS amp is much greater than it is in the tube amp. An fact, the class A tube amp makes little sense, unless it is a very low power amp. The idea of making a hot amp in general does not make sense to me in unless it is strictly a winter amp and you can supplement the work that your furnace has to do. To use that amp when you need to apply air conditioning to cool it is stupid. How does the noise of the blower affect your room noise level and your attempt to achieve good sound? We all also need to think more responsibly when it comes to our energy use. We have limited resources and we may be doing damage to our climate with excessive energy use.

"i'm thinking the tube amp, solely b/c the tube is the hottest part, and its failure is accomodated for in the design (you simply plug in another tube). a hot running SS amp will eventually burn out resistors / transistors, and joe audiophile will be forced to send that to the factory for replacement."

You are correct. In addition, some solid-state amps use output transistors that go out of production, so if you lose a transistor, you're in trouble. Caps are no big deal for either SS or tube designs, as they are relatively cheap and easy to replace (you can easily get 20 to 30 years out of them on a very high quality tube amp, even designs biased in Class A). If reliability is your concern and you are looking down the road, buy a tube amp that uses the same output tubes used in guitar amps (KT-66's, EL-34's, 6550's), as the Marshalls and Fenders of the world sell half a million tube guitar amps every year and you'll always be able to find replacement tubes.

In any event, as you noted, a broken transistor amp has to be opened up. A tube amp just requires re-tubing and re-biasing, and once that is done, you've basically got a new amp.

regardless of class or bias, buy what works best with your speakers.