Great post there Joe. About your comment that Class A operation reveals lower-power figures that expected: You are absolutely correct that theoretically Class A should be able to yield higher power because there is no zero crossing. There are three main aspects, as I see it, as to why in reality it doesn't work out that way - and also how they impact tone because your comments about differences in tone between single-ended and push-pull are actually related.
1) Accurate low frequency reproduction is more challenging in tube circuits that slide into Class B because the power continuity gets broken when no tube is operating. This particularly impacts the low frequencies because they are the ones that need the most power. And they are the ones that mainly have to do with tone.
If I may use an automotive analogy, it is like comparing a manual transmission with a dual-clutch transmission, such as the ones in the latest Audis and BMWs. There is no interruption of power in the dual-clutch setups since there is always a set of plates that are transmitting full power. This is like Class A. But in the other case, you have to put the clutch in and the RPMs drop like a rock as you disconnect the engine (amp) form the transmission (speakers). This is like Class B. The end result is that tone (mainly bass) is much more easily conveyed in Class A operation since the power is always on tap. There is no denying in the world of automobiles that constant power leads to higher performance. Guess which mode of transmission is used in race cars.
2) The actual power ratings are lower than Class AB because of physical power dissipation limits. To operate with a constant bias requires high heat dissipation from the tube's plate. This would be fine except that the cooling medium inside a tube is vacuum, which is as bad as it gets! A vacuum is a terrific thermal insulator since there can be no convection cooling and thus all the cooling is due to "black body raditiation." I have better stop there before I get into the Second Law of Thermodynamics! lol. So yeah, to keep the tube from overheating you have to cut back on the output power. Basically you trade power for bias, which has of course its known merits in addition to this inconvenience.
3) The output transformers in a Class B amp are not designed the same as in a Class A amp. If the amp is Class AB, I am sorry to break the news to some of you but that means it is a Class B amp as far as circuit design goes. Many people feel it is better to say AB rather than B but as far as the electrical design is concerned, there is Class A and then there is Class B.
In a Class A amp, the output transformers have to have an air gap. This means that the core is not made of one continuous piece like it is in a Class B amp. This is due to the fact in Class A you have constant positive DC current in the primaries of the output transformers that should be equal to half the peak output current. In order to prevent the core from "overloading," technically called "saturation," you have to literally cut a slit into the core somewhere so that the excess magnetic field (that goes above and beyond satisfying the core's inherent magentic self-inductance which is what allows it to work in the first place) gets trapped in the air gap. Since air doesn't magnetically saturate, it is a stabilization method for the core.
In addition, this means that a transformer for a Class A amp must be a lot larger than a Class B one because you can't maximize the use of the core since the polarity is always positive, and, the air gap adds its own detriment to the performance of the core.
In a Class B (aka Push-Pull), you don't have to worry about these problems because of what I pointed out in 1) above: the net DC current in the output transformer is essentially zero because the tubes each turn off at the end of their respective cycles. They aren't "on" all the time like they are in Class A.
I feel certain that all these differences account for the changes in sound and tone between a Class A amp and a Class B amp, in addition to the differences in circuit topology of course. If you can live with reduced output power, higher temperatures, higher cost, and higher weight, then Class A has definite advantages. :)
I have two push-pull amps now and I am dying to get a Class A SET because I finally have speakers that can live with their downsides. Im still in the process of figuring out which one to get. As far as differences between different push-pull amps, that lays in the gray area of performance overlap between designs. Which is better will depend on the type of tube, the power supply design, circuit design, personal preferences, speakers, and room.
As I said before, the sonic results of all these technical details can only be adequately assessed if there is an experienced human in the feedback loop. Only then are all the requisite variables fully taken into account.
Arthur
