To re cap: Class A amplifiers –—single-ended or push-pull— suffer from maximum dissipation at idle. All the steady state DC power passing through the tubes goes up in heat. Only when that power fluctuates (like at A-440) does some of it convert to useful audio power to drive the speaker. Power that flows through the tubes and drives the speaker is power that is not dissipated as heat. Yet the dissipation rating of a tube is what limits its power capability much more than the useable power that can pass through it. Like the clutch analogy, it’s the slippage converting energy into dissipated heat that kills it, not the power flowing through it when it’s fully engaged.
Long before Class A power became sexy in the retro-vintage sense that we all dig, the direction of amplifier technology was to deliver more power with greater economy. There was no mystique about Class A, only an awareness of its shortcomings and a desire for more clean horsepower with less waste heat.
Other classes of tube operation were developed to do just that. Class B and Class C are both highly efficient and cool running, and work great for radio transmitters where vast amounts of power are required. These classes are biased so there is virtually no idle current, and nearly all of the juice passing through the tubes is converted to useful output.
Unfortunately, those classes aren’t suited for audio: Too much distortion caused by that “turning on and off” phenomenon. So designers in the late 1930’s started to come up with an exciting new configuration that helped overcome the waste of a Class A operation and the distortion of Class B. It’s the “push-pull” we’ve been talking about and it makes possible Class AB operation with high efficiency and low distortion.