Hi David, Your comment that the 6AS7 is not that linear is not really true. The curve has a lot in common with a 300b and also 2A3s; in that regard the 6AS7G is in good company. On that point the idea that the amp is class A due to the non-linearity of the tube does not hold up. BTW if you study the RCA documents that they released on the tube back in the early 50s, RCA called the tube an audio amplifier *and* VR (Voltage Regulation) tube. FWIW for best VR operation linearity in the pass element (in this case the 6AS7G) tube is important!
While power supply stability is important, I don't think I would say that is why you do class A. IMO, you do it because of the increased linearity of the device. Particularly in push-pull, very low zero feedback distortion levels are possible. Achieving low distortion without feedback allows for lower amounts of higher-ordered harmonics, which are loudness cues for the human ear. Sort of a have your cake and eat it too thing.
Our power supply voltages are not as high as you describe, and it is a fact that the tubes operate class A2, which is something that I am happy to point out should anyone ask; like, right now :)
In A2 there is substantial grid current during part of the waveform, similar to class AB2. Our driver circuit is designed to handle the current, similar to the way Fisher did their class A2 amp back in the 50s. The 6AS7G is remarkably linear in the A2 window, like a lot of power triodes are (we built a 300b OTL once just to see if there was any advantage; the 300b has a similar A2 window BTW).
If the bias and B+ points were a bit different (IOW if the tubes went into cutoff before clipping; right now they cut off only *after* the amp clips), the amp would be class AB2, not A2.
I am if the opinion that class AB is harder to design for and get right since the driver waveforms have to be larger amplitude and the driver power supply has to be more stable. If you are trying to set things up without feedback, which is what I would do, class AB gives you less opportunity for distortion cancellation so you really have to have your ducks in a row to make it work.
While power supply stability is important, I don't think I would say that is why you do class A. IMO, you do it because of the increased linearity of the device. Particularly in push-pull, very low zero feedback distortion levels are possible. Achieving low distortion without feedback allows for lower amounts of higher-ordered harmonics, which are loudness cues for the human ear. Sort of a have your cake and eat it too thing.
Our power supply voltages are not as high as you describe, and it is a fact that the tubes operate class A2, which is something that I am happy to point out should anyone ask; like, right now :)
In A2 there is substantial grid current during part of the waveform, similar to class AB2. Our driver circuit is designed to handle the current, similar to the way Fisher did their class A2 amp back in the 50s. The 6AS7G is remarkably linear in the A2 window, like a lot of power triodes are (we built a 300b OTL once just to see if there was any advantage; the 300b has a similar A2 window BTW).
If the bias and B+ points were a bit different (IOW if the tubes went into cutoff before clipping; right now they cut off only *after* the amp clips), the amp would be class AB2, not A2.
I am if the opinion that class AB is harder to design for and get right since the driver waveforms have to be larger amplitude and the driver power supply has to be more stable. If you are trying to set things up without feedback, which is what I would do, class AB gives you less opportunity for distortion cancellation so you really have to have your ducks in a row to make it work.