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In Reply to: RE: SET Power supply in series with output trans posted by cpotl on October 30, 2016 at 05:45:58
SETs in general are borderline AB - that is, while the output triode is never really cut off, its average current does increase with power. It's not as severe as typical PP amps, but it's not insignificant either.
For example, in the 1950 data sheet for the 300B (which is the one I have at hand this morning) there are two suggested operating points. One has 62mA no signal and 74mA at full power; the other is 60mA to 77mA.
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"SETs in general are borderline AB - that is, while the output triode is never really cut off, its average current does increase with power."
Isn't that another way of saying that SETs generate a large amount of even-harmonic distortion? To the extent that the average current increases with the power output, this means that the current on the positive halves of the cycle increases disproportionately more then current decreases on the negative halves of the cycle.
That is, it seems to me, the essence of even-harmonic distortion.
If the distortion is so great that the average current draw is increasing significantly with loudness, then one is probably talking not so much about high fidelity reproduction as about sound effects.
Chris
You can exaggerate the effect by reducing the quiescent current without changing the plate voltage or transformer impedance. There are some on this forum who like that effect.
It's a legitimate question whether the brief against even-harmonic distortion is due to the distortion itself, or the effect on envelope modulation. It will be a long time, if ever, before measurements catch up with psycho-acoustics.
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