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In Reply to: RE: What do you suppose is the ave. and max THD for good/great commercially sold DHT SETs? posted by Paul Joppa on May 23, 2009 at 13:15:39
Both Donald North and Thom_M make good points. In fact, two stages of purely second harmonic produce both third and fourth harmonic. Certainly in a two-stage amplifier (all triodes, no feedback, etc..) there will be some harmonic cancellation unless the first stage has no distortion at all! Philosophically, I prefer the idea of keeping the driver distortion as low as possible in the hopes that the distortion signature is dominated by the output triode.
In practice that's hard to do, since hardly any suitable driver tubes are anywhere near as linear as the classic output DHTs (45-2A3-300B-845). And often there are other reasons which affect the choice of driver tube. This is the case with my own use of the 12AT7, not a very linear tube actually and not what I might have chosen from a purely distortion standpoint. But then, all good engineering is a balance of very many considerations, not all of them even conscious much less analytically measured!
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