Quote:
Originally Posted by maxhifi
...One strike against the 6AS7 for audio is it's not that linear - sure, all the second harmonic is cancelled by push-pull, so it still does decent with distortion measurements this way, but have a look at the characteristic curves, they are not evenly spaced at all, and lots of "knee" near cutoff. In comparison to a linear triode such as a 6SN7 or a 2A3, they are very compromised.
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The 6AS7 is not as linear, no, but the distortion that is does generate is mostly second and fourth harmonics, which get cancelled away as you said. In simulations using LTSpice, I'm able to get lower distortion from push pull 6AS7Gs than 2A3s, and for the simple reason that the 2A3, while producing less distortion overall, does produce more 3rd and 5th order.
You also have to keep in mind that the 2A3 is a filamentary type tube. There is much more 60 cycle hum to contend with than the 6AS7G, and that will mix with the actual audio signal to produce more IM distortion. The 6B4G is even worse, the 6A5G and 2A3H types are the only 2A3 variants, aside from the 6AS7G, that I know of that are relatively free of this pitfall.
Attached are some plate curves, the 2A3, 6AS7G, and 300B in that order, generated from Ayumi's extremely accurate tube models; they agree with the plate curves published by RCA and Western Electric. Sorry, they're kind of dark.