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In Reply to: RE: the 12AU7 as a design has poor distortion performance. posted by Eli Duttman on July 07, 2015 at 18:58:45
"The non-linear 12AU7 makes a poor voltage amplifier."
Just curious... do you feel the same about 12AU7s used in Williamson-type designs where the amp as a whole is "linearized" with NFB? I wonder how important individual tube linearity really is in such a case. Commercial manufacturers have used many tube types through the years that I wouldn't personally consider desirable in an audio amp, but apparently with little or no degradation to specs or sound. Again, I'm only referring to amps with NFB around those tubes.
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Follow Ups:
In something like the Heath W5M, 'U7 non-linearity may well be of little consequence. Still, less open loop distortion has to be better. FWIW, focus on the HF error correction signal, which (if substantial) can induce slew limiting.
BTW, I calculate that the W5M has 800 mA. of spare heater current. Pull those damned 12AU7s and install pin compatible 12BH7s. Of course, minor Noval socket wiring changes allow use of the 6CG7/6FQ7 and the 6GU7.
Eli D.
" less open loop distortion has to be better. "
Yes, that certainly seems a logical conclusion. However, I was also thinking about this issue from the perspective of the non-linearities of pentode output stages that have to be assuaged by the loop. Relatively speaking, the 12AU7s may contribute so little to that mix as to be really insignificant. Of course, this might not be true if the outputs are triodes. In that case, the degradation created by the 'AU7s might be plainly audible. I admit that I haven't seen or built a Williamson design like that (triode outputs) in quite some time. The temptation of creating more power for free always sucks me in. It would be interesting to know whether the 12AU7's deficiencies are audible in a UL design like the one you posted. The reduction in GNFB that often accompanies UL might allow better tubes to shine through.
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Buy Chinese. Bury freedom.
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