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the controversial 12au7

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In Reply to: RE: Pinging airtime and others regarding the 12au7 and its favored/disfavored applications posted by JoshT on March 15, 2023 at 08:15:58

the waveform produced can run 'distorted' if implemented with the non-linear portions of the curve. It doesn't sound distorted per se, just 'non-linear' when run into the extremes of both ends. How to avoid this depends on the circuit requirements.

I suspect most designers know this but use the tube because of costs - it's a lot less expensive than a 12sn7. It has a sound that is 'compressed' in comparison and some might like that, others won't. If you are a dynamics fan with classical and jazz, you would prefer the 12sn7 for fidelity. Strictly rock music might prefer the 12au7.

Follow Ups:

RE: the controversial 12au7 - Tre' 10:27:08 03/15/23 (1)
"the waveform produced can run 'distorted' if implemented with the non-linear portions of the curve."

I have looked at the plate curves for the 12au7 and I can not find a linear portion. [that is not true, there is a limited area where the tube could be used and give reasonably linear results as long as you have a horizontal (high impedance) load line] The grid line spacing it wide to the left and narrow to the right for the entire width and that will lead to high harmonic distortion.

The 12au7 works great when there is a lot of feedback applied, like in a cathode follower circuit.

As a gain stage, when there won't be much feedback applied, they should be avoided.

For comparison take a look at the 6sn7 (or 6cg7) plate curves. Do you see how the grid line spacing is so much more even and how they don't scrunch together as you move to the right and down? With this tube you can run at many different operating points and get a linear (low harmonic distortion) result. You don't even need a horizontal load line to get good results, although a horizontal load is always better.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
- know all about it as discussed before - Story 10:33:53 03/15/23 (0)
  and people use it for the reasons I said, popular with guitar amps
  
  I don't have anything 12au7, but I do have 12sn7 and 12sl7's in a few pieces
Interesting. Thanks! nt - JoshT 09:45:46 03/15/23 (0)
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