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Old 29th November 2012, 10:00 PM   #101
DF96 is offline DF96  England
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Originally Posted by Rod Coleman
I have in my possession a number of ex-British Post Office STC 3A/109s. These DHTs were especially designed for telephone repeaters, and the samples I have were made in 1968.
This appears to be another 1930s low mu design? The 4021A data sheet is dated 1937. This is equivalent to 3A/109A. Presumably the 3A/109 is slightly earlier.
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Old 29th November 2012, 10:04 PM   #102
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Originally Posted by Merlinb View Post
Also, the 300B needs about twice the drive voltage of the KT88, so really we have only shifted the linearity problem from the power valve to the preamp.
Yes. The reductio ad absurdum is a cathode follower output stage.
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Old 29th November 2012, 10:40 PM   #103
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This discussion of low mu triodes being more linear than otherwise equivalent higher mu triodes is new to me. We can all easily name several exceptions (12AU7, 12AX7, etc.) so I wonder what's the theoretical underpinning.

Thanks for any help,
Chris
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Old 29th November 2012, 10:41 PM   #104
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Originally Posted by Chris Hornbeck View Post
This discussion of low mu triodes being more linear than otherwise equivalent higher mu triodes is new to me. We can all easily name several exceptions (12AU7, 12AX7, etc.) so I wonder what's the theoretical underpinning.
Geometry. Fields and distances.
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Old 29th November 2012, 10:42 PM   #105
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Well, remember that low mu triodes have higher feedback intrinsically. High mu triodes can have the same feedback applied externally to reduce the mu to the same value, and generally the result is better linearity.
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Old 29th November 2012, 10:42 PM   #106
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Originally Posted by Wavebourn View Post
Geometry. Fields and distances.
You're not giving me much to work with here....

Thanks,
Chris
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Old 29th November 2012, 10:44 PM   #107
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Originally Posted by Chris Hornbeck View Post
You're not giving me much to work with here....
It's the starting point. And I believe, the major one in the question.
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Old 29th November 2012, 10:47 PM   #108
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Originally Posted by SY View Post
Well, remember that low mu triodes have higher feedback intrinsically. High mu triodes can have the same feedback applied externally to reduce the mu to the same value, and generally the result is better linearity.
I could more easily accept a correlation with 1/gm than with 1/mu. Here's a pair that's observably the same except for grid pitch: the 211 and 845. What can we conclude from that pairing?

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Chris
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Old 29th November 2012, 10:47 PM   #109
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You're not giving me much to work with here....
That's the origin of the intrinsic negative feedback.
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Old 29th November 2012, 10:53 PM   #110
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The low mu theory doesn't explain the poor performance of the 12AU7. And the EL84 has relatively high mu for an output tube yet is still quite linear.
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