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Old 6th August 2008, 02:56 PM   #11
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"According to Max Robinson, use of a CCS in the tail of a tube-based LTP allows NFB to be returned to the second grid without problems."

With the lowish loop gain used in tube amps, the input diffl. pair has to work over a larger region of it's characteristic curvature (than SS amps.), so matching of the tubes will be important so they both operate with equal currents and equal transconductances.

The CCS will need to be a cascoded one to avoid variable junction capacitance causing a "transistor sound" insertion at the tail. Even with ideally matched tubes and CCS, the diffl. pair still generates a little odd order distortion at signal peaks as its composite gain drops off a little.

Resistor differencing to the same input (ie, inverting node summing) avoids this odd order problem. But input Z goes down of course.

Neg. feedback to the cathode also has some subtle distortion concerns, due to the input grid/cathode transfer function not being perfect. In SS designs, neg. feedback to an input emitter or source (so called "current feedback" amps) are known for having poor distortion characteristics. But tubes have a more gentle non-linearity characteristic.

Don
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Old 6th August 2008, 04:07 PM   #12
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Whatever distortion the cathodyne has is minimal since, like a cathode follower, it has a large amount of negative feedback.
I'll 95% agree. But if one makes the total load too small and the mu is too low, it can have appreciable distortion. It's easy to get it right, though; to a good approximation, one can estimate the distortion by looking at the same tube in common cathode, with the total load in the plate circuit. That distortion is then divided by mu + 1.
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Old 6th August 2008, 04:51 PM   #13
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Whatever distortion the cathodyne has is minimal since, like a cathode follower, it has a large amount of negative feedback
50% iirc

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Old 6th August 2008, 08:41 PM   #14
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Originally posted by SY
I'll 95% agree. But if one makes the total load too small and the mu is too low, it can have appreciable distortion. It's easy to get it right, though; to a good approximation, one can estimate the distortion by looking at the same tube in common cathode, with the total load in the plate circuit. That distortion is then divided by mu + 1.
You have the same problem with cathode followers. The CF makes a good Lo-Z source for a Hi-Z load. The more heavily you load it, the less NFB you have, hence more distortion. With a cathodyne, it shouldn't be all that difficult to ensure it looks into a Hi-Z load. That's what the Williamson does.
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Old 6th August 2008, 08:51 PM   #15
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Originally posted by ray_moth
According to Max Robinson, use of a CCS in the tail of a tube-based LTP allows NFB to be returned to the second grid without problems.
That's what I did with one design: cascoded 6BQ7s to form an LTP with cascoded BJTs in the tail for an active load. Connected the gNFB to the grid opposite the signal input. There were no problems.
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