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Tube diode for cathode bias?

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I know the datasheets for tubes with integrated diodes (say 6sq7) recommend cryptically against doing this. "Diode biasing of the internal triode is not recommended". Might be just fine for two seperate tubes though, since 6sq7 and its like share a cathode (weird dc feedback conditions in that case, I imagine).
 
The dynamic resistance of vacuum diodes is fairly high, which makes them pretty lousy constant voltage sources.

not only high Rdyn, but nonlinear with current ;-)

Here's an example of this being done:

Axiom 300B Tube Amplifier Schematic

They claim that the nonlinear dynamic resistance of the vacuum diode results in some distortion cancellation.
 
not only high Rdyn, but nonlinear with current ;-)

Here's an example of this being done:

Axiom 300B Tube Amplifier Schematic

They claim that the nonlinear dynamic resistance of the vacuum diode results in some distortion cancellation.

Actually I would expect the distortion generated in the stage to decrease with increasing dynamic resistance, since dynamic resistance in the cathode is a form of negative feedback.

But of course that has nothing to do with whether or not it's nonlinear. That is a claim that would need some more explanation at least for me.
 
Well that's a good point.

300B Axiom Single Ended Amplifier

The claim is that the dynamic resistance is less than the equivalent static resistance, not that it is nonlinear per se.

However, it's still not clear to me how it lowers distortion other than through simple degeneration. The vacuum diode's decrease in dynamic resistance with increased current in the triode's cathode branch should have an additive effect with the triode's increased gm with increasing current i.e. more nonlinear current swing, more 1st order distortion.

Clearly more thought or explanation is needed here...
 
Well that's a good point.

300B Axiom Single Ended Amplifier

The claim is that the dynamic resistance is less than the equivalent static resistance, not that it is nonlinear per se.

I'm confused. If this is the goal of the author, there are better devices around (LEDs for one).

I can accept his statement where he says he prefers the sound of the vacuum diodes. But the above claim doesn't make much sense to me.

However, it's still not clear to me how it lowers distortion other than through simple degeneration. The vacuum diode's decrease in dynamic resistance with increased current in the triode's cathode branch should have an additive effect with the triode's increased gm with increasing current i.e. more nonlinear current swing, more 1st order distortion.

Clearly more thought or explanation is needed here...

Yes...

-Kenneth
 
I tried this with the wonderful 6CN7 (shhhhh), the hurdle is finding a matching diode drop. While I can see how, if the phase relationships are right, injecting 2nd harmonic into the cathode can lower the overall THD number it must come at the expense of significantly raising higher harmonics. I'll take the 2nd.
 
"Clearly more thought or explanation is needed here..."

I think I can give a theoretical explanation for the reduction in distortion with the diode/cathode setup. But I have not seen or done any actual test to prove it.

The triode Mu derives from the ratio of two nonlinear internal gm's (or 1/gm resistances) The grid 1 gm and the plate gm. Ideally, these would both be 3/2 power V/I non-linearities. The Mu would stay constant (with a CCS load) if these then track. Unfortunately, the grid 1 gm suffers from "island effect" due to very near proximity to the cathode (higher gm tubes more so for a similar grid design). This makes its variation closer to 4/2 power, as seen in power tubes with maximized gm. This mis-tracking then causes the Mu to vary with current (the tilting plate curves too).

The diode in the cathode circuit adds in a clean 3/2 power resistance (adds in series to both gm1 and plate gm), which dilutes the 4/2 power g1 mistracking back closer to 3/2 power. Since it affects gm1 more than plate gm, it also lowers the Mu and the effective gm1 of the tube. And the diode needs to have the right voltage drop at idle for the desired bias.

One could I suppose use a CCS on the diode/cathode conn. to get the diode to bias up further. An awfully complicated way to bias then.

Don
 
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