Tube diode for cathode bias?

[phrarod]

I'm thinking of playing with tube diodes as cathode bias in my preamp. I've read good things about it. I'm wondering if anyone has experience with this practice. I would be very interested to know what the results were.

[tomchr]

The tube rectifiers I have dealt with (OK, not that many but still...) have had voltage drops on the order of 8-10 V. I think you might be painting yourself into a corner with that... Check the load line on the plate curves of your input tube with a Vgk of 10 V.

~Tom

[m6tt]

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).

[kavermei]

The dynamic resistance of vacuum diodes is fairly high, which makes them pretty lousy constant voltage sources.

[kevinkr]

Here's a possible use for 6AL5/12AL5, note kavermei's caveat would apply here too of course.

LEDs (IR and conventional), schottky diodes, and small signal silicon diodes like the 1N4148, 1N4150, and 1N914 all work reasonably well. (Better than tubes IMO)

[Michael Koster]

Originally Posted by kavermei

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.

[kavermei]

For octal fans there is the russian 6H6S (Cyrillic: 6X6C).

[kavermei]

Originally Posted by Michael Koster

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.

[Michael Koster]

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...

[kavermei]

Originally Posted by Michael Koster

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.

Originally Posted by Michael Koster

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