Grounding the cathode in a tube microphone

[vmanj]

I designed a popular mic that had a grounded cathode. It could use a 12AT7 or 12AY7/6072. The sound was great, the noise was low, and I never saw ANY issue with distortions associated with bias being insufficient. I asked David Bock and Oliver Archut about it and they both knew of mics in history that did the same thing. Try it, you might really like it!

Thanks, I’ll definitely try it with triodes too.

 

[LevinGuitar]

When I made experiments with grid leak bias, I liked it too at first, but it lacked more "punch"? that a C12 bias provided. For an easy experiment (not c12 bias) use a potentiometer on cathode and try some commonly used settings: 1.6v, 1.1v and the grid leak your tube. You'll see the difference

 

[vmanj]

Well, of course, before grounding the cathode, I did exactly that with a 0-10K potentiometer in the cathode and with various bypasses
capacitors.

Subjectively, the sound seemed more “bloated” and “powerful” to me.
But everyone has their own taste.

I decided to leave this for variety, in addition to my other “correct” microphones..

 

[terry setter]

vmanj - I hope you're enjoying your grounded-cathode mic. There's some chance that the "bloat" and "power" you mention were your brain's interpretation of the higher gain that a plate-loaded mic will have when reducing resistance between the cathode and ground.

I discovered the "grounded-cathode trick" by accident (like the guy who discovered the adhesive used on Post-It Notes). I would never have thought to even try it, because I used to assume that the distortion in such a configuration would be off-the-charts. When I realized what I had accidentally wired, I made some tests and even under VERY loud sound conditions (with a 797 LDC polarized at 75V driving a 12AT7) the results were very pleasing - to MY ears.

 

[vmanj]

I moved the cathode potentiometer outside the microphone, changed its values from 0 to 10K and compared the sound on the fly.
The character of the sound changes noticeably.

I have already tried it with different tubes 12AX.., EF86, EF95.
12AX..also interesting to my “ear”..
It is clear that the output transformer is not universal for all tubes.