desirable characteristics for microphone tubes?

PRR · Mar 25, 2011

> when talking about pentode tubes wired for triode operation, the data sheets rarely will give numbers for triode operation. So would the information measured with the tubes operating as pentodes even be relevant at all?

Sure. Wire pentode. Measure. Now wire triode, same G1 and G2 voltage.

"Plate" current becomes the sum of plate + G2 current. If it was 10mA + 2mA, it is now 12mA. 1.2 times higher.

Gm is nearly the same amount higher. If it was 5000mMho, now it is near 6000mMho.

Triode Mu is often given on datasheet. For the types suitable for this use, usually 10 to 40. If not, see if a cutoff voltage is given. If it takes -13V G1 to cut-off when G2 is at 250V, the triode Mu is near 250V/13V= 20 or so.

General-purpose small pentodes are often specced for MUCH higher current and voltage than you would ever want inside a microphone. That's what they are good at, and what they were sold for. Also many pentode applications tolerate or demand remote cutoff characteristic; when you work far below the show-off condition extrapolation becomes dubious.

However.

If you want a very-good mike, many common tubes (even the ancestral 6AK6) can work well.

"Undocumented" aspects like microphonics (BTW, mentioned up at post #6) and leakage/hiss may be more important than anything you can gleen with your nose in a PDF.

If you want a very-very-VERY-good mike, buy a bunch of tubes, build a clean yet adaptable prototype, and try-try-try.

gemini86 · Mar 26, 2011

I'm beginning to think I should build my g7 with a trap door install, making it easy to swap out tubes.

Steph.B · Mar 26, 2011

Except electronic characteristics, don't forget that one of the desirable characteristics for microphone tubes is LOW MICROPHONICS !

To know if a tube has low microphonics is like gambling and one have to put it in situation inside the microphone and test it to hear if it rings or not.

Didn't know about the 5840/EF732, thanks for the info folks !

bezen4uk · Mar 26, 2011

Hi soapfoot,
There is a list of US submini tubes that worth to be tried with tube mics:
5703, 5718, 5897, 6946, 6814, 5977, 5904, 6K4, 6AK4.
Note, that many tubes are very noisy at nominal heater voltage. For some tubes it should be decreased to 70%.
Hope it helps.

soapfoot · Mar 26, 2011

Thanks again.

My prototype power supply has a variable heater voltage. For a tube drawing 275mA of filament current, I can adjust the heater voltage anywhere between 4.5 and 6.3V. I'll definitely be experimenting with all of this.