> 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.
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.
