I was reading in Russian forums, that these tubes need to be matched, a pair from around 20 pcs. Also they tend to have microphone effect, that is also resolved by selection.
I and a friend of mine have messed around with these tubes.
I bought a batch of maybe 10, something like 15 years ago. I found that they do tend to be microphonic in the audio range. I had to select for quiet ones. I got a couple pairs that were close enough in gain and gm to be used as stereo pairs.
My friend bought a batch about 5 years ago. He's used them more than I have. He says the ones he got are microphonic when used pentode, but once he triode-strapped them they were quiet. He likes them as the input tube in a phono preamp (used with an FET pre-preamp for MC carts).
Merlin Blencowe mentions in his Designing High-Fidelity Preamps book that the two cathode pins need to be strapped together or you get weird effects. Don't try to strap the suppressor grid to one cathode pin and connect the other cathode pin to the cathode load resistor or ground.
They do have a broad range of gain/gm. Fortunately they're pretty cheap, so you can buy a bunch and match them up.
A similar type is 6J49P-DR. Ale Moglia wrote about it -- 6Z49P-DR/6J49P-DR Pentode – Bartola(R) Valves
I hope that helps.
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I bought a batch of maybe 10, something like 15 years ago. I found that they do tend to be microphonic in the audio range. I had to select for quiet ones. I got a couple pairs that were close enough in gain and gm to be used as stereo pairs.
My friend bought a batch about 5 years ago. He's used them more than I have. He says the ones he got are microphonic when used pentode, but once he triode-strapped them they were quiet. He likes them as the input tube in a phono preamp (used with an FET pre-preamp for MC carts).
Merlin Blencowe mentions in his Designing High-Fidelity Preamps book that the two cathode pins need to be strapped together or you get weird effects. Don't try to strap the suppressor grid to one cathode pin and connect the other cathode pin to the cathode load resistor or ground.
They do have a broad range of gain/gm. Fortunately they're pretty cheap, so you can buy a bunch and match them up.
A similar type is 6J49P-DR. Ale Moglia wrote about it -- 6Z49P-DR/6J49P-DR Pentode – Bartola(R) Valves
I hope that helps.
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9KC6 with +12V on grid 3 gives you almost an equivalent tube with nice square pentode knees. (gm increases from 24000 to around 35000 with the +12V on grid 3 to reduce the suppressor strength) These used to be available for $1 each, until the $1 list disappeared along with ESRC last year. Vacuumtubes.net lists them for $3. They had them at $1 once also. So they must have some quantity.
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