I'm looking at using an interesting/inexpensive triode/pentode dual in a passively equalized phono application. The tube is fairly unique in that it has a high conductance pentode and high transconductance triode in the same envelope. No names, yet, as I haven't done my lifetime buy... Anyway, I have the choice of using the pentode section triode connected (no characterization for that duty as far as I can see, so I'm on my own) or as a pentode. The transconductance is pretty high, so I would have to use a fairly low value of plate resistance to get the gain I want for an RIAA passive first stage (20-25k umhos at 20 mA plate current). For a first stage gain of 40-50, this translates to about 2k plate resistance. Is this kosher with a pentode in terms of distortion? The high plate current means that the signal current is a small portion of the quiescent plate current. This sounds like low distortion, but is the low value of load resistor ok? Pentode gurus please chime in on this one. I realize that the pentode will also have some issues with partition noise, but this is a MM or high output MC application, so I don't have to be an absolute fanatic about noise.
Dumb question: did you run a load line on the curves to see how linear it would be? I suspect that at inputs this low, linearity wouldn't be a problem anyway.
Since I'm considering this tube for the input, it's pretty much a load point rather than a load line. I plan ~200V B+, which puts the Vp for 20ma quiescent current at 160V, a pretty usual-looking operating point, and not anywhere near any funny business. The other alternative would be to strap the pentode as a triode with current source loading. I may try both to see which sounds better. I may not be able to tell the difference...
Uh-oh, I have a suspicion that we both might be playing with the same tube. I found one that fits your discription, but I only bought a few last year. It looks like I might need to get more soon. I haven't tried the pentode with a low value of plate resistance yet, but I have tried it with a heavy dose of Schade.
I was experimenting with some rather unusual circuits and this tube is the only one that works so far. Those experiments got voted off the workbench a few months ago, but I did have a crude breadboard setup that could be used to measure distortion, if I can find it.
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