I also have a number of 5654/6AK5/EF95 tubes, pulled from RF equipment. Given that these are sharp cut-off rather than remote cut-off devices, they stand a fighting chance of making fairly linear triodes, which is how you’d wire them for lowest noise, as SY says. I haven’t tried them for audio, but they just might be worthwhile. The transconductance in pentode configuration is about 5000 uS at the data sheet bias values. With the screen tied to plate, that value would increase to over 6000 uS, a decently high value. Of course a 6DJ8 can double that value, but transconductance isn’t everything. The triode mu of a 6AK5 is around 20, going by memory, which might be wrong. Often, for the first stage of RIAA, you’d want a bit more gain to increase the noise figure, but a medium mu tube is still a possibility depending upon your other circuit choices. I believe that triode-wired 6AK5s were used happily in some tube microphones, but factors like physical size and heater power requirements might have also influenced those choices. Why not give it a try and let us know how it worked?
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