Why no love for 6GK5 as the input stage of a simple RIAA preamp? It seems to have the necessary gain, and also low grid-plate capacitance. It's a frame grid triode with high transconductance. It may not be very linear, but that's not the most important thing with such a small input signal. I've only seen one design using a simple common cathode 6GK5 as the input stage (Jeremy Epstein), but bazillions of preamps using ECC88/6DJ8/6922 as the first stage. Why?
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Very good question. 
As long as you stay reasonably close to the origin on the plate curves, the 6GK5 is quite linear. You get into trouble at the extreme, where variable μ kicks in.
BTW, Poindexter was hand matching 6GK5 pairs and using them in the LTPs of his "musical machine". So, the 6GK5 is good at medium signal levels too.
As long as you stay reasonably close to the origin on the plate curves, the 6GK5 is quite linear. You get into trouble at the extreme, where variable μ kicks in.BTW, Poindexter was hand matching 6GK5 pairs and using them in the LTPs of his "musical machine". So, the 6GK5 is good at medium signal levels too.
Because it's not fashionable and no longer made. I've known about this great little triode for years. It also has reasonably low plate resistance. And while I have dozens of them, please stop talking about them. You'll make them more scarce and raise their price.
You can get the frame grid 6JC6 for $1, without the non-linearity (Mu 60, gm 15K), so why pay 5 times as much for the 6GK5.
https://frank.pocnet.net/sheets/135/6/6JC6A.pdf
https://frank.pocnet.net/sheets/135/6/6GK5.pdf
https://frank.pocnet.net/sheets/135/6/6JC6A.pdf
https://frank.pocnet.net/sheets/135/6/6GK5.pdf
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