@thor.zmt The high perveance 5687 (9H) would be more popular if it had a 12AU7 type (9A) pinout.
I would hate that. As is getting NOS 5687 is still afforable.
The SinoSov 6N6/6N30 can make a standin - with a 6DJ8/6922 pinout.
I like the Bartola DN2540 circuit (that's the one you are referring to?), but the first FET has the input at 0V, and the output is still AC-coupled. The split PSU adds complication, maybe not better than an input cap?
I referred to Bartola's site for his notes on the slew rate limiting and the excess HF distortion all those large die FET's produce.
I personally use the STN1HNK60 (SOT223 SMD, there are TO-92 and TO-220 version with the same die), with Crrs 3.8pF and Ciss 156pF is a better choice than most, but it still looses to most tubes.
Using current sinks in cathode followers may have some application, I prefer a resistor, a chassis mounted one may be needed as it worked out better in my DC coupled 211 driver. I could not see any improvements in my circuit with a current sink.
Well, if (say) we have -12V (heater) and +120V (anode) available, connecting a CRD will allows around +20dBu signal swing, without loading the cathode. Do that with a resistor.
And as CCS it is 1 part, compared to 4 for a ring of 2 and it is 2-terminal.
A application I really like is shown here (tube part):
The DN2540 does not really fit into the "Large HV MOSFET" category, and what happens to the bootstrapped input capacitance in a follower?
Crrs is not that low. And yes, Ciss is bootstrapped, but depletion FET's have fairly low transconductance compared to enhancement types, but where does the current to charge and discharge it come from?
Are you sure you read the articles at the Bartola Site? Start here:
https://www.bartola.co.uk/valves/2016/01/09/slew-rate-part-iv-and-source-followers/
His latest anode load gyrators use DN2540 to bootstrap a J-Fet at the bottom... Kind'a (yes, I know it's different) like I do with a Tube instead of DN2540.
Thor