Hey-Hey!!!,
To the CCS, there are several levels, from the simplest current regulator diodes, to the depletion-mode MOSFET cascode circuit detailed in the original Merlin. I left out the PS as well as the bias supply, so there should be a few additions in the follow-up schematic. I suggest the Merlin MOSFET cascode. It is simple and easy to implement and with -20V. Unfortunately the 2A3 will need ~75 volts negative...I am not sure what to do about that. Perhaps a doubler, and take the CCS to the first half.On the OPTx, I have several to work through. I would hazzard a guess that the Peerless S-230-Q is going to win the tasting. The 2A3's will run this circuit to less than 10W, so it is not too critical about what OPTx, as long as it is able to deal with the 60-70mA of idle per leg.
On the grid choke, the HTS-7547 should do quite well. Call for no spacer and minimum-gap butt stacking to present the pentodes with minimum load.
cheers,
Douglas
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