Both my pairs of used 807s have seen a great deal of action and I've noticed, over the last few days, the sound deteriorating, turning a bit fuzzy and vague. This is unfortunate as I have no more 807s.
Also it has been difficult to find anything in my stash of old used ECC82 and ECC81, that were not past their best. I did manage to find a NOS Brimar 13D9 with two decently matched triodes in it, and giving a healthy output from both sides. The best ECC82 I have, with enough welly for the CF position is a new production, Electro Harmonix ECC82EH.
Alas, with no more 807s at my disposal I've ended up completely redoing the output stage; sockets, heater wiring, the lot, in order to accommodate standard 6L6GC types. A right pain in the arse, with much cursing and burned fingers!
I have four new production Tung Sol reissue 7581A and two of these are now in service as the output stage. The only NOS valve left in the audio circuit is the Brimar 13D9, but needs must.
I also took time out to fix the value of the potential divider to the grid of the error amplifier in the series pass regulator. I don't like pots in those kinds of situations with DC sitting on them. So the grid bias of the error amplifier is now fixed, resulting in 277V of output from the regulator, giving 258V at the plates of the 7581As. Perfectly serviceable in this circuit.
The bias on the 7581As has been changed to -14.5 volts by the addition of a 47R series resistor to the existing 150R cathode arrangement. This is as close to the book values for operation into a 2k5 load as I can get. This mode of operation gives, in tetrode mode, the lowest 2H distortion out of the three suggested scenarios on the 7581A data sheet.
Easy listening: high fidelity, low anxiety.