I could probably, screw a little bit more power out of it with a GZ34, but I don’t want to push these modern production valves too far, so that’s the output stage rectifier rolling and general p*ssing about pathway shut down too. Anyway, as it stands, I think I’ve probably reached the best compromise between power output, distortion and valve life using the voltages and currents provided by a GZ37.
It also means the the amp only works with KT77s as EL34s over-dissipate with those cathode resistors and that HT. There is little room for manoeuvre, valve wise now, as there are only two current makers of the KT77: JJ and the New Sensor Corporation, under the Gold Lion brand name. I might try a couple of JJs at some point. It’d be churlish not to.
The KT77 also works properly with 2K5 output transformers, which is not the case with EL34s or the 6L6 types, so making the thing for KT77s only, ought to have been a no-brainer, and that was the original intention, but you know what I’m like when I can’t leave well enough alone.
Anyway, deliberately using 270R cathode resistors has shut off firmly, any more messing with the power stage, as has using a 4V UU5 on the input stage PSU. The rectifiers are sewn up, the output valve type is set. The only real opportunity for tube rolling now is with the 6SL7 input valves.I’ve put the schematics below along with the brands/types of resistors and caps used in the PSUs and the audio circuit:
