" I was really curious about the simplicity of the meq" ... "I was wondering if the 150 acts as cathode resistor for v1a, and the 20k for the 6aq5, or whether the DVR for the output transformer primary is more important there."
Yes the 150 R is V1a's cathode R, and the 20K is predominately the 6AQ5's cathode R. DCR of the OPT has no role in the biasing.
But we also have DC feedback from the 6AQ5's cathode to V1a's cathode which will act as a partial "forced bias".
It's a nice symbiotic arrangement in my opinion
Haha! And no way, literally up the road from me, I actually work down by the canal (at least, pre lockdown, still remote at the moment) so I know the area quite well
Yep, up/down the road. A stone's throw.
doesn't necessarily have to be continuously variable but I think having a few set gain settings could be useful to allow for variations in input. It could still be 'set and forget' for the most part but you could still choose an alternative gain if you remember to kill your monitors or do it when the power is off.
If you put the +tve of a capacitor (220uF/6V3 is probably close enough for rock 'n' roll) at the junction of the 120R and 1500R of the MB-1's schematic, you could then use various value resistors from the cap's -tve to ground in order to determine the amount of bypassing that the 1500R has.
Unless I've lost the plot, the ratio of 120R and 1500R gives, approx. a 20dB boost between an unbypassed 1500R (cap -tve left open, or with some arbitrary high value) and a fully 220uF bypassed 1500R (cap -tve shorted to ground).
Then there'd be no nasty DC on your switch to cause cracks and pops.