Hi, thanks for your help. Mine is the mig-50, without the master volume. I’m pretty sure the one with the master is the midget-50H, which is a different model and schematic. I’ve had a bit of a think. I suspect Elan’s drawing is the true mig-50 plan, that was done at or for New Sensor Corp in New York as the drawing says. Sovtek in Russia, probably due to availability and/or financial issues made the amps with whatever components they could get that would do the job. Bruce’s drawing is an “as built”.

Therefore, I’m picking the new EH mig-50 would be closer too or the same as Elan’s plan rather than Bruce’s and so that’s the one I’m going to follow. This could also be why the new EH mig’s are considered better; they were actually done to spec! Here’s an example:

On the right are two linked resisters going into valves 4 and 5. Elan says each one is 750 ohm, Bruce says each are 470 ohm. My amp has 510 ohms. They both have signs of heat damage so I’m going to change them as per Elan to 750’s.
I was looking online for 750 ohm 2 watt resisters. They seem to be relatively uncommon. This may support what I said about availability in 90’s Russia…