No update for a week because I switched my multimeter to diode test instead of high voltage, then checked the HT in the SC200 which made it light up pretty well. Was waiting for a new one to arrive and doing boring guitar setups.
Now new one has arrived, tested relevant stuff like the bias and HT and various resistors in the SC200 with no valves in it. Everything looked fine there. Loaded it up with a different phase inverter valve and it runs fine again, even with two blackened KT88s, guess they’ll need changing, might get them tested, see how they have lasted. Looks like it was a short phase inverter causing the trouble, same thing I had with my first Sound City 120. I sort of suspected but thought it was unlikely. One more guitar to wire up which is being a pain, and I can get on with the Burman, yay.