[thejazzageuk]

Thanks. I've now carried out the next tests suggested by Techman.

check the voltage on the anodes of the two output valves (where R47 & 48 go to the valve base pins) to make sure it's near enough the same on each valve.

They are almost the same 154v and 155v.

Also check the voltage on the other end of both these resistors on the output transformer side.

Just 1v difference (see diagram).

I have already dome some transformer tests earlier in the post. However after measuring the anode voltages, I then removed the rectifier and powered the amp up, still with the lamp limiter in place. The heater elements on the PX4s (and other valves) glow dim-ish orange, and the lamp in the limiter is not noticeably illuminated at all.