[Bookman]

Thanks GJ,

The situation described is under “no input” conditions which would suggest minimal induced magnetic fields derived from the transformer magnetising current plus the valve heater tap and the GZ-34 conductance. Moreover, there is no hum produced by these micro functions without the M3 connected as described below.

I have produced a sketch that shows a common supply rail derived within the twin wall socket.
With plug B and the Dynaco switched on there is no hum. However, if I then switch on plug A but leave the M3 in the off position that is when the 50Hz hum starts.
I would imagine the M3 is switched on the internal transformer primary side however there is a power on amber LED (implies rectification?) that connects to the protection circuit. When the on switch is pressed the protection relay picks up after a couple of seconds and a blue light then shows.

Apologies for quality of sketch
Joe