Big oops!

Just looked at the picture again and it's not a standard D plug, but a
Sennheiser socket, and it is an output rather than an input.

Looking at the circuit, I see in that case it is cap-coupled and only
carries 200V AC.

Alarm bells rang when I realised the significance of the PS caps being
on the amp. That would mean that both plug and socket would need to be
shrouded so you can't touch either when unplugged, because both could
be live.

Sorry for my original misinformation...my only defence is that it is a
very unusual amp and it really does look like a D connector in the
photo.

cheers, Ian