Chris F. Inscribed thus:

At this point, I think you're right - it's probably issue with the B+
supply, probably a combination of poor filtering and inadequate output
current. After tracing out the schematic that's about the only
conclusion I can come to.

Since you get clicks when the tube is not connected, have you checked
the EHT supply. It probably has bad caps in there. Something breaking
down as the EHT rises.

--
Best Regards:
Baron.