PB Wizz:

It's great that you're doing this!

Remember that 866s need heating time to vaporize the Mercury before
applying High Voltage.
When I built the HV power supply for my "big rig," I put in a
thermostatic time delay relay (which triggered a high current relay) to
disable the plate transformer primary for about a minute, after the
filaments went on.

Good Luck & 73

afcsman--W9NPI