Jim, re “ …. This would put perhaps more then a KV difference ?between one 5 volt winding and the second HV winding sitting just below it ?

I wonder where you get the voltages from . ?we can see on the tube voltage chart 760 VDC on the filament ?but nothing about the plates and nothing about the transformer voltage that I can see. ??And I could not understand some reported voltages and I can’t see some in the halli-manual ?but I finally found this:

Presumably 880 is a no load voltage so it will run slightly lower depending on operating mode etc

After much head scratching it seems that 880 VAC should boil down to something like 760 VDC for a choke loaded ?rectifier so now I “see” 1760 rms volts across the HV terminals and that would be the prime stressor of that insulation, but under your construction idea, which must be close to the actual the 1760 will only take place between the 5V wdg and the nearby HV wdg , as you implied I think. So I am of the same thinking other than your? “ more then a KV”

it could be at the end of a layer and the wdg to wdg insulation ?is not wide enough, and there is a creepage path ?around the end

and perhaps corona.? Of course the 1760 could just breakdown? the insulation, as most people know the very ends have higher field intensity

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