• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Unused pin 866A

Quick question(s). Can I use one of the unused socket pins on a 866A for a lug to extend my HV going to the top cap? Just 1 & 4 are for the filaments. 2 & 3 are unused so I was hoping to use one of those as a terminal. Related question is how far away from the surrounding metal structures would be a safe distance assuming a working voltage of 450V? Currently, I have about 0.1" or 2.6mm.
 
Usual number is 5,000 volts maximum per inch of separation. Allows for high altitude and humidity. 10,000 volts per inch often works.

What is the worst that can happen? Blow up a tube, zap a power supply etc.

But you might try a small high voltage ceramic capacitor to the lug as a stand-off.
 
I have some of these. I'll either use them or the tag strip between the chokes shown.
 

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Since MV rectifiers really need to work into a choke input, and the world is messy, you really can't have too good insulators when playing around at the high impedance end of these things. Ordinary materials get weird at kilojolts, and you'll have enough pucker factor from the unknowable insulations inside the choke. Why tempt the goddess to smite you? Be conservative and then some more. You'll be rewarded in Valhalla. Maybe.

All good fortune,
Chris