With the generator running well and the interconnects made for the power supply and control box I decided to power the thing up and see if it worked. I fired up the generator and flipped the breakers on the front panel. Filament voltage was good. After letting the tube warm up I satisfied the interlocks and hit the HV button. The volt meter immediately pegged out. Thats not good. I put a HV probe on the output and yep, full voltage with no regulation.

Looking through the schematics I see that the screen supply voltage is way low. It turned out two of the four of the screen bypass caps were shorted out. While checking voltages I also found that there was no grid voltage at the socket. This was from a open 10k, 30W resistor on the grid supply to the tube. So with no screen and no grid the tube was just a big diode. I ordered replacement caps and resistors from Mouser and installed them. Powered it all up and it works, fully variable from 0-10kv. While I was waiting for parts I picked up a used tube (4CX15000A) off ebay from a very helpful seller who used to work with these kinds of tubes in the past. It looks like I dont need the tube, yet, but it is nice to have a backup.

I have no idea why these caps shorted out or why the grid resistor blew. I guess I will have to wait and see.

The offending caps: