As far as cap boards go, how do some get by with a 500v rating and the use of diode rectifiers?
There is a time-honored technique, borrowed from the early days of the Manhattan Project, called "Tickling the Dragon's Tail". In their case, the test was to see just how close to "critical" they could get a mass or Uranium or Plutonium. Scarey stuff...
Running 500V caps in an ST70 or MKIII with SS rectification can be akin to skating on thin ice, picking up live rattlesnakes, juggling hand grenades, or placing too much refined radioactive material into too small of a space, all of which are strongly not recommended... :o
A properly designed cap board uses 300V (or higher voltage) caps paired in series, with a 300K resistor across each cap in order to help share/equalize the voltage This is a good technique, and applicable for higher B+ voltage levels 600V, 800V, 900V, > etc...
/ed B in NH