Ralph Mowery wrote:

The handbook lists peak voltages for the plate capacitor spacings.

1000 volts .015 inch spacing
2000 .05
3000 .07
4500 .125
6000 .15
9000 .25
13000 .5

Also it states that is for peak RF volts, if the DC plate voltage is across
the capacitor it must be added. Plate modulated finals take about 4 times
the plate voltage.

The rule of thumb I learned for HV conductor spacing is 30 KV per inch
in dry air with smooth surfaces. Rough surfaces, points, sharp edges,
etc. all reduce the arc-over voltage, as does increasing humidity.

I'd go with the handbook's table above, as it allows a safety factor
and probably will prevent embarrassing events involving smoke.

--
I, for one, do not like to damage my precious equipment, especially
when it involves repeatedly hitting it with a computer.

-- Peter Corlett, about his hammer