Jay (W7CW) found these capacitors. They are Murata (now Murata-Erie)
rated for 20 KV DC working voltage. They are 4800 pf (0.0048 uF). Those had
a Z5V ceramic dielectric, with a peak in capacitance at about 15 degrees C.
The capacitance drops as temperature increases.
The dielectric loss is high, dissipation factor of 1.5%. It drops with
temp., but rises with frequency.
These capacitors were rated for DC, and have curves as high as a few MHz.
where the loss rises and DF is higher than a percent. I would be extremely
wary of operating them anywhere close to maximum voltage with RF. However,
if you seriously derate them, you could probably block 500 volts or so,
with Rf applied. You will find that they will heat, and not be stable in
value as they heat.
Jennings, High Energy, and others sell the ceramic doorknobs that are
really made for RF, the HT57, 58 types.
I did see these Murata caps used as the RF bypass to ground on the center
tap of a push pull output transformer for a big amplifier. They
occasionally blew up, but worked sometimes. I replaced them with real
capacitors, made by Draloric of Germany. They can be gotten from Richardson
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