[exray]

Those rated at 1000 and 1200v were mostly rated so because of internal leakage and the higher voltage caps had less leakage. So you need to pair it up on the schematic when you encounter this. AC line bypass and AF output plate (tone) caps could dictate a special consideration.

A 1200 volt rating on a grid coupling cap was a rating because of deficiencies in paper caps.

[Ron Ramirez]

Right. For AC line bypass replacement, use .01 uF X1/Y2 safety caps.

[brownie45]

Never ran across them in a block, but the 1KV & 1.2 KV could be
for across the audio output Xfmr to kill spikes & in case the
Xfmr went open for some reason??
Jim

[morzh]

today's X bypass filter caps are rated 275VAC and this is plenty, and my Y caps are 300V.

X and Y ratings are more important than AC volts as long as the volts are adequate for MAINs voltage. Different technology, different safety standards.
They did not call those 1200V caps in Y filters "death caps" for nothing.

[Arran]

I've noticed that the old cardboard tubular paper capacitors also often have a two digit number inside, usually on a piece of tape that sticks down the outside of the windings of paper and tin foil. Perhaps the number dictated the capacity in microfarads and the voltage of the capacitor before it was stuffed into its' shell, like a #30 would be a .05 uF 600 vdc unit and a #25 would be a .003 uf 400 vdc unit?
Speaking of tone correction capacitors, I have noticed that Philco had a preference in many of their sets for connecting such caps from the plate of the audio output tube to the cathode rather then between the plate and the screen grid. With such a setup they could get away with using a regular paper capacitor rather then having to use a 1 kv unit like some sets did.
Regards
Arran

[codefox1]

Also heard that wax paper or whatever used in multiple cap windings long ago was used as ratings meaningless ot actual opeating conditions. Surprised? That said, new 630 VDC components for everything for the pennies more.