[Amps] Specification for 3-500 grounded-grid grid bypass caps?

[Geert Jan de Groot]

For restauration work on a SB220 I'm looking for specs of
suitable replacement caps for the grid bypass.
In the SB220, each of the three grid-pins is connected by a
200p mylar cap to ground.

mylar caps are not easy to obtain these days.
I have no idea what kind of volts, amps or other specs these caps
need to be?

I'm a bit surprized by the circuit anyway - can someone please explain
why the grid of a G-G amplifier isn't connected to ground, but
connected by a 1mh choke and bypassed by 3 200p C's instead?

73,

Geert Jan PE1HZG