[R-390] Cap Question

[Barry]

Looking at the caps in my "new" R390A RF deck, I'm considering replacing the
three brown beauties in it.  I'm curious, though, about two of them.  They
are used in the heater thermostat circuit of the crystal oscillator can.  In
the picture referenced below (thanks, Don), they are the big, yellow ones in
the upper right corner:

http://militaryradio.com/Images/390rfd.jpg

The heater voltage is 6.3VAC and I'm wondering why the designers used 400V
or better caps here.  Is this necessary because the noise spikes are
considerably larger than 6.3V or is it possible that these were just handy
since there were so many other 0.1mfd @ 400V caps used in other places in
the radio.

Any comments?

Thanks,

Barry(III) - N4BUQ