Tube filaments on 230v? What do I need to know?

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I recently built up a hybrid valve/mosfet amplifier running of 12vdc.
I couldn't get it to play ball and someone suggested a higher dc voltage.
So I tried 30 volts only to suddenly realise the heaters were running off 30 volts too !
Luckily the heaters survived it. So it looks like heaters can take a bit of abuse.
 

There is something additional, that was explained to me *decades ago* by a tube engineer: the maximum filament to cathode breakdown voltage.

For instance the 12AX7 / ECC83 tube has only 180 volts peak rating, which very likely could be exceeded on a direct 230 volt line.

The 50C5 which was designed for the "All American Five" transformerless radios, has a slightly improved margin of 200 volts peak rating. But the line voltage is only 120 volts AC.
 

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