Ross, NS7F wrote:
In a circuit with two 6.3V tubes, can I just series the filaments so
that I can use a 12-13.8V supply? Or are there subtleties that I'm
missing?

You can, but the tube filaments won't last as long because of the turn-on
surge. If it's a circuit where the tubes fail with poor emission long before
the filament fails, that's fine.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."