No problem as long as they take the same current, i.e. 300 ma . If they take
different currents then you have to bridge the one taking less current with
a resistor so that the series current of the highest filament is equal.
Otherwise you'll burn out the filament with the lower current requirement

Ralph Cameron
"Ross, NS7F" wrote in message
oups.com...
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?
Thanks,
Ross NS7F