Highland Ham wrote:
Being in series means they would all be used with the same filament
current which can be easily implemented with a circuit employing a 78xx
device in constant current mode (obviously in a DC environment)

Frank KN6WH / GM0CSZ


Andy adds:

That's true as long as the current requirements were the same. For
instance, putting a 12V/1amp filament in series with a 12 V/200ma
filament will not do well on 24 volts.... Just like putting a 100
watt
and a 7 watt light bulb in series across 220 volts....., (guess which
one will be the brightest, and for how long :)))) ?? )

The currents were matched in the 5 tube AC/DC radios, but generally
you have to check the filament current rating as well as the
filament voltage rating in order to do this...... It ain't rocket
surgery, but you can't put tube filaments in series indiscriminately.

Andy W4OAH