Of course, a lot of TV sets were AC/DC, so the filament voltages are
all over the map since you'd put them in series to run directly off
the AC line. Unless they start with "6" or "12" the tubes
may not be so useful.
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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