[R-390] Another ballast question

[Barry]

Some have suggested using a 12V tube's filament as a "ballast".  I can't
seem to locate a 12V tube, but I do have a dual 6V tube (a 6201).  Measuring
the filaments in series, the resistance is 15 ohms (7.5 ohms for each
heater).

If a 12V tube drops the same voltage as a 45-ohm (or approximately that
value) resistor, then why does the filament only measure 15 ohms?  Does the
filament resistance increase as it heats?  It would make sense as I *think*
resistance increases with thermal activity, but not sure about that.

Can someone enlighten me?

Thanks,

Barry - N4BUQ