As I understand vacuum tubes (like triode / pentode) usually are designed to be heated with certain current through the filament.
And usually it is about low-voltage and comparatively high-current. E.g. 6.3 V at 300mA.
Schematics I usually find around use transformer to get this heating voltage. I wonder, whether I can instead connect heater filament directly to 220V AC via suitable capacitor to limit current. E.g. I think 4.7 uF should provide about the said 330mA. And it can feed several lamps in chain.
What could be the drawbacks? I can suppose that temperature will be pulsating slightly (like incandescent bulbs do about 5% light pulsation) and this can affect the anode current... But vacuum tubes have slower temperature time-constant...
UPD: some clarification - this scheme of course should only be considered for isolated filament (how do we call it... indirect cathode heating?).
Also "simplified" calculation
I=Uac*(w*C) which I mean is correct only while total voltage on filament (or chain of filaments) is small in comparison to full Uac.