Oh well, then those voltages for the filament in the datasheet must have been RMS voltages. A miniature I was actually reading that document yesterday, they recommend AC drive to avoid brightness gradients which can supposedly happen with longer tubes. I didn't observe this while testing and the filaments were glowing so faintly that I had to cover the tube with my hands to barely see them glow. 2.45V RMS AC and 2.45V DC shouldn't be any different in terms of power dissipated in the filament, right? Does driving the filament with DC really wear the cathode out faster and reduce the lifetime of the tube? Sorry if I'm getting repetitive, it's just a little confusing because every single VFD clock that I've seen uses a low voltage DC supply to drive the filaments.
Well in your specific case using 4 independent tubes that should be a easy decision...
But unlike this particular individual case.. VFDs share a common cathode (unlike regular tubes)
and MULTI DIGIT VFDs can not be driven with UNEVEN POTENTIAL
Quoting NORITAKE
Luminance varies with the filament voltage (Ef) as shown in Fig.10. Since the lifetime of a VFD is dictated by the extent of evaporation of oxide materials coated onto the tungsten filament wires, it is critical that the filament voltage is supplied within the specified ratings.
Current drain from the anodes and grids to the filaments can cause ghost illumination so a bias voltage is applied to the filaments to raise them above ground.Old school VFDs like this one I shot below.. if driven with DC POTENTIAL
will have UNEVEN brightness and the higher potential will erode the cathode sooner...
Along with the AC EVEN drive.. you also need a bias offset to prevent artifacts...
Your INDIVIDUAL tubes should not be that hard...
Paul