VFDs usually need AC to operate properly. But what if we do not have a transformer to provide AC in our circuit? This is where the magic of discrete components comes into play...
For a simple, single-digit VFD tube the size of a normal nixie tube, we are fine using around 1 volt DC, but for large VFDs, we need to make an H-bridge that can oscillate back and forth between the two filament pins. The circuit is shown below...
This circuit uses R-C oscillation to make a +V to -V peak-to-peak AC square wave to drive the filament. For medium-sized VFDs, Vfil should be around 3 volts, and for large VFDs it should be around 5 volts.
The reason we need AC on the filaments is that, if driven by DC, a voltage gradient will be evident on the filament by how the display appears — there will be a brightness gradient from the ground side of the filament to the positive side. With AC on the filament, the brightness appears uniform.