depending on where you got your "new" display tube, it could be as old and as used as the one you replaced. there are, however techniques for "rejuvenating" the filament to improve the cathode emission (the filament is the cathode in a display tube), which usually work by running the filament 10 to 15 percent over normal voltage for about a minute or two in order to get fresh material to the surface of the filament. this works with vacuum tubes that have filaments that are hot enough to glow orange. in a display, the temperature of the filament is below the point of emitting much visible light. there are some methods for doing this with VFD tubes, with a word of caution: if the filament has a physical weak spot in it, rejuvenation may burn the filament out. the most successful method i've found is to use a constant current supply to run the filament up to a temperature where it's bright orange, yellow, or white hot for a few seconds, with one method leaving it in white hot operation for up to a minute, or others where the temperature is ramped up slowly to white hot, then ramped back down after being white hot for a few seconds (ramping the temperature may allow for thermal expansion/contraction of the filament without sudden mechanical stress). i highlighted constant current supply, because there is one slight thing that could go wrong using a constant current supply to heat up the filament: there are several sections of filament in parallel in the VFD tube, if one strand fails because it was ready to go anyway, the current will redistribute through the other strands suddenly, and you might end up with all of them burnt out. i think this would actually be done better by an adjustable voltage supply. this should be done with no other voltages applied to the tube, preferably with the tube removed from the circuit board. you can find various methods for doing this, but i would avoid methods that keep the filament white hot for more than a few seconds. with the diameter of the filament wire used in VFD tubes (it is very small compared to other vacuum tube types), if it doesn't happen in a few seconds, it won't happen (i.e. burning off "poisoned" material, or boiling fresh material to the surface of the wire). also, every VFD tube, like regular vacuum tubes, have an area of getter material (looks like a metal coating on the inside of the glass, some people mistake it for the tube being "blown"). if the getter material has visibly reduced in area from when the tube was new (you can see the outline of the original getter spot) or the getter spot has turned transparent or white, the problem is not the filament, the problem is oxygen in the tube.