An open filament can be caused by a number of things (from most likely to least likely):

1. Improper connection between the HV cable and the cathode receptacle.
2. An uncalibrated generator (leading edge mA filament starting values).
3. A worn out filament (either a very old tube or a high capacity clinic).
4. A tube not properly maintained that had an arc, damaging the filament circuitry which in turn damages the tube filament.

Prior to replacing a tube due to an open filament, the FSE must verify the proper operation of the X-ray filament circuit using collimator bulbs and an amp meter. At prep you should have 4-5 Amps. If the current is out of range consider replacing filament circuitry as well to ensure your new tube does not get damaged.