a ballast circuit includes a control loop to reduce a filament heating voltage below after the lamp filaments have been pre - heated . a filament cutback circuit includes a filament cut - back inductive component magnetically coupled to the resonant inductive component in the inverter to receive a filament cutback control voltage associated with an ac voltage for powering the lamps . during the pre - heating period , the filament cutback control voltage is not high enough to charge a chargeable component to a switch threshold level . however , during lamp ignition , the filament cutback control voltage is increased and charges the chargeable component to the switch threshold level . this causes a switch device to operate in a conductive switch state and the filament cutback circuit suppresses the pre - heat voltage .