An instant-start ballast for a multiple gas discharge lamp fixture has a single half-bridge inverter that is used to drive at least two series resonant output circuits. Each series resonant output circuit is used to drive two gas discharge lamps. A single output voltage limiting circuit is provided with a microprocessor that monitors the combined output voltages of the series resonant output circuits and uses a single voltage limiting transistor to control the output voltages of the circuits. A capacitor is associated with each gas discharge lamp socket in the fixture such that a voltage across the capacitor indicates that a lamp has been installed in the socket associated with the capacitor. The ignition sequence will only be initiated if the capacitor voltages indicate that a lamp has been installed in each socket of a fixture.