Abstract: X-ray pulses at two different photon energy levels are produced with an x-ray tube that has an adjustable resistor element in a circuit that connects the anode and filament of the tube to a high voltage supply. Two bias voltage supplies are connected in additive fashion between a control grid element of the tube and its filament to enable control of voltage drop and current through the tube. Switch means are provided to selectively shunt either or both bias voltage supplies in a repeatable order to thereby alter the bias voltage on the control grid and enable producing x-ray pulses having energies corresponding with the voltage drop and current through the tube at a selected bias voltage in accordance with the load line on the anode characteristic curve plot of the tube as determined by the value of the adjustable resistor element.