The design and operation of a miniature x‐ray tube used as a calibration source for a rocket borne x‐ray astronomy experiment are described. The tube utilizes a miniature lamp filament and a 0.05 mm aluminum foil as a transmission target. Anode current of 1 &mgr;A and accelerating potential of 4 kV produce approximately 1000 1.48 keV photons/sec. Shelf life of the tube is greater than three months and the operating life is several hours.