A grid imposed between the filament and target of a demountable high vacuum type x‐ray tube is modulated in synchronism with the full‐wave rectified voltage impressed on the tube, by means of a square‐wave generator. The grid potential varies in such a manner that the electron current flows only when the voltage is sufficiently high to excite theKseries. For the MoK&agr; radiation at 30.8 kv peak voltage an increase in efficiency of 29 percent is realized with the grid modulated. With a peak voltage of 38.2 kv a gain in efficiency of 18 percent is noted, or the x‐ray intensity may be increased by this amount without altering the target heating.