A sampling servo-system suitable for accurate control of the speed of rotation of a large disk is described. A speed in the vicinity of 1 rev/sec can be controlled within a 10% range as a linear function of an applied voltage, and rates of change of speed of up to 20% per sec can be followed with a 20 msec lag. With small rates of change the accuracy of speed control is 0.2%, and is limited by mechanical irregularities associated with the driven disk. These are related to disk position and are systematic enough to permit control of speed to better than 0.1% at any given disk angle.