The frequency of a vacuum tube oscillator depends not only upon the constants of the oscillating circuit but also upon those of the tube itself. In a tuned plate circuit, operating with minimum coupling the influence of the tube on frequency depends only on its plate resistance, and decreases as the plate resistance is increased. The steady drift in frequency usually observed is due mainly to the effect on the plate resistance of progressive changes in the filament. Due to their rugged filaments the UX852 and the screen grid UX860 have been found particularly suitable for steady oscillators. Two methods of compensating for small changes in plate voltage and filament emission are described. A curve is included showing the effect of temperature on the frequency of the oscillator. The apparatus used for this precise study of small frequency changes is described. One of its most valuable features is an audio oscillator which is controlled by a vibrating string.