A phase shift oscillator with a frequency range 0.06 c/s to 18 c/s is described for use in obtaining the frequency response of components of servo-systems. The oscillator incorporates an amplidyne generator and will develop a peak current of 3 A in a 17 ω load at all frequencies. The amplidyne with its associated electronic amplifier has a local feedback loop to minimize the effect of non-linearities caused by saturation and hysteresis of the magnetic circuit.