Abstract: A power supply is provided for an electromechanical device of the type employing ultrasonic frequency vibratory energy for bonding materials. An automatic frequency control varies the output frequency of the power supply until the ratio of the maximum to minimum amplitudes of a standing wave produced in the mechanical vibratory member falls below a pre-set maximum. The frequency of the power supply is set by a voltage controlled variable frequency oscillator. A sweep generator produces a periodic voltage signal having a frequency substantially less than the frequency range of the voltage controlled oscillator.Means are provided for coupling the voltage signal from the sweep generator to the control input of the voltage controlled oscillator whenever the standing wave ratio exceeds a pre-set value, and for holding the last voltage signal received by the control input of the voltage controlled oscillator whenever the standing wave ratio is equal to or less than the pre-set value.