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 . thus , the power supply frequency will be automatically varied to maintain the standing wave ratio below a pre - set value which is deemed to be an acceptable value for efficient transfer of power .