An inverter arrangement including controlled rectifiers, the inverter being supplied by a regulatable direct voltage supply which includes an on-off switchable element and a filter. A control circuit connected to the output of the supply generates and directs first control pulses to the switchable element, and a pulse generator also connected to the output of the supply generates and directs second control pulses to control the rectifiers in the inverter. The control circuit includes a step regulator having an amplifier, a feedback path and apparatus for producing an oscillation, the step regulator having the effect of altering both the frequency and the width of the first control pulses when the voltage at the output of the supply changes.