The power delivered from a three-phase A-C power source and through a network of SCR's (connected, for example, to form a full wave rectifier bridge or to provide an A-C switch) to a load circuit is adjusted by controlling the conduction angles of the SCR's. The control is achieved by developing, from only one of the three alternating phase voltages provided by the A-C power supply, a reference square wave signal of the same frequency and whose amplitude excursions are precisely synchronized or keyed to the zero voltage crossings of the selected phase voltage, even though that phase voltage may include undesired harmonics, noise, transients or other distortion components. Since the influence of the undesired signal components has now been eliminated and since the reference square wave signal also has a fixed phase relationship with the other two phase voltages, logic circuitry may be controlled by the reference square wave signal, and by a d-c control voltage representing a desired power flow to the load circuit, to produce gating signals for triggering the SCR's into conduction at the angles required to translate the desired power to the load circuit. With this arrangement, control of all three phases is made immune to undesired signal components in any of the phase voltages.