A sample of the voltage magnitude across a load, connected in series with an A.C. source and a load-current-flow-control subcircuit, is taken during at one of the time intervals when load current flows during a source waveform cycle. The sampled load voltage is compared to a reference level and the difference in magnitude between the sampled and reference voltages is integrated, with respect to time, to provide an adjustment signal. The adjustment signal is provided to a subcircuit for timing the conduction period of the load-current-flow-control subcircuit, by varying the conduction-termination time after each conduction-commencement (at each source waveform zero-crossing) to vary the selected one of the half-cycle-average (mean absolute deviation--MAD) or RMS voltage magnitude across the load to a value such that the sampled load voltage magnitude is substantially equal to the selected reference voltage magnitude. Several presently preferred embodiments are described for controlling the load magnitude in accordance with this method.