The invention of mercury arc rectifiers in the 1920s made it possible to control a load voltage by varying the conduction angle of a rectifier with a low-power control ignitor. Later years produced the gas-filled thyratron that extended this capability to lower-power equipments. Motor drives, welders, controllers, and a host of other applications followed, but the limitations in reliability and efficiency, coupled with the relatively high cost of this equipment, precluded widespread use. The market was confined to special applications where the advantages outweighed the disadvantages.

A second approach to phase control involved magnetics in the form of saturable reactors and magnetic amplifiers. Saturable reactors had been used for many years in applications that could tolerate a relatively slow response time, and advances in metallurgy brought "square loop" magnetic materials that improved the characteristics of these devices. In the early 1950s, the combination of improved magnetics and low-leakage selenium rectifiers had advanced the art of magnetic amplifiers to a point where they were used in autopilots for military aircraft, but their slow response and high weight were far from ideal.