1. A multistage magnetic amplifier having an a.c. line, each stage comprising a pair of closed magnetic circuits and two branch circuits connected across said a.c. line, each branch circuit including in series a reactor winding respectively disposed on one of said magnetic circuits an impedance and two unidirectional devices, said devices in each of the stages being poled in the same direction and the devices in successive stages being respectively oppositely poled, each stage also comprising a control circuit including two control windings, one control winding being disposed on each magnetic circuit, one control winding being arranged to aid the flux induced by the reactor winding of one core and the other control winding being arranged to oppose the flux induced by the reactor winding on the other core, means conductively connecting the control circuit of the stages after the first stage to the branch circuits of the preceding stage at points between the two unidirectional devices in each branch circuit of the said preceding stage, and a single biasing resistor connected in shunt relation to the two unidirectional devices and the control circuit connection in one of said branch circuits.