A spacecraft includes a plurality of paralleled RF amplifiers. Each paralleled power amplifier (206) includes a signal splitter (222) which produces two nominally equal-amplitude signals, and a signal combiner (242) which combines appropriately phased signals to produce a sum signal at a sum port (250). The signal combiner also includes a null output port (248). A pair of elemental amplifiers (236, 238) is coupled between the splitter outputs and the combiner inputs. In order to allow for phase drift of the elemental amplifiers, at least one phase shifter (258) is associated with one of the elemental amplifiers. A phase control loop is coupled from the null output of the combiner, around the paralleled amplifier arrangement, and controls the phase shifter to maintain the phase within bounds, to maximize the signal at the sum port of the combiner. In order to provide for redundancy, at least one channel (271; 272) of the paralleled amplifier includes the cascade of a phase shifter (258; 259) and a switching arrangement (281) which allows a failed elemental amplifier to be replaced by a redundant unit. No additional phase shifters are required, since the loop phase shifters compensate for differences between failed and redundant amplifiers.