An electron tube suitable for phase-modulating a wave by large amounts, in which a pattern of beams emitted radially from a central cathode passes through a slotted cylindrical anode to a coaxial second anode, the pattern of beams being both rotated at a steady rate by a three-phase supply and modulated by a varying axial magnetic field that advances and retards the beams and thereby also the phase of the current at the second anode.