An amplifying arrangement employs both density and velocity modulation of an electron beam in order to produce amplification of a high frequency signal. In one embodiment, density modulation is produced at the cathode region, a first carrier signal being applied between a cathode and a modulation grid to produce bunching of electrons. The density modulated electron beam is then velocity modulated at a first resonant cavity. It is preferred that the first carrier signal is twice the frequency of the carrier signal producing the velocity modulation. The arrangement may be such that, at the final resonant cavity, adjacent electron bunches produced by density modulation combine, thus giving a high efficiency amplifying arrangement.