Abstract: A microwave amplifier tube having a first and a second ring resonator of which the first serves as a driver resonator and the second serves as an output resonator. By means of a cathode system an electron beam rotating about the ring axis at the frequency of a control signal is generated. The electron beam is accelerated by a direct voltage and enters the second ring resonator which is tuned to the same frequency as the first resonator. The electron beam influences a high-frequency electromagnetic field in the second resonator and delivers a part of its energy to said second resonator. In order to facilitate equalization of the angular phase velocities in the two resonators, the ring resonators are provided above each other in the direction of the ring axis and the electron beam passes through the first and the second ring resonator parallel to the ring axis.