Abstract: The klystron amplifier comprises an input cavity a plurality of intermediate cavities and an output cavity which are arranged along the path of an electron beam for focusing it, and an output waveguide coupled to the output cavity through a coupling opening. The product G.sub.0 (R/Q)Q.sub.ex is selected to be0.4.ltoreq.G.sub.0 (R/Q)Q.sub.ex .ltoreq.0.7where G.sub.0 represents the DC conductance of the electron beam, R/Q the characteristic impedance of the output cavity as defined at the gap between the edges of the drift tubes therein and Q.sub.ex the external Q determined by the size of the coupling opening.

Abstract: A traveling-wave tube has a slow wave circuit consisting of a constant phase velocity section extending from the electron gun side of the tube, and a phase velocity tapering section extending from the collector side of the tube in which the phase velocity of the electromagnetic wave is lower than in the constant phase velocity section. An attenuator is located at about the center of the constant phase velocity section. The ratio of the length of the phase velocity tapering section to the distance between the end of the attenuator which faces the collector side of the tube and the electron gun side end of the phase velocity tapering section is equal to, or less than 0.4.

Abstract: In a velocity modulation tube comprising a floating prebuncher and at least one final prebuncher, frequencies of respective fundamental modes of resonance of the input cavity, the floating prebuncher, and the final buncher are adjusted to the lowest frequency of the passband of the tube, adjacent to the highest frequency of the passband, and higher than the highest frequency, respectively. Furthermore, the Q-value of the floating prebuncher is made equal to or lower than that of the input cavity. Naturally, the output cavity has its fundamental mode of resonance approximately at the center of the passband.