A tube for amplifying high-frequency multikilowatt, amplitude-modulated signals utilizes a linear beam of electrons which is density-modulated by a permeable control grid 24 spaced close to a thermionic cathode 22. The beam is focused through a drift tube 17 having two axially spaced gaps 18, 35, each coupled to a resonant circuit 84,26 such as a hollow cavity. The first circuit 84 is tuned to a resonant frequency higher than the signal frequency to produce velocity-modulation bunching of the beam electrons in phase with the density-modulation from the grid. The second circuit 26 is tuned to the signal frequency and its energy is coupled out to an external load.
The grid modulation is Class B or Class C so there is no current between the electron bunches. The floating bunching circuit 84 can thus, by velocity modulation, produce very dense bunches to excite the output circuit 26, providing very high conversion efficiency.
The tube is particularly adapted to amplitude-modulated signals such as television, for which a conventional klystron yields very low average efficiency.