Beam tube with density plus velocity modulation

Abstract

A tube for amplifying high-frequency multi-kilowatt, amplitude-modulated signals utilizes a linear beam of electrons which is density-modulated by a permeable control grid spaced close to a thermionic cathode. The beam is focused through a drift tube having two axially spaced gaps, each coupled to a resonant circuit such as a hollow cavity. The first circuit 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 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 can thus, by velocity modulation, produce very dense bunches to excite the output circuit, 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.