The paper describes a tube in which the intensity of a short hollow beam of electrons is modulated by varying the electric field in the emitter-collector space. The modulating electrode (gate) surrounds the beam but intercepts only a marginal number of electrons because an axial magnetic field prevents divergence of the beam. In spite of the relatively low gm, very high power gains are possible in the kHz and MHz region. At higher frequencies, transit-time effects reduce seriously both gain and efficiency. Operation at UHF is possible when high voltages and close spacings are used but this necessitates the use of high current density cathodes such as porous tungsten stypes. Rather low knee voltages can be obtained if returning electrodes are reflected towards the center of the emitter. Field-Effect tubes show advantages above gridded tubes in applications where wide load-impedance fluctuations can be expected and where extreme ruggedness is required such as sonar, induction, and dielectric heating.