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The fundamental principles involved in the operation of velocity-modulation tubes are discussed with particular emphasis upon physical concepts. The relation of these devices to familiar low-frequency tubes is examined. The operation of several devices for the production of velocity modulation is considered, and several methods of converting a velocity-modulated beam into an intensity-modulated beam are explained. The behavior of the Klystron amplifier is considered on a kinematic basis, and is illustrated by means of the Applegate diagram. Phase shift in the Klystron is discussed in terms of electron transit time, the mechanism of bunching and catching, and the orientation of the coupling loops. The Klystron oscillator is considered in terms of general oscillator theory. The values of accelerator voltage required are developed in terms of the feedback coupling and the dimensions of the tube. The operation of the cavity resonators is explained in a qualitative manner. (A more detailed discussion of cavity resonators appears in an earlier chapter of the book.) The inductive output amplifier is discussed and its operation is compared with that of true velocity-modulation tubes.