Apparatus for counteracting the cathode current increase occurring during warming-up in a travelling-wave tube in response to variation in the grid-cathode distance

An apparatus for compensating increase of cathode current occurring during the initial operation period of a travelling-wave tube including a cathode and grid when, during warming-up, the grid, to which a grid voltage is applied, first approaches the cathode and then assumes a nominal distance therefrom, said apparatus comprising a distance control means for measuring deviation between actual grid-cathode distance and the nominal grid-cathode distance and means responsive thereto for generating a control signal proportional to the deviation for reducing the value of the grid voltage, said distance control means comprising means for multiplying instantaneous values of the cathode current and grid voltage to produce a signal corresponding to the grid-cathode distance, comparator means for comparing said signal with a reference signal corresponding to the nominal distance, and means operative in response to said comparision for generating a control signal for reducing the value of the grid voltage.