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Transit-time effects in ultra-high-frequency triodes and tetrodes used in class-C operation are analyzed. Effects discussed are electron-transit reactance; electron-transit phase-delay angle; cathode back-heating; use of a screen grid to improve efficiency; changes in optimum shunt impedance; secondary emission; and anode back-heating by secondary electrons. It is pointed out that by increasing voltage and current density simultaneously the frequency and power can be raised without sacrificing efficiency or bandwidth. An equivalent circuit is described which takes account of certain important transit-time effects.