The Operation of Electron Tubes at High Frequencies

Electrons passing from cathode to plate induce charges on all electrodes. When the electron current has an alternating component, induced alternating currents flow to all electrodes including negatively biased grids. These currents may have resistive components because of finite transit time. Calculations of the alternating currents to electrodes in diodes and amplifier tubes and of the resulting impedances are compared and interpreted. Measurements of diode impedances and of the input impedance of amplifier tubes confirm the analysis up to 300 megacycles. Tubes with normal space-charge control show positive input resistance and capacitance change when placed in operation. Tubes whose static characteristics are concave downwards have a negative input resistance and a negative capacitance change. Confirmation is presented by measurements on diodes and on hexodes with current-distribution control. The negative resistances found may produce high-frequency oscillations.