Reduction of Distortion and Cross-Talk in Radio Receivers by Mean of Variable-Mu Tetrodes

In attempting to control the audio output of a radio receiver employing the present types of tubes by varying the control-grid bias or screen-grid voltage distortion, due to nonlinearity of the output-input voltage relation for the tube, and cross-talk are encountered at the higher signal voltages. Both effects are largely due to the rapid increase in the higher-order curvature parameters of the tube characteristic which occurs as the grid bias increases negatively, or screen-voltage decreases. Two tubes, designated as Types 550 and 551, have been developed to reduce these effects. They are shielded tetrodes of which the characteristic has been specially shaped to reduce the higher-order curvature in relation to the transconductance. The desired shape of characteristic is attained by a composite structure by virtue of which the tube acts as a high-mu tube at normal grid biases and automatically changes into a device of low-mu as the grid bias increases negatively. The 550 tube will handle an input voltage of 15, and the 551, 7 volts, with negligible distortion. These represent improvements of 50 and 25 times over present 24 tubes. Cross-talk is reduced to 1/500th that obtained with the present type at input voltages of 0.1 volt. A general discussion of the problem of distortion and cross-talk is given from both the theoretical and experimental viewpoints. Both effects are shown to be approximately proportional to the ratio of the third derivative to the first derivative of the plate current-grid voltage characteristic.