Detection Characteristics of Three-Element Vacuum Tubes

The change of grid potential in a grid-leak grid-condenser detector can be determined by considering a fictitious "rectified voltage" acting in series with the grid resistance. This equivalent voltage is inversely proportional to the tube "voltage constant" v, which has the value v = 2 R
g
/ (dR
g
/dE
g
), and can be readily measured by an a.c. resistance bridge. The rectifying action of different tubes can be compared on the basis of the respective voltage constants at grid resistances inversely proportional to the size of grid condenser. Tubes are then compared under conditions of equal detector distortion, and the change of grid potential is inversely proportional to the voltage constants. The voltage constant of ordinary vacuum tubes at first drops rapidly as the grid resistance is increased, but soon flattens out and becomes constant at grid resistances above 50,000 to 150,000 ohms. The highest audio frequency that can be satisfactorily reproduced with the detector adjusted to full sensitivity is inversely proportional to the grid resistance at the lower end of the flat part of the v-R
g
characteristic. It was found that tubes of the same type had uniform detection characteristics, that age, use, plate voltages between 16 and 122, and filament voltage (above the minimum necessary to give electron saturation) had little or no effect on the rectifying ability of high vacuum tubes at a given grid resistance in the useful range of operation.