Some Principles of Grid-Leak Grid-Condenser Detection

The action taking place in the grid-leak, grid-condenser method of detection is reduced to an equivalent circuit consisting of the grid-leak condenser impedance in series with the dynamic grid resistance. The effect of applying a signal voltage to the detector, as far as the rectified grid current is concerned, can be represented by the introduction in the equivalent circuit of a fictitious voltage that is determined by the signal, and by a single tube constant called the voltage constant. The voltage drop produced by this fictitious voltage across the grid-leak condenser impedance is the change of grid potential resulting from the detector action. A bridge method of measuring all detector constants is described. The effects of grid leak and condenser sizes are discussed, and the factors involved in the selection of conditions for best detection with telephone and telegraph signals are considered.