A New Type of Practical Distortion Meter

This paper gives a description of a distortion meter embodying circuits which differ somewhat from the types previously employed for this type of instrument. It consists essentially of a bridged-T audio-frequency bridge circuit, in which the inductance element is replaced by a reactance-tube circuit. Because of the flexibility obtainable in vacuum-tube circuits, it is a relatively simple matter to vary the effective inductance continuously over a fairly wide range, and thus allow the distortion meter to be used at any frequency in the audio range. Certain precautions must be taken in a circuit of this type in order to avoid difficulties due to nonlinear action of the reactance-tube circuit. Application of negative feedback to the reactance-tube circuit effectively reduces the nonlinearity, increases stability, and at the same time keeps tube noise and hum at a minimum level. An analysis of the reactance part of this circuit, together with formulas for calculating the effective Q and the optimum operating conditions are included. Although the study of the properties of the reactance-tube circuit with negative feedback was limited to features applicable to the problem at hand, some of the information obtained may be of use in other fields.