Two circuits are described which act as medium‐speed, single‐pole, double‐throw switches to connect two independent voltages to the input of a single‐beam cathode‐ray oscillograph. In each, a square wave is applied to the cathodes of two high‐mu pentodes, biasing them beyond cut‐off on alternate half‐cycles. The independent inputs go to the control grids and the output is taken from the common plate connection. The circuits differ in the means of generating the square wave. In one, a high voltage transformer connects to the plates of two diodes which are run at reduced heater current in order to secure voltage saturation. In the other, a transformer swings the grids of two high transconductance, sharp cut‐off television amplifier pentodes, overloading the grids on positive peaks and cutting off the plate current on negative peaks. The relative merits of the two circuits are discussed. A complete bibliography is included. A simple mechanical switch serves to change the circuit into either a two‐stage, audiofrequency amplifier or into an electronic mixer.