A New Method for Determining the Efficiency of Vacuum-Tube Circuits

A new method for determining efficiency of vacuum-tube circuits at both high and low frequencies has been described. This method consists of the application of a surface pyrometer which indicates the temperature of the glass walls of a vacuum tube. Readings are made of the temperature of the tube walls when the circuit is oscillating together with notation of d.c. plate watts input and then oscillation of the tube system is stopped and the plate input watts changed until identical pyrometer reading is obtained. The difference in wattage between the two measurements represents the radio-frequency output power. The simplicity of the method and the short time required for the measurement recommends it for serious consideration where air-cooled tubes are employed.