The output voltageepof a thermionic tube operated over a suitable restricted range of grid voltagesegmay be represented by the parabolaep=b0+b1eg+b2eg2. By adding to this in opposite phase the output of another tube, operated on the straight line portion of its curve, we may balance out entirely the termb1eg, so that the characteristic of the combination of tubes isec=b0+b2eg2. The system will then operate as a frequency doubler, for the output is proportional toeg2alone. In the experimental set‐up described, with a 201A tube, an output of 0.175 volts of frequency 2fis obtained from an input of 1.0 volts of frequencyf. It is found that when the characteristic of the system is plotted, the curve is never a symmetrical parabolaec=b0+b2eg2over a very wide range of grid voltages. This is because the coefficient of the cube term in the equation of the tube characteristic is not negligibly small, and the equation must be writtenec=b0+b2eg2+b3eg3. It is shown how the coefficientb3may be determined by measuring the dissymmetry of the curve.