triangular wave form having a constant amplitude with a frequency proportional to the output signal of the pitch setting means 14 if the frequency of the triangular wave generated as the output from the voltage frequency transducer 15 is expressed by f it may be defined by the equation f v p 39 hz output signals of a duty setter 18 and a duty correction setter 19 are respectively added by an adder 20 which forms a first operator the output signal of the adder 20 is applied as one input to a comparator 21 fig9 is a detailed view showing the construction of the duty setter 18 the duty correction setter 19 and the adder 20 the duty setter 18 is a voltage setter which is formed with the use of a resistance the duty correction setter 19 is formed of an attenuator using a resistance and is adapted to change its output in response to the detected voltage of the tachogenerator 13 the adder 20 serves to add the set voltage of the duty setter 18 to the attenuated voltage of the duty correction setter 19 and is composed of an amplifier circuit the comparator 21 functions as shown in fig2 to compare the triangular wave from the voltage frequency transducer 15 with the output from the adder 20 and is adapted so that the output voltage from the adder 20 causes the generation of a.