The oscillator provided with the set has been designed to use the latest developments in this type of instrumentation. A signal of 750 c.p.s. is generated by using a power transistor switching circuit. This arrangement eliminates troublesome vibrators, buzzers or other moving part elements. The Transistor Oscillator converts low voltage (twelve volts) d.c. to stable audio frequency a.c. directly and hence, by a highly efficient method, the input current to the Oscillator is only 1.7 amperes for a full output of fifteen watts, a conversion efficiency of better than 80%. In order that a maximum of energy can be transferred from the Oscillator to the pipe, the output of the oscillator is provided with taps so that voltages of 2.5. 5, 7.5, 15, 50, and 100 volts are available to match the load. An interrupter is provided to make the signal more easily recognized.