English: A vacuum tube signal generator from 1931, the Model 303 made by Wireless Egert Engineering, New York. 10x8x7¼ inches. It uses a dynatron oscillator circuit, an uncommon circuit that uses the negative resistance of an early tetrode tube caused by an effect called secondary emission. The dynatron circuit was used in precision oscillators from about 1930 to 1945 because of its frequency stability, which reportedly was compared with crystal oscillators. Plug in coils (one shown at bottom) allowed it to cover the 165, 80, 40, and 20 meter bands, from 1.8 to 15 MHz. The meter was used to keep the space charge in the tube constant.