Abstract :
Electron tube amplifiers now form an important part of practically all radio receiving sets except the most simple types. Storage cells, which have heretofore usually been required for operating amplifiers, require constant attention, are bulky and heavy, and have other serious disadvantages. The dry cells, which are often used as a source of plate voltage, have a comparatively short life and are expensive and inconvenient. It would be much more convenient to use an amplifier which could be supplied with power from 100-volt, 60-cycle mains. This paper describes a five-stage amplifier which operates satisfactorily on 60-cycle supply for both filaments and plates. This amplifier has three radio-frequency stages and two audio-frequency stages, and uses a crystal detector. A special transformer with five windings is used, the primary being supplied with 110-volts a-c. The 60-cycle current when used in an ordinary amplifier circuit introduces a strong 60-cycle note which offers serious interference. This has been practically eliminated by balancing resistances, grid condensers and special grid leaks of comparatively low resistance, telephone transformer in the output circuit, and crystal detector instead of electron tube detector. In the final form of the amplifier, there is only a slight residual hum which is not objectionable. The amplification obtained with a-c. supply was as good as that obtained with the same amplifier used with d-c. supply. The complete unit is light, compact, and portable. For the reception of damped waves, the amplifier as constructed operated most satisfactorily for wave lengths from 200 to 750 meters. This range was determined by the working range of the radio-frequency transformers used. By using suitable radio-frequency transformers, it is expected that the amplifier will be effective for the reception of damped waves and undamped waves as long as 10,000 meters. For the reception of undamped waves, a separate heterodyne should be employed. T- e paper gives circuit diagrams, and states the values of the condensers, resistors and inductors used.
