WIRELESS.

Star (Christchurch), Volume LXIV, Issue 757, 10 May 1933, Page 4

WIRELESS.

The Advantages of Push-pull

Amplification. - ; - A STABILISING EFFECT. I (By “AERIAL.”) Push-pull amplification is generally; admitted to possess many advantages, which cannot be gained by other methods. In circuits of this type two valves are used to do the work of one: but. due to certain characteristics of the circuit, the output of the two output of either valve when used alone. Secondly, push-pull amplifiers are not affected by slight voltage variations, such as frequently take place in the output of plate power-supply units. Thirdly, another cause of frequent distortion (namely, saturation of the cores of audio transformers) is eliminated in push-pull transformers because of the fact that the plate current flows in opposite directions in the primary wtnding. And fourthly, the use of the circuit causes a stabilising effect, similar to that of a voltage-regulator valve, and as a result improves the operation of the detector and radio-frequency amplifier valves. Choke Coil as Grid Leak. In impedance and resistance-coupled amplifiers, an impedance coil instead of a resistor may often be used to advantage as a leak in the grid circuits of the amplifying valves, especially in that of the last valve. The secondaries of some audio-frequency amplifying transformers are suitable fc-r this purpose. Radio Symbols. A symbol (as used in the familiar schematic wiring diagrams) is a sign which in itself is simple and easily made, yet represents a radio part or device which may be complicated in its actual construction. The use of symbols greatly simplifies all kinds of radio diagrams, making it possible for radio fans and experimenters tc* trace the circuits easily. A glance at a schematic; diagram shows in an instant the nature; of any circuit, -which would be difficult to trace from a pictorial representation, or the apparatus itself. Every listener; should make himself acquainted with the standard symbols. There are not many—less than sixty. Short-Wave Reception. Using a s.w. converter with a 5-valve A.C. set last Sunday, WBXK. Pittsburg, on 48.86 metres (relaying KDKA) was reproduced at good loud-speaker strength from 4.3 5 to 5 p.m., when the station broadcast English. Canadian, American (U.S.), French and German sports result's (football, tennis, etc.). GSB, Daventry, on 31.55 metres, was just audible at 10 p.m. VK2ME, Sydney, on 31.28 metres, was heard about 10 p.m. and at very good strength. The best reception obtained that evening was that of Khabarovsk (RVIS), Siberia, on 70.2 metres. It looks as if the short waves are resuming their interesting phase, after a few weeks’ dullness—the annual “ blot out” usual at the beginning of our radio year out here.