Value of Short Wave

Radio Record, Volume II, Issue 9, 14 September 1928, Page 27

Value of Short Wave

HE chief characteristic that must be looked for in a short-wave valve is that it shall not be noisy, either in operation. nor by reason of its genera~ tion of a hum due to microphonice action. Bright-emitter valves are, generally, better in this regard than the dull filament kind; but the glowless, or semi-glowless valves have now been developed to such an extent that the microphonic noise can be usually reduced to almost nothing by the use of a good non-microphonic valve holder. Incidentally a properly made holder of this type will also have a low capae ity across the sockets, which helps greatly in a short-wave receiver. Unfortunately, however, the use of an anti-microphonic holder that is very free in its springs will bring another trouble in its train when the set is being used on 20 metres and below. Swinging Detector Valve. HE detector valve will start swinging as the tuning dial is revolved and will continue to swing, with the .esult that there is a continuous small change in capacity. This is quite sufficient to set up a kind of "high-speed fading" that will make distant and

faint telephony stations very difficult to read. It has not so bad an effect when the receiver is oscillating for the reception of morse signals; but when the detector is nearly on the point of oscillation quite small capacity changes will affect it badly. The cure for this trouble is to "damp" the valves by means of small pieces of cotton wool, placed so that they will stop the swinging without bringing back the microphonic noises. With regard to noisy working, that is a matter of test. Valves that are of the same make and class often differ quite a lot one from another. In most cases a valve that is suited for a particular position in an ordin ary receiver will work quite well in the same position in a short-wave set. For leaky grid rectification any valve with an internal resistance of from 11,000 to 20,000 ohms will do quite well. Smaller Grid Condenser. T is usual, however, to use a rather smaller grid condenser than is nor mal to the broadcast receiver, and to put up the value of the grid resistance

very considerably. Up to 10 megohms ean be°used with effect and a highresistance connected to the positive side of the filament will often be effective in reducing or removing the annoying "grid howl" that sometimes completely spoils reception by bursting out when the set should be most sensitive.