A Corner for Beginners

Radio Record, Volume II, Issue 47, 7 June 1929, Page 12

A Corner for Beginners

Power for Dynamic Speakers. DXNAMIC cone speakers are now becoming very popular, and with their popularity problems concerning them arise. The power required by these speakers to excite the field magnet ranges usually from’ half an amp to an ampere or more. This places a fairly heavy drain upon the 6-volt accumulator, and so some provision has to be made in order to keep the "A" battery up to scratch, There are two methods: 1. The provision of a charger with a charging rate of about 3 amps, to recharge the "A" battery as soon as it is finished with, and (2) keeping a trickle charger either on the battery when this is connected with the speaker or by connecting the battery charger directly to the speaker winding. Where a battery charger haying an output of half an amp. upward is obtainable it can be quite well connected between the electric light mains and the speaker. The only difficulty here is that if the charging rate is not high enough the field magnet. will not be saturated and hum will result, although this is not very distracting providing the field ean be relatively well provided with current. Connecting a trickle charger to the battery and the battery to the moving eoil loudspeaker as well as to the set has a slight disadvantage in that it is liable to cause the set to hum, but this again very much depends on the particular conditions in which the set and the speaker work. It is sometimes stated that this type of speaker requires ‘ery high plate voltage on the last valve and is less sensitive than an ordinary cone speaker. This is not necessarily the case. It is sometimes, and the intending purchaser should ascertain the sensitivity of the speaker he intends purchasing. Recently, the writer tested a good moving coil speaker comparing it with a cone type speaker for both sensitivity, tone and quality. It was found on all these three points that the dynamie cone was far superior, it being quite as sensitive as an ordinary speaker even on distant stations and with only 150 volts on the plate of the last valve, When operating on the local station, it was found that the voltage on this valve could be reduced to 100 without a great deal of loss without quality. The Care of Wet Batteries. i the set is to function well it is essential that the wet batteries, whether "A" or "B," are carefully attended to. The principal points in the care of the batteries are as follow: (1) Keep it fully charged, (2) keep the liquid at least jin. above the plates (3) handle it carefully. When the battery is being charged the sulphuric acid which has passed from the electrolyte into the plates is driven out, thus raising the specific gravity of thi electrolyte, and one recognises the condition of a full battery. This is the explanation of the hydrometer testing apvaratus, whereby various specific gravities are tested. As a battery slowly discharges, sulphurie acid passes from the electrolyte (the solution) into the plates, and unless this is thoroughly driven out by regular recharging the plates become loaded with sulphuric acid, and sulphation takes place. Acid should never be added. If the battery becomes weak, and the full specific gravity cannot he real-

ised, it should be charged as fully as possible, and the electrolyte drained off and fresh solution of the specific gravity of 1.8 (1800) added. It is important that the solution be kept covering the plates. This is a point very liable to be overlooked. A case was noted by the writer recently where a listener complained that his battery would not come up’ to the right charge, and dropped very rapidly. On examination it proved that the solution was not covering the plates. In fact, two inches were uneovered, ‘This is liable to permanently ruin a battery, for in discharging sulphurie acid enters the plates, the electrolyte evaporates slightly, and on recharging the sulphuric acid cannot ‘ driven out, because there is nothing drive into, hence in time the plates come ruined. Distilled or rain water collected in a earthenware vessel can be used. . Correspondents quite frequently complain that in spite of regular charging and keeping the electrolyte to its proper level the battery rapidly runs down, indicating a short circuit.- Presuming this is within the accumulator (which can be tested by isolating the accumulator from the set, and leaving it for some time, and testing), it is caused probably by parts of the plates becoming dislodged, falling to the ‘bottom, and short cireuiting. This is the outcome of either age or rough handling. In addition, rough handling causes leakage. Concerning Audio Transformers. [HE several advice regarding these. is to avoid small transformers, the reason being that unless special material is used in the core and in the wire of the windings, various faults will arise. ~ Particularly is this the ease when more than one transformer has to be used. Probably the most common fault in the audio amplifier is audio frequency oscil-lation-a high-pitched squeal that altogether spoils: reception. This is sometimes very difficult to eliminate, and the writer had a quite interesting experience recently. An ‘amplifier using high voltages was employed, but it was found that, whenever a high voltage was applied to the plate of the last valve a high-pitched howl, which developed into a_ roar, drowned reception. It was caused probably through interaction between the transformers, an over-saturation of the last vaive or grid plate interaction. There are several cures for this type of audio oscillation, and from time to time, we have recommended them to corresnondents through "Questions and Answers." The usual practice is to reverse the primary leads to the transformer, or to place a resistance in shunt, that is across the terminals of the secondary of the last transformer (G to GB or C minus). This resistance may be grid leak, and the most appropriate resis« tance is found to be usually half megohm, If the whistle persists, resistance may be lowered, Such a resistance may be placed across the secondary of the first transformer. This will reduce volume, but considerably increase tone. By placing the grid leak in series with the grid, a preventative is assured. Short grid and plate wires well clear of one another should be employed. Transformers should be at right angles to one another, and it is sometimes necessary to earth the cores. When this howling is noticed with a crystal and amplifier, A minus should be connected to earth.