QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 25, 28 December 1934, Page 41

QUESTIONS AND ANSWERS

J R.C. (New Plymouth): I have just *"* built the "Conquerox Superhet," described in the 1984 "Radio Guide," and it is giving me splendid results. Why is it necessary to have a step-down tranéformer between the driver valve and the class "B’" output stage? A.: During the half-cycle in which 5 grid of a class "B" valve becomes positive, the grid to filament resistance becomes very low, and grid current flows. If the effective internal resistance of the secondary winding of the transformer is high, this current will reduce considerably the voltage actually applied to the grid, and the wave-form will be distortet. Consequently, it ig necessary to keep the effective internal resistance of the secondary winding as low as possible, This effective resistance depends on the impedance of the driver valve, as well 2% on the actual ohmic resistance of the winding. By using a step-down transformer, the impedance of the valve is stepped down to-the required value for current operation. Af R.K. (Christchurch): I have a fila- * ment transformer with a 74-vou:t winding. I am using a 171 valve in the last stage of my set, and I would like t» use this transformer to supply the fila: ment current. I have tried placing a rcsistor in series with one side-of the winding, but this produces excessive hum. A.: In order to keep the filament current. of the valve in a balanced condition (which is necessary to keep hum at a minimum) the potential on each side of fhe centre tap of the transformer must be the same. If resistance ig.added-on one side, the voltage on that side of the filament is reduced and, the filament is unbalanced. If the centre-tap on the transformer is not used, and a variable voltage divider or potentiometer is employed for the centre-tap of the filament circuit, the adjustable arm of the resistor may be used to counteract the unbalancing. The best way of overcominz the difficulty, however, is to place two resistors in the circuit, one in each side of the filament supply. In this way the voltage on each side of the centre-tay is kept the same, and hum is reduced to a minimum. The actual values of the resistors depend on the filament current aniéi the amount of voltage reduction required. From Ohm’s law, the resistance equals the voltage drop divided by the current in amperes, i.e., R-=2.5 -- 25-10. Thus. a 5 ohm resistor should be connected: in each side of the filament. "OBT" (Auckland): You would probae ably find that by connecting the pick-up across the load resistor of the 2B7, as shown in the "Comet Superhet Five," described in the September "Radig Times," the leads to the pick-up terminals would be much shorter. In this case, the ‘terminals should not be shorted. when the set is being used for radio reception. Af C.P. (Westport): I have built the * "xplorer Four" shortwave set described in the "Radio Times," and am getting splendid results from it. However,- lately I have been troubled with an annoying crackling, which I cannot . _ trace, .

A,: Crackles and similar noises may be due to a defect in the receiver, in the aerial, earth or batteries, or to atmospheric disturbances or interference due. to electrical machinery, A brief test may be applied to the receiver by disconnecting the aerial and earth leads, and if the noise then ceases it may fairly safely be assumed that the receiver is in order. It should be noted that this (oes not mean that the aerial and earth themselves are in order, and it is advisable to examine the lead-in wires, insulators, and the joint between the earth lead and water-pipe or earth tube. The intensity of the noise caused by atmospherie disturbances varies from time to time. It is less in winter than in summer. Interference caused by electrical apparatus is often similar to an "atmospheric" background, and may be picked up by the aeriai or earth. Crackling due to a fault in the receiver may be due, amongst other things, to bad switch contacts, defective components or valves, bad wiring connections or faulty batteries, The spring contacts of the wavechange and on-off switches should be examined to make sure that a firm, springy grip is made, and if the contacts appear dirty they should be cleaned carefully with very fine sandpaper. The valve pins should be cleaned, and care Should he taken to see that they fit tightly in the valve holder sockets. In some cases’ the internal connections of valve holders develop faults, and it ie worth while examining these. The vari-

ous joints, terminals and insu] ated wires in the receiver should all be ] ooked over and remade or tightened up where necessary.

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