“A” CURRENT FROM MAINS

Sun (Auckland), Volume III, Issue 628, 3 April 1929, Page 14

“A” CURRENT FROM MAINS

DEVICES NOW IN USE After many years of research the problem of suppling current to light receiving valve lilaments from the electric light mains has at last been effectively solved. Three different methods of employing the mains for this purpose are now available to the amateur. Two of these involve the use of special valves and cannot easily be applied to a complete receiver without substantial alterations to the wiring, but the third, which involves the use of a battery eliminator identical with a high tension eliminator can be employed to supply any type of valve receiver. Alternating current valves are of two types. In the first the filament is replaced by a sheath of very light foil impregnated with a thorium compound. Inside this sheath is placed a small heating unit resembling a miniature electric radiator. This heater is connected to a small transformer operated from the electric supply mains. It heats the outer sheath to a dull red heat, and thus enables it to produce an emission of electrons necessary to maintain the operation of the valve. The second type of alternating current valve is simpler. It resembles an ordinary valve, but the filament is slightly more massive. Alternating current is applied direct to the filament, but because the filament is fairly thick its temperature does not fluctuate with the alterations in the supply current. Thermal inertia causes it to maintain a uniform temperature, and thus to provide a uniform emission of electrons. In this type of valve, however, it is necessary for the return leads from transformers and tuners to be connected to the midpoint of a resistance across the filament. Unless this is done a laud hum will be produced in the loud speaker. The adjustment is a delicate one, and in most home-made sets will probably be found to require a good deal of skill and patience. For most purposes the new filament supply eliminators will be found to give excellent results. The parts for these components can now be purchased for about the same price as the parts for a high tension eliminator, and they may be readily assembled. Current from the mains is reduced to an appropriate working pressure through a small transformer. It is then rectified. The most suitable rectifier for this purpose is the copper oxide dry rectifier, which, operating on the same principle as the crystal detector, was first developed for use in trickle chargers. It has now been produced in a form large enough to maintain the filament current required to operate an eight-valve set. The smoothing equipment consists of a choke, suitable types of which can be purchased cheaply, and a bank of the new dry electrolic condensers. These condensers are made in sizes, having a capacity of about 1,500 microfarads, but they are little larger than a four microfarad paper condenser. The output from the smoother consists of a steady, direct current, which can be applied to the filament terminals of a receiver in the same way as the output from an accumulator battery.