Radio Broadcast (May 1928-Apr 1929)

A KEW WAT TO RECLAIM OLD SETS The use of the screen-grid tube in an external amplifier unit constitutes one of the simplest and most efficient ways of giving new life to receivers that are in need of a little more r. f. amplification. High gain, easy operation, and the elimination of the necessity for neutralization are among the features of this unit An Extra IL R Stage for Any Receiver By The Laboratory St THERE is hardly any receiver that cannot profitably utilize a little more radiofrequency amplification to enable its owner to reach out for those stations which ordinarily are just beyond the set's range. If, at the same time, selectivity can be added — and all this very cheaply — it is small wonder if many thousands of the one-stage screen-grid tube radio-frequency amplifiers described here are not built. It may be added in front of practically any receiver, and with those which are none too selective or sensitive now, the difference made on weak signals after they have passed through this amplifier is extraordinary. Such an amplifier as is illustrated in Fig. I is quite simple to build. It adds a stage of radio, adds selectivity, adds dx-getting ability, and with simple sets like the one tubers, the crystal receivers, or the reflex receivers, makes all the difference between a local or one-station receiver and a high gain modern receiver. The apparatus is as follows: a screen-grid tube, a coil and a condenser with a dial, a socket, an r. f. choke and a coupling condenser. The tube requires 3.3 volts at 0.132 amperes for the filament, 45 volts for the screen grid and from 9Q to 135 for the plate. The bias on the control or signal grid is obtained by taking the drop across a resistance in the negative filament lead of the tube. The circuit diagram is given in Fig. 2, and any one can put the apparatus together in about an hour witiL the certainty that it will work. If desired a box can be built around it, or the whole outfit can be placed in an aluminum or copper can or — as was done in the Laboratory — it can be built in the simple manner shown in Fig. 1. When this amplifier is used with a receiver which is not shielded, and which may already have considerable r. f. gain in it, it may be an advantage to shield the external amplifier, either by putting it into one of the Alcoa or similar box shields, or by placing a metal box around the coil. Under some conditions the amplifier may cause the receiver to oscillate, or the amplifier itself may oscillate. In the Laboratory no MANY owners of radio receivers desire to add a stage of radio-frequency amplification in front of their set. Such an amplifier with a tube of the 201 a type has a tendency to oscillate and create trouble if it has any r.f. voltage amplification in it. The answer is to use a screen-grid tube, which does not oscillate so easily. Such an amplifier is described here. In the Laboratory it has been possible to hear stations with its use that were inaudible without it. When the unit is used with the crystal receiver described in June Radio Broadcast, the receiver changes from a one-station set to one which will not only separate wjz from weaf under the very severe receiving conditions mentioned in the article, but will bring in all the other New York stations as well. No receiver has been found in the Laboratory which could not be used with this additional amplifier— it seems to be a universal unit. — The Editor. trouble was had at all, even when the external amplifier was used with receivers which had considerable gain. In the Laboratory the amplifier was used with the crystal receiver described in the June Radio Broadcast, making it possible to receive wjz, which is about thirty miles away, without getting weaf, which operates within 50 kc. of wjz and is only five miles away. Ordinarily wjz could not be distinguished through weaf on this crystal set. It was also possible to hear good signals from the other New York and New Jersey stations when the amplifier-crystal detector receiver was operated, although they could not be heard at all without the amplifier. In view of the extremely strong field in the vicinity of the Laboratory from weaf, the gain in selectivity evidenced by this additional tuning circuit and its amplifying tube is considerable. how the unit is operated TTHE operation of such an amplifier is simple. The antenna is removed from the receiver and attached to the amplifier. Then the output wire from the amplifier is attached to one of three places on the receiver. On most sets it is merely necessary to attach the output wire to the antenna binding post. On some sets which have an additional tube with an untuned antennaground circuit ahead of the r. f. amplifier itself, better signals will be received if the output wire is attached to the grid terminal of the first or second tube socket, or to the rotor or stator plates of the first tuning condenser. The best 217