Radio Broadcast (May 1928-Apr 1929)

FRONT VIEW OF SKYSCRAPER" RECEIVER The "Skyscraper vale FOR a long time the neutrodyne type of receiver, consisting of a two-stage neutralized radio-frequency amplifier using 20iA-type tubes, a non-regenerative detector and a two-stage audio-frequency amplifier, justly has been considered an excellent set — sensitive, selective, and easily controlled. The receiver described in this article has these plus some additional good characteristics. This set uses two stages of r.f., detector and a two-stage audio amplifier, and to this extent is to be likened to a neutrodyne — but here the similarity ends. In this receiver two 222-type screen-grid tubes are used in the r.f. amplifier and, as a result, it becomes unnecessary to neutralize the r.f. circuits and greater amplification is obtained. The use of the screen-grid tube in the r.f. stages makes it essential that shielding be used and that all the plate and filament circuits be bypassed and filtered to prevent common coupling between the various circuits, which would result in oscillation. As part of the shielding, it is recommended that Remler tube shields be placed over each of the 222-type tubes. Starting with the antenna connection we will review the circuit arrangement used in this receiver, and in this way we will be able to bring to the attention of the reader the various interesting design features which the set possesses. The circuit diagram of the receiver is given in Fig. i. The input circuit to the receiver is arranged so that either an antenna or a loop may be used. The loop is connected to the two terminals marked "Loop" in which case no ground need be used, although a ground may be connected to the ground terminal if one desires. The antenna connects to the first coil Li, through a small 0.0001-mfd. fixed condenser, Ci. The switch SW2 adapts the set for use on either a short or a long antenna, and, when the set is put into operation, reception should be checked on both positions of the antenna switch. In locations where there are several near-by broadcasting stations and a selective receiver is essential, the antenna switch should be thrown to terminal No. [. With the antenna switch in this position the set is very selective; if the switch is thrown to terminal No. 2 the volume will increase but By CLIFFORD DENTON Ferranti, Inc. the set will lose some selectivity. In out-of-town locations, where very sharp tuning is not required, the set generally will give most satisfactory operation with the antenna switch on terminal No. 2. The output of the tuned circuit L1C2 feeds the grid circuit of the first 222-type tube which in turn feeds into the r.f. transformer consisting of L2L3. This transformer is wound on a threaded hard-rubber tube. The plate coil is wound first with a small-size wire in the bottom of the grooves and on top of this primary winding the secondary is laid, the secondary wire being of a CT'HE receiver described in this article is a development of Robert Arnold, in association with the laboratory of the Ferranti Company in the United States. The set was examined and tested in operation in New York City by a member of the staff of Radio Broadcast Laboratory and it performed very satisfactorily. The set is quite selective and the fidelity is excellent. Radio Broadcast will be pleased to hear from those who undertake the construction of the set. Constructional data is not given in this article for complete information of this sort may be obtained directly from the Ferranti Company. —The Editor. size such that it lays on top of the groove. The turns ratio is 1 to 1. The coils are mounted on a standard four-prong base and plug into sockets mounted on the sides of the shields, as indicated in the picture of the receiver. This secondary, L3, of this transformer feeds into the second radio-frequency tube whose output circuit supplies energy to the next transformer consisting of L4L5; this transformer is similar in construction to the preceding one. A C-bias detector is used; the plate is supplied with 90 volts through the 50,000-ohm resistor, Ri, and minus 6 volts is used on the grid. The output of the detector goes to the first audio transformer, Ti, across the primary of which is connected a jack, Ji, to which a phonograph pick-up uni't may be connected. The output circuit of the audio amplifier is push-pull with 171 a 95 type tubes. The output transformer, T3, should be of a type designed for use with the particular loud speaker which is to be used with the set. The designers of the receiver, feeling that many experimenters might have available an audio amplifier and, therefore, desire only to construct the radio-frequency and detector circuits, have shown the jack J2 in the plate circuit of the detector. If a separate amplifier is to be used it should be connected to this jack. As we mentioned in a previous paragraph all the battery circuits of the receiver have been filtered carefully in order to prevent common coupling which is often the cause of oscillations and motorboating. The screen-grid circuits of the two r.f. tubes are filtered by the 10,000-ohm resistors, R2 and R3 and the condensers, C5 and C6. The plate circuits of these tubes are filtered by resistors R4 and R5, each with a value of 50,000 ohms, and the by-pass condensers, C7 and Cg. The detector circuit is filtered by the 4-mfd. condenser C9 and the resistor Ri. Filtering in the detector circuit is especially important since even a small amount of coupling at this point will affect the characteristics of the audio amplifier and either make it distort or hum badly, and in some cases the coupling may be sufficient to make the audio amplifier oscillate. 1 Three filter circuits are located in the audio amplifier. The grid circuit of the first audio-frequency amplifying tube is filtered by a 2-mfd. condenser, Cm, and a 50,000 resistor R7 The plate circuit is filtered by a 20,000-ohm resistor, R8, and the condenser Cn with a capacity of 2 mfd. The third filter circuit is located in the grid circuit of the input push-pull transformer, T2, and it consists of a 10,000-ohm resistor, R9, and the condenser, C12, whose capacity is 2 mfd. [Push-pull amplifiers frequently have a tendency to oscillate especially if the power tubes have slightly different characteristics. If the amplifier does oscillate it can be overcome by placing a 50,000-ohm resistor between the center-tap of T2 and R9, in the grid circuit of the input push-pull transformer. This resistor should not be by-passed. The use of this resistance will not in any manner adversely affect the quality. Editor.]