Radio broadcast .. (1922-30)

A TWO-TUBE SCREEN-GRID RECEIVER Maximum efficiency is obtained by using metal front and sub-panels while there is also a metal plate between the r.f. and detector tube circuits. The active parts used in this receiver are listed on page 435, and the circuit diagram is also given there. A copper cylinder is used to shield the tube An Experimental Screen-Grid Receiver THE introduction of the screen-grid tube is most opportune for that section of the radio world which is constantly in search of a new plaything. The ux-222 (cx-322), or screen-grid tube, offers the advantage of an extremely high gain per stage when used with the inside grid as control grid and with a steady polarizing voltage impressed on the screen. As is to be expected, the high amplification factor is accompanied by a very high plate impedance, necessitating a high impedance in the coupling unit if the advantage of the high amplification factor is to be realized to its fullest extent. While the screened-grid tube requires no neutralization, careful shielding is necessary, particularly if several stages are used. Elaborate shielding is not required if only a single stage is used. For this reason, a single- stage amplifier adapts itself to preliminary experiments with the new tube. The set described here is purely experimental and no claims are made as to the results obtainable. Tests have shown that it is stable, and results in operation approach those obtained with three tuned cir- cuits and fixed input in a three-stage amplifier using standard tubes. The selectivity without regeneration is comparable to that obtained in a normal two-circuit system but the figure of merit of both circuits is somewhat reduced because the Charles Thomas first circuit is coupled to the antenna and the second feeds the detector. Two-circuit selectivity is insufficient for broadcasting reception in this country, hence the detector is made regenerative. Regeneration more than compensates the losses TlflTHOUT a doubt tie screen-grid tube is rr attracting the attention of every serious experi- menter and engineer in the radio field. Readers of RADIO BROADCAST are by this time familiar with the theory of this tube, and something of its operation and application has already been in- cluded in the contents of this magazine. The follow- ing brief description of a two-tube tuning unit, to which may be added any audio amplifier, is the forerunner of a construction article telling exactly how to build the receiver. This article, and the com- pleted receiver, which is being thoroughly engineered, are products of a well-known engineer, whose real name, unfortunately must be hidden with a pseudo- nym. Articles on the new screen-grid tube appeared in RADIO BROADCAST as follows: "Applications of the Four-Electrode Tube" December, 1927; "The Screened-grid Tube" January, 11)28; The Screened-grid Tube" February, 1928; "A Four- Tube Screened-grid Recever" March, 1928 — THE EDITOR. 434 in the tuning unit feeding the detector and im- proves both selectivity and sensitivity. The shielding required, as shown in the photograph, is not very elaborate. Brass panel and baseboard are used, and a brass partition between the two stages is also advisable. The screen-grid tube is enclosed in a copper cyl- inder which fits closely around the tube and extends about half an inch above it. It is also necessary to shield the short lead between the plate of the screen-grid tube and the detector. In general, shielding is required between all parts of the plate circuit and all parts of the control grid circuit. The circuit of the experimental set, which includes only detector and radio-frequency amplifier, follows conventional lines. The an- tenna is coupled through a tapped coil to the control grid of the screen-grid tube. The position of the tap controls to a certain extent the gain and selectivity of the set. Its position must be determined experimentally. The plate of the radio-frequency amplifier is coupled to a tuned impedance. Parallel coupling is used, the d. c. plate circuit going through a radio-frequency choke to the plate supply. The method of regeneration control is some- what unusual. The regeneration coil L t is not appreciably coupled to the grid circuit. Re-