Radio Broadcast (May 1928-Apr 1929)

A Two-Tube Screen-Grid Tuner By Glenn H. Browning Browning-Drake Corporation THE ARRANGEMENT OF APPARATUS PROVIDES FOR A SYMMETRICAL FRONT PANEL LAYOUT TTATELY there has been put on the market a tube especially designed for r.f. amplification and which is known as the screen-grid tube. So numerous have been the requests for descriptive matter relative to the use of this tube with the Browning-Drake set that it has been decided to give all possible information on the subject at the present time. The circuit built around this tube has been experimented with for the last two or three months but until the writer had worked out the idea of neutralization, the tube did not perform in a manner which came up to his expectations. With the use of neutralization and a regenerative detector, the radio-frequency amplification obtainable is so augmented that signals inaudible before are as loud as locals, providing the noise and interference is not such as to drown them out. The circuit finally decided upon by the author is given in Fig. I. The selectivity of the receiver with the average antenna is somewhat inferior with the screen-grid tube as a radio amplifier, but with a short antenna, signal strength is decreased very little and selectivity is improved. Although the grid-to-plate capacity of the screen-grid tube is very small indeed it is not eradicated entirely. Consequently, in a circuit employing regeneration on the radio-frequency transformer (detector input circuit) it is the writer's opinion that it is necessary to neutralize. This may be readily seen by reference to Fig. 2, where the reactance of the tuned circuit is plotted against the ratio of the resonant and non-resonant frequencies. The equations for this curve are given on the diagram and it will be noted that the resistance of the circuit enters in such a way that the lower the resistance the higher the positive reactance. Thus, with regeneration, one can get almost infinite positive reactance just before the circuit goes into oscillation. As all radio experimenters know, the higher the positive reactance in the plate circuit of the radio-frequency amplifier tube, the greater the tendency there is for that tube and its circuit to go into oscillation, due to the feed-back through the capacity between plate and grid. Thus, even if the capacity between plate and grid is extremely small, the circuit before the one regenerated will oscillate unless this capacity in the radio-frequency amplifier tube is neutralized. Incidentally, the capacity in the ux-222 type tube which must be neutralized THIS article describes Mr. Browning's version of how a screen-grid tube can be used in the well-known Browning-Drake circuit. The reader will note the following facts: The receiver is shielded; the amplifier is neutralised, and is coupled to the detector through a tuned impedance; a trimmer condenser is used to keep the amplifier input circuit in tune with the detector circuit; the plate voltage is fed to the first tube through a choke coil — shunt fed, as the amateurs would say. This tuning unit, composed of two tubes, requires an audio amplifier, and any system of amplification with which the constructor is familiar will be satisfactory. The constructor will find this arrangement interesting and it is as can be seen, one version of the use of the screen-grid tube in a tuner unit. Mr. Millen's article, beginning on page 20, provides a complete receiver using a somewhat similar r. f. circuit and an excellent audio channel: The choice is largely a matter of personal preference. In the Laboratory, the operation of the receivers was identical. — The Editor is much smaller than that of the 199 type tube. The screen-grid tube has the advantage of a high amplification factor, due to the effect of the screen grid on the mutual conductance of the tube. This large amplification factor makes possible great signal amplification. The plate impedance of the 222 type tube is very high, and greater efficiency can be obtained by using direct coupling into the tuning circuit, as shown in Fig. 1, than by the use of transformer coupling. This makes it altogether imperative that the set-builder use some system of parallel feed, such as that shown in the accompanying diagram. A parallel feed system has been already adopted in connection with the Browning-Drake this year for the reason that it keeps the radiofrequency current out of the plate supply, and makes neutralization with the ordinary 201-A type tube considerably easier; consequently, adopting the 222 type tube for an r.f. amplifier entails very few changes over the present Browning-Drake circuit. Another essential part of the set built around the 222 type tube is complete shielding. Any magnetic feed-back from one tuning circuit to the preceding one creates a tendency for oscillation to take place, and this must be prevented at all costs. There are numerous other advantages of the. use of shielding with which the reader is more or less conversant, so they will not be outlined here. In the filament circuit a 1 5-ohm resistance must be placed in series with the rheostat specified for the ordinary two-tube Browning-Drake receiver, for cutting down the five volts which the 201-A type of tube uses, to three and three tenths volts, which should be applied to the filament of the 222. A bypass condenser of 0.5 mfd. should be con OBt15 OB + 90 6 Gnd FIG. I 27