Radio Broadcast (May 1928-Apr 1929)

HEW LIFE FOR A WEAK RECEIVER The screen-grid tube can be added to any receiver, and — at the cost of one additional control — will add a respectable amount of sensitivity and some selectivity. Directions are given for using this unit with any receiver A Screen-Qrid Booster Unit for Any Receiver THE introduction of the screen-grid tube has opened a new field in r.f. amplification, for not only does this tube make possible at broadcast frequencies an amplification much greater than is possible with a 201 -a type, but because of its very small capacity between control grid and plate, relative stability as an r.f. amplifier is secured. As most experimenters know, the screen-grid tube has two grids, one forming a nearly complete shield around the plate and known as the screen grid, while the other is practically the same as the grid in the ordinary tube, and is called the control grid. The screen-grid has two effects; it shields the plate as its name indicates, and also increases the mutual conductance in such a manner that the amplification of the tube may be at some frequencies as high as 100. The function of the control grid is, as in the 201 -a tube, to regulate the flow of electrons between filament and plate. The construction of the tube may be seen in the accompanying photographs. The metal cap on top of the tube is the terminal for the control grid while the prong on the socket, marked "grid," is the terminal for the screen-grid. These connections should be carefully noted. By Glenn H. Browning Browning-Drake Corporation To get the utmost from the screen-grid tube involves careful shielding of the tube and the circuit in which it is used. There are thousands of sets which are not now shielded nor could they readily be shielded. This difficulty, in addition t<asin^ — — i*3** "A/f/?. BROIVNING'S "booster" provides at one time a means of improving the dx ability of any receiver and its selectivity. In the Laboratory, it was possible to hear stations that were not audible without it. It was also possible here to listen to kdka with very little interference from wabc, only 8 miles away, while under usual conditimis this is not possible at all. In our estimation, this booster should be a gold mine to service men — because nearly every one wants some little gadget like this that will make an otherwise almost-dead receiver come to life. We hope that Mr. Browning will design a set of plug-in coils so that this single stage of r.f. amplification may be used with short-wave receivers. It will then eliminate nearly all radiation from these receivers, as well as improve signal strength. — The Editor. e*g>8 ^— — IgV 87 to the fact that it is very rare indeed that any existing circuit can be efficiently and easily altered for the successful use of the screen-grid tube makes it wiser in most cases to find ways of utilizing the high gain to be secured from the screen-grid tube elsewhere than in the receiver proper. For the last year there has been an insistent demand for another stage of tuned radiofrequency amplification to be added to the Browning-Drake receivers. The writer has worked a great deal on this problem only to come to the conclusion that it was not feasible for the home-builder because of the high gain in the Browning-Drake transformers which entails careful construction, and critical adjustment of the neutralizing condensers. However, with the availability of the screen-grid tube, the problem simplifies itself considerably. For some time the writer has been experimenting with a one-stage r.f. amplifier which could be added to the large number of Browning-Drake sets which are in use. This amplifier employs the screen-grid tube, and not only gives a tremendous r.f. amplification, but increases selectivity to a marked degree. It has also been found that the one-stage amplifier can be used