The Screen Grid Browning-Drake

Radio Record, Volume II, Issue 33, 1 March 1929, Page 30

The Screen Grid Browning--Drake

AS§ I am feeling in somewhat of an argumentive humour, writes "Diogenes" (Cromwell), and as I have something in the ‘way of an argumentive subject before me, namely radio technicalities, may I presume to criticise the screen grid Browning Drake as recently published in the "Record." The fact that I do not know a tremendous lot about the subject may add to the interest of the argument, and I am sure I will have great pleasure in learning wherein I err. As far as I can see, to apply the screen grid to the B/D circuit, your article proposes, and in fact, does, cut out the primary. It changes its function altogether, and pretty well makes it useless in order to gain a similar function elsewhere in the circuit. In other words, he makes the circuit no longer a Browning Drake, for what is a Browning Drake beyond a peculiarly. wound primary? My idea on the subject of making a sereen grid B/D is not to alter the circuit at all, but to invent some method of winding B/D coils with an impedence of 200 thousand ohms, and use the valve in the ordinary way. I have heard that such a scheme has been published in a recent American magazine.. Answer to "Diogenes": In correcting our diagram of the sereen grid Browning Drake, "Diogenes" raises a good point, that is that the circuit is not really the Browning: Drake, but merely a screen grid circuit. The difficulty, he has pointed out, is to get an impedance of 200,000 ohms to match that of the screen grid valve. If transformer coupling is to be used, it will be found that the aerial coil may consist of a primary and secondary, as in the ordinary BrowningDrake, ‘but the transformer following the screen grid valve must be of a 1 to 1 ratio, and very tightly coupled. A good plan would be to wind on, say, a primary of 60 turns space wound. In the spaces, wind on the same number of turns to complete the secondary coil. This may be tuned in the usual manner. If the coupling were not so tight as this, the primary also would need tuning. To increase impedance of the radio frequency circuit, the coils should be space wound with silk-covered wire on celluloid formers, and all steps taken to reduce loss. This would raise the impedance, and get back more to the Browning-Drake scheme. --_ ---_-j