Radio Broadcast (May 1928-Apr 1929)

'mi,i^inini^.n,mn FROHT VIEW OF RECEIVER The New A, C* Screen-Grid Browning Drake Receiver THE Browning-Drake Kit-Set has enjoyed a continued popularity since its introduction a number of years ago, doubtless due to the efficiency of the radio-frequency transformers, combined with the simplicity of the circuit in which they were used. During this time some slight improvements have been made. With the introduction of the screen-grid tube, the problem of designing a one-stage tuned radio-frequency amplifier was attacked again from an analytical standpoint and an extremely efficient transformer was developed for this type of circuit. The problem of obtaining, selectivity and gain in a radio-frequency amplifier employing a screen-grid tube differs considerably from that encountered when using a 1 99 or 20 1 A-ty pe tube. This is due to the inherent electrical characteristics of the tubes themselves. For instance, the 2oiA-type tube has a plate impedance of approximately 12,000 ohms, and an amplification factor of 8, while the screen-grid tube has approximately 400,000 ohms of impedance with an amplification factor of from 150 to 300. With the 20iA-type maxirrlum gain could be obtained easily by the proper number of turns on the primary, together with a normal coefficient of coupling which was about 0.5. This, together with a low-resistance secondary, resulted in a gain of about 12 to 15 per stage. However, when using this tube careful neutralization, even in a single-stage amplifier, was necessary to obtain the best results. The primary purpose in the design of the screen-grid tube was probably to make the capacity between plate and grid so small that neutralization was usually unnecessary. In interposing the screengrid between control grid and plate, high amplification in the tube itself resulted as well. PROBLEMS OF DESIGN THERE are two ways of obtaining radiofrequency amplification under the new condition imposed by the screen-grid tube, i e., by an auto-transformer (tuned impedance), or by By GLENN H. BROWNING Browning Drake Corporation the usual tuned radio-frequency transformer, consisting of a primary and secondary winding. These have been discussed at some length in a previous article in this magazine and will not be dwelt on here. It is sufficient to say that tuned impedance has the advantage of slightly more amplification per stage while the transformer gives greater selectivity. The design of such a transformer, however, is not a simple matter by any means. With the increase of plate resistance that the screen-grid tube has over the 201A type, the turns on the primary of the radio-frequency transformer should be increased a great deal for maximum gain, or the coefficient of coupling must be increased, or both. Unfortunately, there is a very definite limit to the number of turns which may be used on the primary of the transformer. This limit is determined by the distributed capacity and inductance of the winding itself, coupled with the capacity placed across it due to the plate to ground capacity of the tube used as the radio-frequency amplifier. These two capacities tune the primary to a definite wavelength, and if this wavelength is 200 meters or above, the transformer as a whole will tend to pass a signal coming in on this wave no matter where the secondary is tuned. In designing a transformer for the screen-grid, the high plate resistance means that primary turns should be increased, but the plate to ground capacity is increased over the 201 a type of tube by a factor of three or four times, due to the proximity of the screen grid to the plate. Therefore, it is essential to increase the coefficient of coupling as much as possible. Some months ago the writer started to. determine just how this coefficient of coupling could be increased from its normal value of about 0.5 to as great a value as possible (the maximum theoretical value is 1). The result was that, with a short winding length for the secondary and a slot wound primary placed in about J" from the low-potential end of the secondary, the coefficient of coupling in "5 creased to 0.91. Thus, with this coefficient of coupling and placing as many turns on the primary of the radio-frequency transformer as possible, consistent with keeping its natural period below 200 meters, a transformer for the screen-grid tube was developed, which has an extremely good gain. However, it might be stated that as far as the writer has been able to determine, it is impossible to get the maximum theoretical gain at broadcast wavelengths from the screen-grid tube because of the limitations imposed on the number of turns on the primary winding of the radio-frequency transformer. THE SELECTIVITY THE selectivity of the transformer under discussion, as well as the amplification, is considerably better than in the case of using the 20iA-type tube as a radio-frequency amplifier. There are two reasons for the increase in selectivity. First, other factors being equal, the selectivity is better in a radio-frequency transformer when the gain is below maximum. Second, for a given amount of gain the higher the coefficient of coupling the greater the selectivity, provided the resistance of the secondary winding of the transformer is approximately the same in both cases. This later statement will probably not be evident but can be proved mathematically or can be shown readily in laboratory measurements. The 1929 Browning-Drake Assembly employs one stage of tuned radio-frequency amplification with either a.c. or d.c. screen-grid tubes with the transformer described. Tickler feed-back is used in the detector as previously. No neutralization or shielding is necessary for efficient operation. The antenna system has been changed to use an untuned primary. This is because the coefficient of coupling between primary and secondary has been increased to 0.91 so that a primary is as effective as direct coupling and aids somewhat in making the kit absolutely single control. The 0.0001-mfd. condenser (Q) is con