Radio Broadcast (May 1928-Apr 1929)

226 RADIO BROADCAST AUGUST, i9a8 better signal strength of dx stations, it has been found that the more conventional 0.0002 5-mfd. condenser with a 2-megohm leak results in more smooth and uniform regeneration over the entire broadcast band. When the 0.0001-mfd. condenser is used the set will not oscillate at the very upper end of the wavelength range. The polarity of the primary connections to the r.f. transformer also has a noticeable effect at times on the smoothness of the regeneration. While generally best results are obtained with the plate lead of the 222 tube connected to the primary terminal nearest the front panel, such is not always the case. In some instances it makes no apparent difference which way these leads are connected, while in other instances the reversed connection gives best results. NOTES ON THE AUDIO SYSTEM THE audio system used in the original receiver, while capable of very satisfactory results, is not the only type that may be employed. Many inquiries have been received from readers who favored transformer or resistance coupling and wanted to know if one of the other forms of amplification could be employed with quite satisfactory results. The answer is decidedly "Yes." If a pair of high-grade audio transformers, such as the National, Amertran, Ferranti, etc., are employed, just as fine results are obtained with the use of one less tube. In the advent of such a change, the f-ohm filament equalizer specified for the 5-tube set should be replaced by a i-ohm resistor. It has been found that with some of the sets made up by different constructors, it was necessary to ground the case of the tone filter to the A -minus lead. This same condition was also encountered in one transformer-coupled set, in which grounding the cases of both audio transformers, as well as that of the tone filter, proved worth while. SOCKET POWER OPERATION \ A/HILE designed for use with a 6volt stor' v age A battery and a good 180 volt B-power unit, a.c. tubes may be used, if desired, for complete a.c. operation of the Screen-Grid Five re ceiver. In such event, the new a.c. type 222 tube, such as the CeCo, should be used in the r.f. stage. Alternating current has been used by some experiments directly on the filament of a standard d.c. type of 222 tube, but the results have never been any too satisfactory, due to the resulting 60-cycle modulation of the signal. The new a.c. 222 tube, however, completely overcomes such trouble, as it is of the heater type, similar to the UY-227. The UY-227 works very well as the detector in conjunction with the a.c. 222 in the r.f. stage. In the first audio stages, either 227's or the 226's may be used. Some tube manufacturers are now making high-mu a.c. tubes of the 226 type, one FREE BLUEPRINTS The interest in the d. c. model of the Rational Screen-Grid Five has been so great that blueprints of this receiver will be sent to all who write for them. Address requeststo Mr. James Millen, care of RADIO BROADCAST of which will be quite worth while for use in the second stage. In the output stage the regular 1 71 a with 5 volts a.c. on the filament is used. The layout of the a.c. set is essentially the same as that of the d.c. model except for a few minor substitutions among some of the smaller parts. For the radio-frequency and the detector tubes, 5-prong sockets should be used in place of the 4-prong type. The two filament ballasts of the d.c. set are omitted. The two grid bias resistors, the power tube grid filter resistor and associated condenser, the two center-tapped resistors, the small potentiometer, the bypass condenser and the small i|-voIt biasing cell for the 222 tube are the special parts required for the a.c. receiver. The parts and connections are indicated in Fig. 1. Flexible No. 18 rubber covered wire should be used for making all the filament connections, which should be run in twisted pairs. The dial light is connected directly across the2.25-volta.c. supply. The leads to the dial light should be twisted together. While a center-tapped resistor may, if desired, be used across the 15-volt filament line, it will be found in most cases that the use of a potentiometer, or other form of adjustable resistor for this work, will result in less a.c. hum from the finished receiver. The average value of the resistor used for obtaining the grid biasing voltage for the first two audio stages is about 1000 ohms, but it may be found with some sets that a slightly higher value will give better results. Varying the plate voltage of the first a.f. stage is also something well worth trying. The power unit for the operation of such a receiver may comprise either a standard highgrade 180-volt B unit and separate filament heating transformer, or else one of the special combination A-B units, such as the National No. 7180 A-B unit, in which the filament windings are part of the B-supply transformer. This later arrangement makes a compact and easily connected power supply. A list of the additional parts necessary for a.c. operation is given on this page. Other makes of parts may be substituted if the set builder desires. LIST OF ADDITIONAL PARTS FOR A.C. OPERATION 2 General Radio 5-prong sockets 2 25-ohm center-tapped resistors 1 2000-ohm resistor 1 1000-ohm resistor 1 .i-megohm resistor 3 single mountings 1 Carter midget 20-ohm potentiometer. 2 Tobe i-mfd. condensers 1 i| volt C Battery 1 CeCo a.c. 222 tube 1 CeCo N 27 tube 1 CeCo M 26 standard tube 1 CeCo AC Hi-Mu tube 1 Ceco J 71 a tube The accessories for a.c. operation are a standard 180-volt B-power unit, and a filament heating transformer.