Radio Broadcast (May 1928-Apr 1929)

THE A. C. MODEL OF THE NATIONAL SCREEN-GRID FIVE Operating the National Screen-Grid Five WHEN the National Screen-Grid Five was designed, it was with the idea of making available to the radio public a receiver that could be easily constructed by the average fan; that would involve no difficult mechanical work in its construction; that would prove sufficiently selective to bring in a few distant stations while the locals were still on early in the evening; that would be easy to tune; and that would not require the use of a conventional antenna. That these results and even more have been successfully achieved is well proved by the many enthusiastic letters received from readers who have actually built the receiver. One New York City resident wrote that he received kfi, Los Angeles, the first night he tried the set, while letters and phone calls were also received from a number of New England fans who were consistently obtaining far better dx reception than the designer had even hoped possible. As was pointed out in the previous article, the set itself is an improvement on the original National Browning-Drake Circuit, in that the r.f. transformer has been redesigned to use the new ux-222 screen-grid tube; and the layout and design of parts have been altered to provide for single control tuning and simplicity of wiring. From the contents of a number of the letters received, it would seem that there are a few questions of rather common interest regarding accessories, operation, and variations in design of the Screen-Grid Five. THE CORRECT RESISTORS OERHAPS one of the most important ques1 tions that have come up is in regard to the values of filament equalizers or ballasts specified and indicated in the circuit diagrams. The values given were in ohms, which is the system being used by most filament resistor manufacturers at present. The *-ohm resistor is one which when used with five j-ampere tubes, will drop the 6volt filament supply down to the 5 volts required. While only four ;-ampere tubes are used By J amies Millen JN THE May issue of Radio Broadcast Mr. M Men described the construction of a Five Tube Screen-Grid Receiver. Since this article appeared Mr. Millen has received many letters from builders of the set many of whom desired further information on how to operate it. The operating suggestions and trouble finding hints which are given in this article should therefore prove interesting to those of Radio Broadcast's readers who have constructed this set. The description in the May issue described the d. c. operated model. The availability of a.c. screen-grid tubes has now made it possible to operate this receiver from the light socket, and the circuit diagram and photograph of the a.c. model are to be found in this article. — The Editor in the set, the current drawn by the 222 plus that drawn by the dial light is equivalent to an additional J ampere. The 15-ohm equalizer is used to decrease the five volts available across the filaments of the large tubes down to the 3.3 volts required for use across the 222 tube filament. In a few cases where trouble has been reported due to the receiver lacking sensitivity, it was found that the difficulty was caused by the use of a 22-ohm filament resistor rather than a 15ohm unit. The 22-ohm filament ballasts are made for use in cases where the ux-222 is operated directly from six volts and not from five volts, as in the case of this receiver. In other words, the number 22 designation should not be misconstrued to mean that this resistor is the proper one for use with the ux-222 tube under all conditions. When a type 22 filament ballast is used with the National Screen Grid Five, the filament voltage on the 222 is too low and the receiver is insensitive. Another cause of lack of sensitivity, which at the same time also results in lack of selectivity, is the failure to "line up" the two tuned circuits. 225 Perhaps, in order for the operator to become fully impressed with the importance of this adjustment, it would be well to loosen the condenser rotors from their shafts and tune in several local and semi-local stations by separately adjusting each condenser. Then try detuning one of the two condensers and note the difference in results. It might also be well at this same time to study the operation of the antenna variometer or trimmer. With the antenna variometer rotor in mid position detune the larger variable condenser and then retune by means of the trimmer. Before fastening the condensers to the dial shaft again, tune in a local station with a wavelength somewhere around 360 meters. The antenna trimmer should be in mid-position. what tubes to use \ A / HILE the use of a good 200-A detector * * tube (when the grid leak return has been shifted from the positive to the negative side of the filament circuit) results in increased sensitivity, the lack of stability and the noisy operation encountered offsets these advantages. Where a sensitive detector tube is desired for dx reception, it is recommended that a highmu tube such as the ux-240 be employed. While not generally considered as satisfactory as the ii2a for all-around use, the 240 will be found of worth-while aid on out-of-town reception. Due to the difference in inter-electrode capacity of the 1 12A, 200A, and 240, it is necessary to readjust the trimmer and the tuning condensers when changing from one detector tube to another. In some instances, it may even be found necessary to reset the position of one of the variable condenser rotors, as previously described. There seems to be a trend upon the part of many set builders at present to use smaller values of grid condensers with correspondingly higher values of grid leaks. While the use of a 0.0001-mfd. grid condenser with a 4 or 5megohm leak will generally result in slightly