Radio Broadcast (May 1928-Apr 1929)

78 RADIO BROADCAST SEPTEMBER, 1928 i ■ ■■ September, 1928. Radio Broadcast's Service Data Sheets on Manufactured Receivers rHIS Service Data Sheet is devoted to a description of one of the leading receivers now being manufactured and sold in England. The design of this receiver affords some interesting comparisons with American receivers of similar design, which should prove interesting to many of our readers. Screen-grid tubes were available and in use in England for several years before the advent of the type 222 tubes in America, and the double-ended construction adapted by our British friends has some advantages in simplifying receiver design, which are pointed out below. 'THE most interesting receiver in the British radio field at the present moment is undoubtedly the Marconiphone Model 61. This six-tube set has three screen-grid tubes, a C-battery type detector and two resistance-capacity-coupled audiofrequency stages. The latter are normal in construction, but the radio-frequency end of the instrument is decidedly unique and gives many valuable lessons to the experimenter. The tubes employed are of the double ended type due to Captain Round, the famous Marconi engineer. Their construction is shown in the photographs below. It will be seen that a "V" filament and standard grid are mounted on one glass foot, and the plate with the screen grid on a second, the electrodes then being assembled in a glass bulb and the tube finished off with two caps. The anode and screening grid are flat, the latter having a "skirt" approximately i" wide brought as Screen grid CONTROL GRID 1\ FILAMENT THE ENGLISH SCREEN-GRID TUBE closely as possible to the glass to prevent edge effects. The great practical advantage of this construction is shown in the top view of the receiver, where it will be seen that the tubes are mounted horizontally with their screening grids in the planes of the inter-stage partitions, which are cut away just sufficiently to allow for the diameter of the glass. The whole of each stage is entirely enclosed by the screening when the cover is closed. Furthermore, by an ingenious process the magnesium "getter" used to remove the last traces of gas from the tube is limited to the plate end of the bulb and cannot therefore form a conducting path past the screening grid. A 6-volt, 0.25-ampere filament is used, and at the standard operating voltages of 120 on the plate, 80 on the screening grid and minus li on the control grid, the amplification factor is 110 and the plate impedance 175,000 ohms. Full advantage of this high mu value cannot be taken when three stages are in use, as instability is inevitable. A step-up of 30-35 per stage is actually achieved in Model 61. Incoming signals are therefore amplified 30,000 times before reaching the detector. Multiply this by a conservative 25 as being the stepup of the detector and two audiofrequency tubes and we theoretically obtain a total magnification of nearly three quarters of a million times between aerial and output! With this stupendous increase it is clearly essential not only to adopt the most perfect screening but also to take precautions against coupling through the common wiring and batteries, in order to obtain stable operation. THE GENERAL LAYOUT OF PARTS The system of screening is shown in the top view. Front, back, baseboards and sides of the cabinet are copper lined and there is a partition between each r.f. stage. A broad lip is formed round the upper edge of the screen and in this is laid a length of cable covered with braided copper and bonded to the metal at several points. To the underside of the cabinet top is secured a copper plate of sufficient size to complete the screening. When it is closed this plate is pressed into firm contact with the braided cable throughout its length, thus insuring a perfect electrical joint. The four circuits — loop and three tuned circuit — are tuned by separate condensers arranged in pairs with edge control dials so placed as to be easily controlled with two fingers. As the circuits are accurately matched and not too sharply tuned (the selectivity of this receiver is probably insufficient for use in the United States — Editor) it is a simple matter to find a station by synchronizing at approximately the correct wavelength and operating all simultaneously. The dials are calibrated in meters. There are two sets of astatic tuning coils covering 250-550 and 1000-2000 meters, the change-over being effected by multiple switches controlled by the "Range Switch" on the front panel. THE CIRCUIT OF THE MARCONIPHONE MODEL 6l