Radio Broadcast (May 1928-Apr 1929)

100 RADIO BROADCAST JUNE, 1928 ■ FIG. IA-B Details for mounting and tapping the coils on in a clockwise direction. The coils in each assembly should be arranged so that all of the arrow marked sides face in the same direction. The two groups of coils should be mounted as far apart as mechanically possible, and at right angles to each other. After the coils have been mounted and the positions of the other pieces of equipment have been determined, the inside and outside leads of each coil may be brought to the most convenient spoke of the spider-web form, and fastened through a hole in the spoke by means of a screw and nut (see diagrams). Then the spokes may be cut off to within \" of the wire on the coil, thus reducing the over-all size of the coils. For connections, the outside and inside leads of coils are indicated on the wiring diagram by "O" and "I" respectively. The center point of the NP is coil indicated by "M." The coils as they appear on the wiring diagram, Fig 3, from left to right are as follows: Antenna, Li, antenna secondary, L2, NP, L3, and L6, secondary, L4, and above the secondary, the tickler coil, L6. The taps on the antenna coil may be connected to an inductance switch if desired, but a simpler method is to place a test clip (See Fig. 1 A) on the end of the ground lead, and clip to the tap which gives the best results. The longer the antenna in use, the fewer turns will be required on the antenna coil. One tap will be found which will give satisfactory results on all wavelengths. The method of obtaining the mid-point "M" of the NP coil may require some explanation. "M" is obtained by connecting the inside end of one winding with the outside end of the other winding. Two different colors of v/ire may be used to avoid confusing the two windings, or a flashlight bulb may be lighted through each winding to locate the corresponding inside and outside ends. ■ LOCATING THE APPARATUS THERE is nothing special to be said about the locating or mounting of the remaining equipment, with the exception, of course, that the grid and plate leads should be kept short. The photographs give an idea of a good baseboard layout which was designed to fit a phonograph cabinet and to include all the a.c. power supply apparatus. The panel layout suggested may easily be modified to suit the size and shape of the panel used by the constructor who duplicates this receiver. The switch mounted on the panel controls the line current supply and should be of a size and capacity equal to wall switches used for house lighting. A simple method is to use the body of any ordinary no-volt tumbler switch and mount it directly on the panel with a slot cut in the panel for the lever. No rheostats are needed when the a.c. tubes are used, and the volume control may be a 25,000-ohm potentiometer in the antenna circuit or the volume control may be as indicated in the model described here, i.e., a variable resistance such as a Clarostat or Bradleyohm across the secondary of the first audio transformer. No loud speaker jack is shown on the panel, as modern practice tends toward mounting this at the back of the set if it is used at all, with the antenna and ground binding posts. As to the make of tubes to be used, there is little choice between standard, reliable products. There has been considerable doubt on the part of prospective set-builders as to whether the a.c. tubes will give very long usage. For those who prefer to make certain, a written guarantee is given with some makes to replace tubes free of charge if they fail to function for one year. This should satisfy the demands of the most exacting buyer. This article describes the use of a.c. tubes which employ the Radiotron type bases, although the set will work excellently with the Sovereign or Kellogg a.c. tubes. In the event that the builder uses this type of tube, the major difference between that construction and this is in the a.c. filament circuit. No rules need be observed in the assembling of the set, except that the filament leads should be isolated as far as possible from all other leads, and especially from grid leads. The pho Base Board FIG. 2 The coils and their relation to the panel tographs show how this may be accomplished, it is undoubtedly wise to complete all other wiring of the set before starting the filament wiring, to avoid any possibility of connecting filament leads to any other part of the circuit. The filament current may be obtained from a filament transformer. If a 1 12 or 171 type tube is to be used for the last amplifier, the filament transformer should have three voltages; namely, 1 J volts, 25 volts, and 5 volts. Some B power units provide a 5-volt winding which may be used for the amplifier tube, in which case only the two lower voltages need be supplied by the filament transformer. If a 210 type tube is used for an amplifier volts are required for the filament, and this voltage is usually provided in the power pack or powerizer being used for plate current. Any good filament transformer may be used, some makes furnishing mid-taps for the 25-volt and 5-volt windings. It is not wise to use a midtap in the transformer on so low a voltage as 1 1, so that instead a mid-tap resistance or a potentiometer, R6, is used to obtain the mid-point of the 1 f-volt winding. The mid points of the 2j-volt and 5-volt windings may be obtained in the same manner if the transformer is not provided with mid-taps. SECURING GRID BIAS FOR grid bias on the reflexed tube, the center point of the mid-tap resistance is connected to one side of a 1000-ohm resistor, R4, The other side of the resistor is connected to ground and there is a 1.0-mfd. condenser, C7, across the resistor. For grid bias of the power tube, the midtap of the filament transformer winding supplying this tube is connected to a resistance of 2000 ohms, R3, the other side of which is connected to ground. A 1.0-mfd. condenser Cs is placed across this resistance. This value of resistance is satisfactory for 112 and 171 type tubes. Some tube manufacturers recommend a positive grid bias for the heater of the detector tube, and some recommend negative bias. In this circuit no bias has been found necessary. The midtap lead of the 2^-volt winding may be connected to plus 45 instead of to ground in the event that there is excessive hum in the output of the receiver. A 227 type tube may be used in the reflex position if desired, but satisfactory results should be obtained with the 226 type, and this type is somewhat cheaper than the 227 type. Each pair of filament leads must be twisted with about two or three twists to the inch to avoid hum. Likewise, the leads which go to the tumbler switch on the panel should be twisted, as well as any other a.c. leads in or around the set. For filament wiring, no smaller wire than No. 14 should be used, as the a.c. tubes draw a much heavier current than the d.c. tubes. For 0 FIG. 3 Diagram of connections. Those who desire to convert their d. c. Roberts sets can follow this diagram and instructions on this page and the one following