This article describes the construction of a miniature radio set combined with a table lamp--the lamp serves as the limiting resistor in the filament circuit, the shade doubles as the speaker cone.

Click on any illustration with this article to view a larger version.


Clyde Fitch, Radio-Craft, May 1933

Fig. A: Front view of the Radiolamp ready for operation.
Looks interesting, eh? Well, it is interesting. [click on image for larger version]

In the February, 1933 issue of Radio-Craft there appeared the first complete description of one of the new miniature radio receivers. It seems as though, overnight, this type of set has earned its spurs, and has swept the country by storm. It not only fills a great public need, but, to the ingenious radio fan, it offers unlimited possibilities for home-made sets of novel design. For example, the "Radiolamp," illustrated herewith, was made possible by following the general principles of miniature set construction and combining them with a lamp. Also, the variety of new tubes now available, mainly the 6-volt, .3-ampere automobile type, which is used in this set, makes possible many interesting set combinations for the home constructor.

Fig. 1: Schematic circuit of the receiver. Note its similarity ti the International Kadette. SW and R2 are ganged. [click on image for larger version]

The Radiolamp, see Fig. 1, contains three tubes and a rectifier. The first tube is a type 39, R.F. pentode amplifier, coupled to a type 36 screen-grid detector, which is, in turn, resistance coupled to a type 38 output pentode. The rectifier is a type 12Z3. All of the tubes except the rectifier have a 6.3-volt heater which operates at .3-ampere. The rectifier has a 12-volt filament also rated at .3-ampere.

This tube has an indirectly heated cathode, and, although of the high-vacuum type, it has a low internal voltage drop, which does not cause any appreciable reduction in voltage available for the plates of the tubes.

Miniature radio sets are made universal; that is, they operate on both A.C. and D.C. The filaments of the tubes are connected in series and are connected directly to the 110-volt line through a limiting resistor, thus avoiding the use of filament transformers. The limiting resistor causes a voltage drop of some 85 volts, and, consequently, at .3 ampere, it must dissipate about 25 watts of electrical energy in the form of heat. For this reason many miniature sets get hot enough to fry eggs on! This condition is one important reason why the combined set and table lamp is a logical combination, as the limiting resistor is eliminated and the lamp bulb takes its place, thus converting energy which was formerly wasted, into useful light.

A table lamp requires a shade. By making the lamp of modern design, the shade may be in the shape of a cone, thus making it suitable to serve as a loudspeaker diaphragm. Without the limiting resistor and the loudspeaker in the set, the set itself can be made smaller, so that there will be ample room in the base of the lamp for the set without making the base unduly large. Another important reason for combining the set and lamp is that a much larger cone can be used than is ordinarily used in a miniature set, and the tone quality is thereby considerably improved.

The Radio Chassis

Fig. C: View of the receiver with all aprts labeled for convenience. [click on image for larger version]

Figure 1 shows the schematic diagram of the receiver. The circuit is conventional and is typical of the type used in many commercial miniature sets. It is a simple set to wire, and one should have no difficulty in making the connections. One point to observe is that the type 36 detector heater is connected to the negative side of the line, assuming the set is plugged into a D.C. outlet. This is important in order to reduce hum to a minimum when the set is used on an A.C. line. From the negative side of the line the current passes through the detector heater, then the type 38 R.F. tube heater, into the type 78 A.F. tube heater, and through the rectifier tube filament into the lamp filament and back through the switch to the other side of the 110volt line. A 40-watt Mazda lamp is the correct size to use with this set.

We can also trace the plate supply from one side of the line through the switch to the rectifier plate, out through the cathode of the rectifier to the filter choke L5, and on to the other tubes. About 95 volts are available for operating these tubes, whether used with an A.C. or a D.C. light line supply. Another point to observe is that the screen of the detector tube is connected to the cathode of the type 38 tube so that a.positive potential is obtained for the screen which is equal to the grid bias of the type 38 tube.

All of the parts are marked in the diagram with the same symbols as used in the other illustrations and in the List of Parts. Note that the metal chassis is insulated from all of the wiring by means of the insulating condenser C3. This procedure is followed to avoid any possibility of a short circuit or serious shock should the metal chassis become grounded accidentally, as usually one side of the light line is grounded, and the negative "B" supply is connected directly to one side of the line. For this reason it is important, in mounting the two-gang tuning condenser, C1 and C2, and the electrolytic filter condensers; C9 and C10, to use insulating bakelite washers under the screws and make sure that the mounting screws do not touch the metal chassis. The volume control, R2, is mounted on a bakelite support for the same reason. For the values of the parts, the reader should refer to the List of Parts. For the locations, the photographic illustrations, mainly Figs. C and D, should be observed.

Fig. 2: Construction details of the "shade" pictured abovew, in Fig. B. [click on image for larger version]

The metal chassis and lamp frame-work are clearly illustrated in Figs. 2 and B. Fig. 3 shows a plan view of the radio chassis with the location of the holes for the tube sockets indicated. One sixteenth-inch aluminum shield can material is used throughout. All of the important dimensions and other information are given in Fig. 2. Note that the holes in the top and bottom part of the lamp base, the bottom of which is also the radio chassis, are threaded so that the sides, front, and back plates may be removed easily. The top of the lamp base is made exactly the same size as the radio chassis, or bottom. The small chassis mounted on top helps support the two rods which support the loudspeaker unit and the wire frame for the shade and, also, improves the general appearance of the lamp.

The “Radiolamp”

Fig. 4: Construction details for the cone-shade (left). Main drilling layout for the chassis (right). [click on image for larger version]

The combined lamp shade and loudspeaker cone is cut from a piece of white drawing paper to the size shown in Fig. 4. Ordinary lamp shade parchment should not be used as the oil it contains deadens the sound. The white drawing paper works very well for both lamp and speaker, and may be given a coat of white shellac to improve its appearance. The shellac should be applied after the border design, if any, has been painted. The design itself may be made to suit the builder's fancy. After the paper is cut out as shown in Fig. 4 and decorated and shellacked, it should be glued along the seam with rubber cement, forming it into a cone. The tip of the cone is clamped with metal apexes similar to those used in cone-type loudspeakers. This allows it to be attached to the drive pin of the loudspeaker unit.

Fig. B: View of the speaker-lamp construction with the "shade" removed. [click on image for larger version]

When assembled, the cone rests on a ring support made of No. 10 brass wire covered with a soft rubber tube. This ring is 13-1/2 inches in diameter and is supported at the lamp base in four places as shown in the photographic illustration, Fig. B. This wire support is soldered together.

The loudspeaker unit was taken from a Premier miniature cone speaker and mounted directly on the threaded rods as shown, using the mounting holes already in the unit. The drive pin was extended to fit the new cone by soldering a longer one to it.

In assembling the complete unit, the radio chassis, of course, should be built and wired first. Two wires are run from this unit up through holes in the top to the loudspeaker, and two more run up to the lamp. A lamp cord enters the back of the chassis through which connection to the line is made. The aerial lead is attached directly to one of the coils and extends through a hole in the back.

Fig. D: Under-view of the receiver and aluminum case. All values are shown.

The loudspeaker lamp shade assembly may then be mounted to the top of the lamp base and the two side pieces attached. This supports everything, and the connections to the lamp and to the unit may be made, making sure that enough slack is left in the wires so that the chassis may be removed without disconnecting them. The front can then be drilled for the volume control and tuning dial shafts, and a window cut in to show the tuning dial readings. The tuning dial is simply a celluloid dial forced onto the condenser shaft and cemented on the shaft. This touch practically completes the construction of the unit. The reader should carefully study the various illustrations and note especially how the coils are mounted directly on the tuning condenser with small brass supports. They are mounted at right angles so as to reduce oscillation.

Operating the Set

Finished Radiolamp. [click on image for larger version]

The operation of this set is quite simple. While it may not be as selective as larger T.R.F. or superheterodyne receivers, or give the power output, it is extremely sensitive and picks up many stations with a small indoor aerial. No shielding of the tubes is shown in the illustrations. In some cases, if oscillation becomes serious, it may be advisable to place a metal shield around the detector tube and also to shield the detector control-grid lead. The condenser has a shield built in it between the two units. This feature and its small size and specially cut plates facilitate tuning considerably, making it ideal for this type of receiver. The coils also are so compact that shielding is usually not required. If a very long aerial is employed, it is advisable to connect a .0001-mf. fixed condenser in series with it.

List of Parts

One Cardwell two-gang midget condenser .00035-mf; C1, C2;
One sprague fixed condenser .1-mf., C3;
One Aerovox fixed condenser .01-mf., C4;
One Aerovox condenser .5-mf., C5;
One Aerovox fixed condenser, .0001-mf., C6;
One Aerovox mica. fixed condenser, .02-mf., C7;
One Solar electrolytic condenser, 4 mf., C8;
Two Aerovox electrolytic condensers, 4 mf., C9, C10;
One Aerovox pig-tail resistor, 150 ohms, R1;
One Acratest volume control with switch, 250,000 ohms, R2;
One Aerovox resistor, 50,000 ohms, R3;
One Acratest resistor, 2.5 megohms, R4;
One Aerovox resistor, R6;
One Aerovox resistor, 1,500 ohms, R6;
Two Find-All aerial couplers L1, L2 and L3, L4;
One 15 henry choke, L5;
One type 39 tube;
One type 36 tube;
One type 38 tube;
One type 12Z3 tube;
One 40-watt lamp;
One Premie; loudspeaker unit;
Three five-prong sockets;
One four-prong socket;
One lamp socket with bushing;
One cord and plug;
One celluloid dial;
Cone apex, cone and fittings;
Two Blan chassis, 5-1/4, by 61/4 by 1-1/4 ins. deep, 1/16" aluminum;
One Blan chassis, 3-1/2 by 3-1/2 by 1 in. deep;
Two Blan sides, 5-1/4 by 6 by 1/16 ins.;
One Blan back 5-3/8 by 6 by 1/16 ins.;
One Blan front 5-3/8 by 6 by 1/16 ins.;
Two No. 10-32 threaded brass rods, 8 ins. long;
Screws, nuts, etc.


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