A COMPLETE SOCKET POWER RECEIVER
Browning-Drake receiver before converting to AC operation
Browning-Drake receivers were sold in kit form for a number of years during the 1920s. The tuning capacitors, coils, and dial mechanisms were made by the National Company of Cambridge, Massachusetts, who supplied these components as a kit.
The receiver was a simple TRF design with a neutralized RF amplifier stage followed by a regenerative detector and two audio stages. As was typical of the time, it was designed to be battery-operated. Regeneration was controlled by a rotating "tickler" coil on one end of the second RF (detector) coil. Adjustment was by means of a shaft on this tickler coil which extended through the front panel just to the right of the detector tuning dial. Both RF coils were somewhat unique in that their primary windings were wound in a slot in a form located inside the ground end of the secondary.
Also typical of the time, the RF and detector coils were tuned by separate dials. Beyond this the design was not much different from that of hundreds of other radio receivers of the period. Performance, though, was above average for a four-tube receiver. The neutralized RF stage coupled to the regenerative detector made it competitive with sets with a greater number of tubes and more complicated design.
Batteries were expensive, though, and anything that could be done to eliminate them was welcome. There were no tubes available that would work with AC on their filaments in the early 1920s, though, so AC operation presented quite a problem. Power packs were available to replace the "B" batteries, and even the "C" (grid bias) batteries, but the "A" battery was a problem.
The following material is reprinted in part from an article in the August 1926 issue of Radio. It describes the conversion of a Browning-Drake receiver to operate completely from the AC line. The filament problem was solved by operating the filaments in series, and also in series with a bank of four mazda light bulbs which served as an inexpensive dropping resistance. Obviously some of the references to parts, then commonplace, will not apply today but otherwise it makes for interesting reading.
Constructional Details for a "A," "B," and "C"
Battery Supplanter and for the Latest Type of Browning-Drake Receiver
The advent of improved apparatus, together with changes which greatly improve the operation of the ABC battery eliminator described in December 1925 and May 1926 Radio magazine, has resulted in the development of a hum-free A and B current supply unit capable of operating any type of receiver now in use with small tubes, and which will supply the necessary C biasing voltages for the grids of the tubes.
In designing the new power plant, as it will be called, the primary consideration was availability of apparatus, simplicity of assembly, and constancy of voltage output even with a considerable fluctuation of line supply voltage. No makeshift apparatus, or specially constructed parts will be necessary, and a factor of safety for the rectifier tubes of at least 50 percent is included, so that the life of the rectifiers will be considerably longer than their normal rating. To accompany the power plant, and furnish the reader with plans for a complete up-to-date type of receiving set, operated without batteries, a Browning-Drake four-tube receiver has been constructed in Radio's laboratory, and embodies the new and improved Browning-Drake coils and condensers, together with associated equipment of latest design and excellent performance.
Figure 1. Schematic of the Power Plant and Browning-Drake Receiver
The Power Plant
The completed receiver is shown in the pictures, and a complete description of how it may be assembled will be given after a theoretical discussion of the power plant. Fig. 1 is a schematic wiring diagram of the receiver and power plant, the dotted line representing the dividing line between the two units, which are assembled separately and connected together by a flexible cable.
The Power Plant
The power plant consists of a transformer, with 120 volt primary, 600 volt secondary with center tap, and a 7.5 volt secondary for lighting the filaments of the rectifier tubes, which are connected so that 300 volts is applied to the plate of both tubes. Each tube rectifies half the alternating current wave, and the output of this rectifier system, which is called a full-wave rectifier, will be approximately 220 volts at 120 milliamperes, the normal maximum permissible drain from the UX-216-B, CX-316-B rectifier tube when used in a full wave circuit. With smaller current drains, the voltage output will be higher, rising to 300 volts at a few milliamperes. The rectifier output is connected to the input of a two-section filter, one section of which is specially designed to be resonant at 120 cycles, so that the 120 cycle fundamental ripple of the pulsating d.c. output of the rectifier will be removed. The remaining section of the filter is of the "brute-force" type, and removes the harmonics of the 120 cycle fundamental, so that the output of the filter is pure direct current, without noise of any kind.
As the rectifier load is limited to 120 milliamperes maximum at 220 volts, we cannot light the filaments of .25 ampere storage battery tubes, nor could we operate more than two .06 ampere dry cell tubes, of the '99 variety, in parallel, so that series operation of all the tubes except the power is necessary. The filaments of the three type 99 tubes, which require .06 amperes (60 milliamperes) each at three volts, are connected in series, so that a total of nine volts at 60 milliamperes is needed for normal operation. Having a source of direct current up to 220 volts at 120 milliamperes, and using but 9 volts calls for a high resistance in series with the three tube filaments to limit the current to 60 milliamperes, this being placed between the positive d.c. source and one end of the series filament circuit.
As the filament supply draws but 60 milliamperes from the rectifier, we have another 60 milliamperes available for plate current. The r.f. and 1st audio amplifier tubes will require about 6 milliamperes at from 80 to 90 volts, so by the insertion of a high resistance between the plates of the tubes, and the positive of the rectifier, the voltage is cut down to the proper value. This makes 66 milliamperes drawn from the rectifier. The detector tube requires 1-1/2 milliamperes at 45 volts, and this is obtained through another high resistance connected between the rectifier positive terminal and the plate of the detector, making a subtotal of 67-1/2 m.a. as a rectifier load.
The power tube used in this particular model is the new CX-371, UX-171, which has a filament requiring .5 ampere at 5 volts, and draws from 20 to 23 milliamperes of plate current at 180 volts. A.C. filament voltage is supplied from a step-down transformer, which has a 6-volt secondary, and as the secondary is not center tapped, a 200 ohm potentiometer is shunted across the secondary and the slider is adjusted until the electrical center of the circuit is obtained. Assuming that the power tube draws 25 milliamperes, at 180 volts, the voltage being cut down through a 600 ohm resistance, our total drain from the rectifier is now say 93 milliamperes.
For the purpose of stabilizing the detector tube, the detector plate voltage is obtained from the voltage drop across a 12,000 ohm resistor placed in series with the variable high resistor shown in the diagram, so that a few milliamperes additional will be drawn from the rectifier, making about 95 milliamperes in all so far.
In order to regulate the voltage output of the rectifier, particularly with reference to the plate voltage of the r,f. and a.f. amplifier tubes, a voltage regulator, or glow tube, is shunted between the positive and negative 90 volt rectifier taps. This tube has the ability to absorb direct current in such a manner that the voltage across its terminals is always 90 volts, within certain limits, so that when the receiver load, or line voltage fluctuates, the tube compensates for these changes, and maintains a constant current supply to the plates of the tubes. In normal operation the current drain of this tube will be 10 milliamperes, so that our total current drain is 105 milliamperes, and the output voltage of the filter circuit will be about 225 volts.
To obtain C voltage for the tubes in the series filament circuit, the tubes are arranged so that the voltage drop across the filament of any one tube furnishes the necessary C voltage for some other tube in the circuit. In this manner, the 1st a.f. grid is negative with respect to its filament by the voltage drop across the r.f. tube filament, and the latter tube grid is 3 volts negative with respect to its filament by the drop across the detector filament. The detector grid is connected to its positive filament, and needs no C voltage. For the power tube, C voltage is obtained by means of a resistance placed in series with the negative B voltage lead from the slider of the potentiometer to the negative B terminal of the rectifier. This resistance is adjusted so that with normal plate current the voltage drop through the resistance is about 40 volts, which is the correct C voltage for the power tube. The actual construction of the power plant is shown in the pictures. Fig. 2 shows a picture of the power plant. The power transformer, filter choke and condenser bank are arranged in three uniform metal cases, at one end of the baseboard, with the two rectifier tubes and the glow tube in the center. On the panel, which is used for the sake of appearance and convenience, are mounted the two variable resistors, the 6-volt step-down transformer and the 110 volt a.c. snap switch.
The four mazda lamps comprise the fixed resistances in the filament circuit. Rather than use expensive wire wound resistances, two 25-watt and two 10-watt mazda lamps are placed in series, providing a total resistance of 4000 ohms. Should it be desired to change the resistance by a small amount, lamps of smaller or greater power consumption can be used, the 10-watt lamp having a resistance of 1400 ohms, the 25-watt of 600 ohms. and the 40-watt lamp 400 ohms. These lamps will safely dissipate a great deal more power than is required in the circuit, and as none of them cost more than 35 cents, their economy is obvious.
The 200-ohm potentiometer C biasing resistance for the power tube, and its associated shunt condenser are mounted at the right-hand end of the baseboard. All connections of the receiving set are made through a 5 conductor battery cable, with the exception of the 6-volt a.c. circuit for the power tube, which is carried through a piece of twisted No. 18 lamp cord, to prevent introducing noise into the other connecting wires.
The power transformer is provided with a flexible cord, with one side of which the snap switch on the panel is placed in series. One of the primary terminals of the 6-volt transformer is also connected to the power transformer side of the snap switch, and the other 6-volt transformer primary lead is connected to the other side of the 110-volt line, so that the snap switch is the main control of the input, and turns everything on or off without extra adjustments.
It is absolutely essential to the success of the unit that a 600-volt transformer with center tap be used, and to employ two half-wave rectifier tubes of the type specified. Any lower voltage for the transformer secondaries, or the use of lower powered rectifier tubes, will mean that insufficient voltage will be available for the 371 type power tube, and the values of fixed and variable resistances will have to be changed.
A 25,000-ohm variable resistor is specified for the C voltage resistance in the power tube circuit, as it gives sufficient latitude in adjustments to accommodate other power tubes having smaller plate current drain, such as the CX-UX-112.
In testing one of the experimental power plants, it was found necessary to increase the capacity of the filter condenser placed at the mid-tap of the filter choke to 4 mfd., in order to entirely remove the 120 cycle hum from the loudspeaker connected to the receiving set. Filter chokes vary in inductance, and while the combination of 2, 2, and 4 mfd. shown in the diagram is normal for the chokes specified if a noticeable hum remains when all connections are made, the extra 2 mfd. condenser should be added between the negative high voltage lead and terminals 2-3 of the filter choke.