SEVERAL newspapers and magazines have published data on constructing the Browning-Drake circuit, but most of these articles have been incomplete, leaving many questions in the minds of the readers. It shall be our endeavor to answer these questions, and give complete constructional data so that the most inexperienced, as well as experienced, radio fan can construct the receiver with some assurance that it will work properly.
It was in August of 1923 that Mr. F. H. Drake suggested a mathematical treatment of a tuned radio frequency transformer, in order to determine the proper constants necessary for maximum amplification. This theoretical investigation proved to be more fruitful than either of us would have believed, for after choosing the constants predetermined by mathematics, laboratory measurements showed that without a "slot" winding for the primary of the radio frequency transformer the theoretical amplification could not be obtained. However, with the correct constants and the "slot" winding 90% of the amplification predicted by theorywas actually obtained when the transformer was, subjected to laboratory tests at Harvard University.
It was noticed by looking over the equations that by adding a coil in the plate circuit of the detector tube signals could be increased 10 to 20 times. Accordingly, a set was constructed, using this additional feature, with surprising results. Comparisons made with several makes of sets showed that the receiver was extremely sensitive and selective. Our first impressions, that the set was very good, were later verified by reports from all parts of the country, constantly comparing the receiver with the superheterodyne, so that it is believed that many others have been surprised with the efficiency of the four-tube set besides the writer.
Fig. 1 shows the layout for the receiver. Much time was given to the placing of apparatus considering both the appearance and also as to electrical characteristics, with the result shown. The dimensions are given for the parts recom- mended. Note that the centers of the two condensers are 8" apart. This liberal spacing makes the task of "Balancing" easier than would be otherwise the case.
Fig. 2 shows the layout behind the panel and the placing of apparatus. This is self-explanatory as all parts shown are drawn to scale.
Both a picture wiring diagram and a schematic diagram are shown so that very little need be said about construction. The layout of the parts entering into the complete receiver has been so placed as to make the connecting leads as short as possible. Thus the RF Amplifier tube is placed between the antenna tuning system and the regenaformer, while the three other tubes are placed at the right of the second tuning condenser. The two variable air condensers which support the coils should be mounted so that the rotor plates move towards each other.
The connections from the .0001 mf condenser to the coil should be made of flexible wire so that connections from point 1 to point 2 and from point 1 to point 3 may be tried to see which gives the better results.
Be sure the stator plates of the condensers go to the grid of the tubes as this will eliminate body capacity when tuning. In connecting up the set, be sure to solder connections directly to the wire of the antenna coil, and the secondary of the regenaformer.
Sometimes the 1.0 mf condenser acting as a by-pass across the "B" battery can just as well be omitted if the leads to the "B" battery are short. This is the reason it is marked optional.
It will be noticed that the rheostats are connected in an unusual way, which facilitates the use of the voltmeter, so that it reads the maximum voltage across the filament of all tubes. This voltage is controlled by rheostat No. 2. Rheostat No. 1 which has 30 resistance really acts as a volume control regulating the filament voltage of the first_tube. More will be said later, concerning the setting of these rheostats.
Many people believe that separate rheostats for the detector tube help bring in distant stations. In the case of DV2, DV3, UV201A, UV99, WD12, and WD11, it seems to make no difference in volume whether the detector has a separate control or not. In fact, it is better to run the detector tube at rated voltage on the filament as cutting down this voltage tends to distort signals. This is why the detector and two audio tubes are connected to the same rheostat.
The ideal arrangement of tubes would be a DV3, or UV199 in the first socket with UV201A tubes in the other three. The 25 ohm fixed resistance shown takes care of the small tubes filament voltage so that if rheostat No. I is never turned on more than half way, there is no danger of burning it out. The picture diagram shows a standard socket for the radio frequency tube. If a DV3 is chosen for the radio amplifier, this will be used as shown. If a UV199 is used, a UV199 socket would be substituted in place of the standard one for the RF tube.
DV3 tubes or UV199 may be used in all sockets satisfactorily, though not quite so much volume will be obtained. In this case, the 25 ohm resistance should be omitted.
When the set has been carefully constructed according to the diagrams given, the next step is to balance it so as to obtain the most volume, and eliminate radiation. We shall assume that a DV3 or UV199 tube is in the first socket with 201A tubes in the other three. After connecting antenna ground and batteries with rheostat No. 1 off, turn rheostat No. 2 until the voltmeter reads five. Then turn rheostat No. 1 about half way on, and tune in a local station. Turn rheostat No. 1 completely off, leaving the RF tube in its socket, and retune for the station. Set the balancing con- denser until changing the setting of the .0005 mf condenser does not affect the signal strength. (This usually occurs when the balancing condenser is set for minimum signal.) The set is then ready to operate at maximum efficiency. Balancing in this way should be very easily done if wiring diagram has been followed.
A few notes on tuning may be helpful though actually operating the set is the only way one can acquire the knack of doing DX work. If the set is performing normally, the volume coil Lp can be tuned to such a position that placing a finger on the stator plates of the .00035 mf condenser gives a "pluck" in the receivers. This means circuit L2, C2 is oscillating. With the volume coil in that position, turn the dial of the .00035 condenser until a whistle is heard. Turn dial of the .0005 mf condenser until this whistle is loudest. Then adjust the volume coil until the whistle disappears, and by slightly retuning, the station should come in.
The receiver described has performed very well indeed from all reports. During the recent trans-Atlantic test, Madrid was received with a three-tube set, and a telegram was obtained verifying the reception. Verified reception has also been obtained from KGO, Oakland, Calif.; KHJ, Los Angles, Calif.; Mexico City; and Calgary, Canada.
The writer is able to receive Chicago stations on the loud speaker with a ten-foot piece of "Silvertone" Radio Antenna. However, the constructor should not be too optimistic until he learns how to operate the set at maximum efficiency.
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