FINDAMENTALS in radio design have not changed much in the past few years, contrary to the, belief of the general public, and receivers which ere originally based upon sound electrical principles have remained practically unchanged. The original Browning-Drake circuit of 1924 has had very few changes made in its essential principles. However, it has been found advisable, from time to time, to change the mechanical design so that the receiver might be brought up to date in appearance and also to incorporate electrical improvements which have developed in the last few years.

Advertisement for the 1928 Browning-Drake receiver kit, now with single-dial tuning.

Due to the fact that many fans are desirous of using power amplifiers which incorporate either 171 type valves in push-pull arrangement or 210 type valves, in order to get a large mount of undistorted energy delivered o the reproducer, it is thought advisable to describe in this article a two-valve Browning-Drake tuner combined with the new AmerTran power-pack amplifier. This makes a fine combination, inasmuch as a tremendous undistorted volume may be delivered to the reproducer.

The Browning-Drake circuit consists essentially of one stage of tuned-high frequency amplification, with a specially built slot-wound high-frequency transformer which was developed by he writer and Dr. F. H. Drake. This combined with a regenerative detector, the stage of high-frequency amplification being neutralized. The resulting combi-nation makes a tuner which is both easy to build and sufficiently sensitive to enable the operator to receive almost all signals which are above the noise level.

The antenna circuit incorporated in the Browning-Drake receiver is a conductively coupled one; that is, the antenna lead connects directly to a tap on the antenna coil through a .0001 mfd. condenser. This system has proven extremely efficient, inasmuch as it has a much more even response over the entire broadcast band of frequencies than any other simple circuit tested by the author. Another advantage is that good signal strength may be secured when using even an extremely short antenna. One disadvantage, however, is that it is extremely difficult to make the two condensers on the receiver run together when both long and short antennas are being used alternately. Dr. Drake and the writer have, for the past season, been working on what might be termed a "single control" for this circuit and have so designed the receiver that; the tuning condensers employed may be attached to one shaft and controlled with a drum-type illuminated dial without making any other adjustments for certain types of antennas. The receiver described, however, employs what is termed a "trimmer condenser" in parallel with the first tuning condenser. The operator will find that, in most cases, it will be necessary to make slight adjustments on this for different stations.

The RF circuit of the Browning-Drake receiver. Basically unchanged from the original circuit, this one now used type 27 tubes. Note the unuslay way they show the cathodes of these tubes.

Another change which has been made is that a neutralization system has been developed in order that a large valve may be used as the high-frequency amplifier. The neutrali-zation system, as will be noted from the schematic wiring diagram in Figure 1, consists of a number of extra turns added on the secondary of the high-frequency transformer, with the end connected to the rotor plates of the neutralizing condenser, the stator plates then being connected to the grid of the first vacuum valve.

In order that the set builder may use shielding if his location is such as to demand it, a metal sub-panel is employed. The grid-leak on the detector valve is suspended by a stiff wire from the stator plates of the second tuning condenser directly to the grid connection on the valve.

Using AC Valves in the Set

If the set builder desires to use the new type, AC valves, he should provide himself with two five-prong sockets, as well as one center-tapped resistor of about 30 to 100 ohms. He should also secure two 227 type AC valves. These are heater type valves and require a step-down transformer which supplies 2-1/2 volts to heat their filaments. The filament connections should be made by means of two wires twisted together. This is extremely necessary if there is to be no AC hum in the completed receiver. All the twisted pairs also should be run underneath the sub-panel.

The high-frequency valve should have from 3 to 4-1/2 volts of negative "C" battery bias. No binding posts are put on the sub-panel for this connection, as it is best to run them out in a cable, preferably using two different colored wires twisted together.

The set builder will find that if he is extremely careful in constructing the receiver for use with these valves he will get no hum whatsoever. It is sometimes necessary, instead of connecting the cathode directly to the center point of the filament circuit, to put in a 22 or 45-volt bias, as recommended in the instructions which accompany the valves. The writer, however, found that in the majority of cases this was not necessary and that the cathode can be connected directly to the center tap of the resistor which is connected across the filaments. A little experimenting on this point is well worth while.

Balancing the Set With AC Valves

In case the set builder chooses to use AC valves with the two-valve Browning-Drake kit set, the balancing or neutralizing of the set should be done as follows: The Clarostat, which is used as a volume control, should be turned clockwise as far as it will easily go. Set the condensers at a low value, say 20 on the scale. Turn the tickler coil down until the second circuit is oscillating. This may be determined by touching the finger to the stator plates of the second tuning condenser (the one to the right as one faces the set). The tickler coil should then be set back so that this circuit just goes out of oscillation. Adjusting the trimmer condenser will, if the set is not neutralized, throw this circuit into oscillation, which may be determined as mentioned above. Set the neutralizing condenser so that turning the trimmer condenser has no affect on the oscillation produced in the second circuit. The set is then completely neutralized.