Radio Broadcast (May 1928-Apr 1929)

Record Details:

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H RADIO BROADCAST MAY, 1928 9000-ohm resistor to the metallic base of the regulator tube from "X" i'n Fig. 21 to "X" in Fig.. 1. The following table shows the value of the resistors designated in Fig. 2: Ri 1000 ohms, 5 watts R.2 2000 ohms, 5 watts R3 50000 ohms, 2 watts R4 20000 ohms, 2 watts R5 9000 ohms, 5 watts R6 2000 ohms, (double potentiometer, such as the Amsco "duostat") The constructor should find no difficulty in following the layout of parts by reference to the photographs. D. E. Replogle, Cambridge, Massachusetts. STAFF COMMENT THE arrangement shown in Fig. 2 may be used with practically any socket power device. As was pointed out in previous articles of this nature in "Our Readers Suggest" Department, however, the C bias arrangement should be employed only with a socket power unit capable of delivering a total voltage output under load equalling the sum of the maximum plate and grid voltage required. The grid bias feature may be eliminated by the omission of C2 and R6 and by the connection of the bottom of R4 to the regulator tube and to "B" on the regular B device. This connection is indicated in dotted lines, the heavy line being, of course, left out Plate Detection THE advantages of using a C battery for bias in the detector circuit may be retained without the use of an extra battery by a simple change in the wiring of any circuit. In the writer's case a resistance-coupled audio amplifier is used for the first stage. The detector grid condenser and leak are no longer used, and are "shorted" out of the circuit. Connect the detector rheostat in the A minus filament line and connect the grid return as shown in Fig. 3, which arrangement utilizes the negative bias obtained by the drop across the filament rheostat. A definite value of resistance in the detector plate circuit will be found to work best with each value of C bias. In this case a one-megohm leak was found to be right when ninety volts was used with a 201 -a type tube. This arrangement was found to be practically as sensitive as the usual circuit having a grid leak and condenser, and at the same time had Detector 1st. AT. This adapter, describedby Mr. Replogle, modernises the old B supply device. A glow tube is used the selectivity and tone qualities gained by employing plate detection. Karl V. Nyquist, Stromsbing, Nebraska. STAFF COMMENT P\ ETECTING on the lower bend of the plate-current grid potential characteristic curve, like most justifiable variations from an average procedure, is characterized both by advantages and disadvantages. The possibility of distortion due to overloading of the detector circuit is reduced in the so-called plate method of FIG. 3 A useful circuit for plate detection AN UNDER VIEW OF THE DEVICE DESCRIBED BY MR. REPLOGLE detection. The sensitivity of the detecting circuit is, however, generally lessened. In the case under consideration, the loss in signal strength is probably negligible due to the fact that the relatively low plate potential is secured by increasing the resistance in the plate circuit of the detector tube to approximately ten times the value employed in the grid current detecting system. Increasing the value of the external plate resistor in a resistance-coupled amplifier, while still maintaining the applied plate voltage at an optimum value for detection, increases the input to the amplifier, which in this instance, partially compensates the loss in detecting efficiency. Adjusting Cone Loud Speakers UOR THE proper adjustment of a cone loud * speaker, it is essential that the pin be exactly centered in the collar at the apex. It often happens that in the rough handling of transportation the movement of the loud speaker is shifted slightly from dead center with the result that there is a strong tension on the pin. This limits the amount of power the loud speaker can handle without distortion. The correct adjustment can be easily made with the aid of such simple apparatus as is generally found in the radio equipped home. A no-volt lamp of indiscriminate wattage, a house current plug, and a 1000-ohm resistor are required to make the adjustment. This apparatus is placed in series with 110 volts a. c. and the loud speaker. The set screw on the loud speaker collar is loosened and the circuit shown in Fig. 4 is closed. A 120-cycle hum will be distinctly heard in the loud speaker. The screws holding the frame should be loosened slightly and the actuating mechanism moved from side to side and up and down until the sound is at a minimum. With the set screw loosened the loud speaker will rattle freely at this adjustment. The current is turned off and the set screw is tightened down upon the pin. Harry Wirth, New York City. Selectivity with A. C. Tubes LJAVING had occasion to alter a half dozen *■ ■* or so battery receivers for a. c. operation, with both the R. C. A. and Arcturus types of tube, I have noticed that the selectivity of the battery receiver has been, in every case, noticeably superior to that of the rewired job. In the course of my experiments, however, I found that the selectivity of the a. c. set could be improved until it was quite on a par with the original 201 -a job, by increasing the negative bias on the r. f. tubes. Walter Bennett, New York City. STAFF COMMENT \A/ITH coils de* * signed for tubes having the characteristics of the 20 1 -a type tube, the substitution of alternating-current tubes of a lower input impedance will necessarily result in the loss of selectivity, generally accompanied with an increase in sensitivity. These effects can be compensated, as suggested by Mr. Bennett, by increasing the grid bias applied to the radiofrequency tubes. It will be found that, at the point at which the selectivity is equal to that of the d. c. set, the sensitivity will also have been readjusted to the same degree characterizing the original battery receiver. -WWW 1000 Q FIG. 4 A simple arrangement for the adjusting of cone loud speakers. Unless properly adjusted, the cone loud speaker will not give its maximum undistorted volume