Radio Broadcast (May 1928-Apr 1929)

DECEMBER, 1928 RADIO BROADCAST No, 13. December, 1928. Radio Broadcast's Service Data Sheets The A. C.-66 Dayton Receiver '"PHIS data sheet is devoted to a discussion of a receiver that utilizes the screen-grid tube as an r. f. amplifier. There are three r. f. stages in the set, the a. c. screen-grid tube being used in the first stage and 226-type tubes in the second and third r. f. stages. The detector circuit uses a 227-type tube, the first audio stage uses a 226-type tube and the power stage employs a 250-type tube. This receiver can be considered unique in that it is one of exceedingly few manufactured receivers that utilizes the screen-grid tube in the r. f. circuit and a 250-type tube in the output circuit. TECHNICAL DISCUSSION 1. Tuning System The four tuning condensers are ganged to a single control. Across the first tuning condenser is placed a midget variable condenser so that this circuit may be tuned to exact resonance. An antenna of ordinary length is connected directly to antenna terminal No. 1 ; a long antenna is connected to terminal No. 2 so that the signals are compelled to pass through a small fixed condenser, Ci, w hich has a capacity of 0.00025 mfd. No neutralizing or stabilizing devices are necessary in conjunction with the 222-type tube. To suppress oscillations in the 226 circuits 900-ohm fixed resistors are connected in series with the grid of these two tubes. 2. Detector and Audio System In the grid circuit of the detector is placed a 2megohm grid leak and a 0.00025-mfd. grid condenser. The detector is a 227-type tube and the output of the detector is bypassed to ground with a 0.001-mfd. fixed condenser, C2. The audio amplifier contains two transformer-coupled stages. A 0.0025mfd. fixed condenser is connected across the secondary of the first audio transformer to improve the high-frequency response. The 250-type output tube feeds into a choke-condenser combination located in the power unit. 3. Volume Control The volume control in this receiver consists of a 100,000-ohm resistor, Ri, connected across the secondary of the first tuned circuit. By adjusting this control it is possible to regulate the amount of energy fed into the r. f. amplifier. In this way the possibility of overloading in any of the tube circuits is prevented. 4. Filament Circuits Filament current for the various tubes in the receiver is obtained from several windings on the power-supply transformer located in the power unit. The 222-type a. c. screen-grid tube obtains its current from a 2.5-volt winding, and 2.5 volts for the detector tube filament is obtained from a winding on a separate filament transformer. The r. f. tubes and the first audio tubes are supplied from a 1 .5volt winding and the 250-type power tube from a 7.5-volt winding. The 1.5-volt winding, is shunted in the receiver by a 20-ohm potentiometer, R2, with two 0.5-mfd. by-pass condensers connected across it, their center point being connected to the 125-volt lead from the power-supply unit. A 65-ohm center-tapped resistor, Rr, is connected across the filament circuit of the 250-type power tube. The 20-ohm potentiometer is adjusted at the factory to the point of minimum hum in the loud speaker. MODEL . Plate Circuits The screen grid of the 222-type tube and the plate circuit of the detector tube are supplied with 125 volts through a 35,000-ohm fixed resistor, R3, which serves to reduce the potential to approximately 45 volts. The plate circuit of the first audio tube is supplied with 125 volts through a 10,000ohm fixed resistor, Ri, which serves to reduce the potential to about 100 volts at the plate of the tube. The plates of the 226-type r. f. tubes and the plate of the screen-grid tube are all supplied with 125 volts. The 250-type tube is supplied with 350 volts from the power unit. 6. Grid Circuit The grid bias on the grid of the screen-grid tube is 1.4 volts, obtained by connecting a 900-ohm resistor, Rs, in series with the plate circuit of this tube and then utilizing the drop in voltage across it for grid bias. The 900-ohm resistor, Rs, supplies 9 volts of grid bias to the 226-type r. f. tubes and the first audio tube. There is no bias on the detector tube. A 1500-ohm C-bias resistor in the power unit supplies 63 volts to the grid of the 250-type power tube. 7. The Power Supply The power supply, not shown in the circuit diagram below, is placed in the cabinet with the radio receiver. The power unit is of conventional design, supplying all the d. c. and a. c. voltages required for the operation of the set. A 281-type tube is used as the rectifier. The following data was supplied by Mr. R. S. Copp, Chief Engineer of the A-C Dayton Company: "The a. c. screen-grid tube is rather new to the public and has only been available to manufacturers a comparatively short time, and, therefore, there is not a great deal of data available as yet. Our Engineering Department has been giving quite a bit of time in the laboratory on this new tube and we have found out a few things which might be of interest to the readers of Radio Broadcast. "The a. c. screen-grid tube receiver, known as Model AC-66, uses one of these tubes in the first radio-frequency circuit only. It is placed in the first r. f. circuit in order to gain sensitivity, especially on inefficient antennas. This tube is then followed by two tuned stages of radio-frequency amplification, using the 226-type a. c. tube. The 227-type tube is used as detector, the 226-type tube as first audio and then for the last stage, we are using the new 250-type super-power amplifier in order to give the best of tone quality with the increased volume obtained. "We are using r. f. transformer coupling on the screen-grid tube with a ratio of one to three. This system is employed in preference to impedance coupling in order to obtain a good degree of selectivity, and yet not destroy the sensitivity which this tube has. " Inasmuch as this new tube is for a. c. operation we obtain our C bias through a 900-ohm resistor in the plate-supply lead which is in series with the cathode and ground. This gives approximately 1.4-volt bias on the grid of the tube, with 125 volts of plate potential with 45 volts applied to the screen-grid element. "The heater element of the a. c. screen-grid tube is the same as the one used in the 227-type tube, and the same methods are used as in the 227 heater circuit. The center tap of the heater circuit is grounded and is not connected to cathode as shown in some circuits. By grounding center-tap connection of heater winding the heater becomes 1.4 volts negative with regard to cathode which is necessary in order to obtain maximum efficiency. "Our experiments on screen-grid tubes have shown us that this tube is very efficient. In fact, so much so in some cases as to prohibit the use of more than one of the tubes in a tuned r. f. set, without decreasing selectivity to an undesirable degree. Then again if several of these tubes are used and coupled properly to obtain the highest order of amplification, the sensitivity becomes so great as to increase background noises which in the end does not give satisfactory results, unless the volume is reduced and then the efficiency is back again to that of one tube, in the first r. f., followed by regular tubes as r. f. amplifiers. "The shielding of a screen-grid amplifier is very important, otherwise, inter-coupling of circuits will develop and cause uncontrollable oscillation. Where only one screen-grid tube is used, it is not necessary to employ extreme shielding, as is the case where two or more are used in a receiver. " With our form of construction and circuit design, we find a gain of approximately twenty in the first stage as compared to approximately eight in the second and third stages of the r. f. circuit, therefore, our gain up to the detector tube is in the order of about 1300, whereas the gain on a straight 226-type set-up of three stages is about 500. This we believe is a sufficient increase over a 226-type set-up, presenting a compromise of amplification and selectivity. " If three stages of screen-grid amplification were used, a gain of twenty per stage could be maintained; the result would be 8000. However, this gain is entirely theoretical and would be decreased by several thousand in actual practice. Granting that we could count on a gain of 6000, what would our chances be in using it all? Atmospheric static and electrical disturbances in the average community are such to-day that this enormous amplification would cause a noise level in the volume of a roar and in order to reduce this roar, the volume control would have to be retarded greatly, so we would be only bringing up something we could not use and there is no object in this." One Dial Control 2 meg f^^l 0.00025 _||_J_ mfd_ THE RECEIVER CIRCUIT