Radio broadcast .. (1922-30)

NOVEMBER, 1926 MODERNIZING THE ATWATER KENT MODEL 20 53 pensively constructed from standard parts in a very short time. It may also be quickly and easily substituted for the regular Atwater Kent amplifier. Figs. 3, 4, 5, 6, and 7 show the con- struction, wiring diagram, and method of installation of such a resistance- coupled amplifier. Four tube sockets, three double resistor mountings, a single detector grid resistor mounting, and binding posts are mounted on the top of a 5 i" x 7!" x ft" bakelite panel. On the under side of this panel are mounted an Amperite for controlling the filament current of the power tube and the sev- eral different fixed condensers, the ca- pacities of which are shown in Figs. 5 and 6. Although four plain ux sockets are shown in the model illustrated, it is preferable to use one of the spring sus- pended sockets for the detector tube. To install the completed resistance- coupled amplifier in place of the old, proceed in the following manner: RADIO BROADCAST Photograph FIG. 7 The wires to be snipped when removing the amplifier from the original Model 20 Atwater Kent Receiver to substitute a resistance-coupled amplifier, are here indicated. Study this picture in conjunction with Fig. 5, whereon the numbered wires correspond to those numbered in this picture FIRST STEP: Cut and tag for indentification the following leads: Detector grid lead, cut at grid condenser. Plus A lead, cut at binding post. Negative A lead, cut at binding post. Plus B 90 lead, cut at binding post. Negative A lead from rheostats to socket, cut at socket. SECOND STEP: Remove the Atwater-Kent amplifier shelf and fasten the resistance-coupled amplifier shelf in its place. THIRD STEP: Solder the leads cut in first step to the new amplifier, as indicated in Fig. 6. FOURTH STEP. Remove the rheostat disc from the front panel (unfasten the three screws behind the panel) and solder a lead to its bat- tery switch side. Replace the rheostat disc and run the new lead to the Amperite. Either a type F Ceco or a ux-iyi power tube should be used in the last audio stage. High-Mu tubes, such as Cleartron, Ceco, or Daven, should be employed in the first two stages. The following is a list of parts for the resis- tance-coupled amplifier. i Bakelite Panel jj x T| x ft inches. 4 ux Sockets, General Radio or Benjamin. 3 Double Resistor Mounts (Lynch). i Single Resistor Mount (Lynch). i Metalized Filament Resistor Pack (Lynch). PO Output from Set 4 mfd.Con densei Type A National Impedance Speaker 8*0- FIG. 9 The wiring connections for the output device. The posts marked P and B plus connect to the set, as indicated in Fig. 3. This output device may be used with any receiver. Mayolian, Gen- eral Radio, and National make complete output units contained in a single metal case RADIO BROADCAST Photograph FIG. 8 The output device mounted on a small piece of board. It consists of a choke and a 4-mfd. fixed condenser l Type 112 Amperite, Mounted. 3 o.oi Mfd. Sangamo Mica Condensers, i o.ooi i 0.00025 8 Binding posts. PARTS FOR THE OUTPUT DEVICE i Base, 4x5 inches. 4 Binding posts. i Tobe or American Electric 4-mfd. Condenser, i National Impedance, Type A. It is important in a resistance-coupled ampli- fier that only the best of resistors be used, as most of the cheaper grades of the impregnated paper types not only deteriorate after they have been used a short time but are also very noisy. The new metalized filament resistors, such as those of Durham, Dubilier, and Lynch, now available on the radio market, give exceptionally fine results in amplifiers of this type. The Lynch resistors are also marketed in small boxes containing a complete set of the proper size units for a resistance-coupled amplifier and, in ad- dition, a two-meg, resistor for the detector tube. The proper places in the different mounts for the several different values of resistors is indicated in Fig. 5. OUTPUT DEVICES AS PREVIOUSLY mentioned, it is well with any type of output tube to employ an output device for keepingdirect current out of the speaker circuit. Direct current in the speaker cord, although perhaps only fifteen or twenty milliam- peres in magnitude, is quite capable of starting a fire, especially where long speaker cords of poorly insulated tinsel are employed. Fig. 3 shows how to connect the output device illustrated in Fig. 8, and the necessary batteries, to the resistance-coupled amplifier. A very satisfactory output device for use with the re-vamped Atwater Kent receiver may be easily constructed from a 4-mfd. fixed condenser and a type A National impedance. The con- denser and impedance may be mounted on a small base, as shown in Fig. 8, or else may be directly fastened to the back of the cabinet. • Output transformers are manufactured by Silver-Marshall and General Radio. A circuit diagram for the output device is shown in Fig. 9, while a list of parts is given at the end of the list for the resistance amplifier. USE B LINE SUPPLY UNITS REGARDLESS of the type of amplifier em- ployed in the re-vamped Atwater Kent receiver, a B supply unit of either the a. c. or d. c. types will give excellent results. As almost all a. c. B current line supply devices supply voltages well in excess of 100 volts, sufficient voltage for properly operating the power tube in the last audio stage is readily obtainable. With the d. c. variety of B units, however, the maxi- mum voltage obtainable is but about 100 volts. In such a case, it is necessary to add a 45-volt B battery in series with the B supply unit, as shown in Fig. 10. O C> 6 6 + 145 FIG. IO If you use a line supply device for your B supply, and it does not give you more than one hundred volts or so, an ordinary B battery may be connected in series to make up for the de- ficiency when power tube operation is required