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350
RADIO BROADCAST
OCTOBER, 1928
FIG. I. TOP AND BOTTOM VIEWS OF THE TUNER
able from the manufacturer of the dial. It will be noted that a single i^-volt flashlight cell is used to obtain bias for the control grid of the type 222 tube. While it is possible to obtain this voltage by means of resistors and a condenser it
was felt that the additional expense of this arrangement was not warranted.
The practice of locating the volume control at the end of an extension cord rather than on the panel proper is becoming quite general, but
whether the actual control is located on the panel or at the end of an extension cord does not affect the type of control which is recommended for use with this receiver. The volume control consists of Ri and R2 in Fig. 3, an arrangement for varying the positive bias on the screen grid of the type 222 tube. Ri has a value of 1000 ohms and R2 a value of 5000 ohms.
THE AUDIO AMPLIFIER
ANY good audio amplifier may be used with the screen-grid tuner as just described. The writer uses the push-pull amplifier illustrated in Fig. 4. This amplifier uses the new National Veritone audio transformers which have cores of a special nickel-steel alloy, for by the use of such a core material it becomes possible to reduce very materially the bulk of the transformer and at the same time to improve its frequency characteristic.
It will be noted from the Fig. 4 that four sockets are provided in this amplifier. The additional socket is of the UX type and is connected in parallel with the UY socket in the first stage. Therefore, when the amplifier is used with a tuner employing, d. c. tubes, a type 1 12a tube may be used in the first stage in place of the 227.
The C-biasing resistors located in the amplifier base are of such values as to provide automatically the proper C bias to either the type 210 or 250 power tubes, regardless of the plate voltage' used. Where high volume is desired and the use of high plate voltage is not objectionable, then it is recommended that a pair of type 210 tubes be used in the power stage. For home use, however, far more than sufficient volume is obtainable when using 250 type tubes at about 300 volts. It is not necessary to use 500 volts on the plates, for with 300 volts as delivered by the power box shown in Fig. 5, it will not be possible to overload the tubes at normal volume levels. The power output will be about 4 watts. One of the great advantages of push-pull amplification, in addition to improved tone quality as the result of the reduction of harmonic distortion, is the fact that a relatively low plate voltage suffices to give a large undistorted power output.
If more than one speaker is to be operated at one time, regardless of the type of amplifier
FIG. 2. THE PICTURE WIRING DIAGRAM
This diagram corresponds with the circuit diagram, Fig. 3, with the exception that the volume control resistors, R\ and are not shown, since, as indicated in the text, some constructors may want to locate this
control at the end of a flexible lead.