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302
Radio Broadcast
A receiver consisting of a regenerative detector and two stages of audio-frequency amplification, as in Fig. i, was put into operation. The amplifying tubes were VT-2's and drew approximately 1.2 amperes on the filament. A potential of about 150 volts was applied to the plate. The signal strength on local stations was tremendous, but the music was distorted. A milliameter was placed into the plate circuit, and with above filament current and plate voltage, the two tubes drew approximately 32 milliamperes normal current. To minimize distortion, the filament current was reduced until the music sounded best (i. e., till amplification was best), yet the tubes drew 22 milliamperes. (It can be readily seen that with this amount of current being drawn from the B batteries, they cannot last very long. That is the case with many amplifiers in use to-day. The operators are told to increase
the plate voltage if they desire greater signal strength. That is true, but the other constants must also be in proportion, if efficient operation is desired.) Now the filament current was brought back to normal — 1.2 amperes — and C batteries were added as in Fig. 4, until maximum intensity was obtained without any trace of distortion. With the C batteries in, greater signal intensity was obtained than previously, due to the fact that we were operating on the straight part of the characteristic curve, and full plate-current variations were obtained. When in this state, the reading on the milliameter was noted, and it was only 5 milliamperes for both tubes. The increase in the life of the B battery is readily apparent.
A great deal of the distortion that is attributed to the loud speaker may be taking place within the second stage of the amplifier. Who can tell?
PRESIDENT COOLIDGE READING HIS ANNUAL MESSAGE TO CONGRESS
The two microphones on his reading desk actuated six broadcasting stations. Behind the President are Senator Cummings, President -pro tern of the Senate, and Speaker Gillette, presiding