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Radio Broadcast
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FIG. 7
A suggested hook-up. The oscillation control condenser is shown as C2. Note that there is no grid condenser or grid leak
the condenser C3, which has a maximum capacity of .001 mfd. The secondary of the radio-frequency transformer is not tuned in any way, but it is connected directly to the grid of the detector tube, as shown. The control condenser in the plate circuit makes it necessary to furnish the B battery voltage to the plate through the radio-frequency choke shown. This is an iron-cored coil. No grid bias is used in this set and there is no stabilizing device such as a potentiometer. It is interesting to observe that the detector tube contains no grid leak or grid condenser. The designing engineer for this company has found that the insertion of the grid leak and grid condenser in the set actually hinders its operation.
The detector tube works with two stages of audio-frequency amplification, as shown. These are of the usual form. The dial marked "•Osc. Control" (Fig. 5) operates the condenser C2 (Fig. 6). The dial marked "antenna inductance" operates the variometer V, and the dial marked "radio-frequency control" operates the condenser C3. These dials are unique in that they operate at all times with a micrometer adjustment; that is, the knob makes several revolutions in order to advance the dial through its full scale. The condensers C2 and C3 are so arranged that the shaft supporting the moveable plates, and running to the knob, is always farthest from the plate side of the condenser; that is, it is always at the potential nearest that of the filament. In this way, body capacity effects are nearly eliminated.
A variation of this circuit is shown in Fig. 7. In this circuit a loop is used in conjunction with a small tuning condenser Ci. The control
condenser is shown as C2, and the transformer tuning condenser as C3. The radio-frequency transformer may be made up of cardboard tubes. Bakelite apparently does not work as well under ordinary conditions. The two windings may be placed one inside the other, as shown in Fig. 8. The inner winding may be used as the primary and the outer winding as the secondary. Both coils should be wound in the same direction. They can be made up with ordinary cotton covered or enameled wire of from Nos. 22 to 26. The primary should have comparatively few turns compared to the
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FIG. 8
A cross-section showing the arrangement of the coils that make up the primary and secondary of the R F. transformer. The two lower leads should go to the filament and B battery if both coils are wound in the same direction
secondary. The exact number can be determined only by practice. Roughly, from 30 to 40 turns on the primary and from 75 to 100 turns on the secondary will probably give good results if the ratio of the diameter of the primary winding to that of the secondary is about 0.6. The radio-frequency choke may consist of an old primary of an audio-frequency amplifying transformer or any iron-cored coil of only a few layers in which the wire is not too fine.
This set is subject to almost perfect control by means of the series plate condenser C2, and with it, radio-frequency at short wavelengths is very satisfactorily accomplished.