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RADIO AGE for February, 1927
•(--UNCONTROLLABLE
FIXED PRIMARYCOUPLING
LINE OF OSCILLATION
LOW
WAVELENGTH
Figure 2 — Theoretical advantages of variable coupling as compared to fixed coupling
so that the higher audio frequencies and their harmonics are passed on to the loud speaker without loss. This results in the reproduction of the higher musical tones of such instruments as the violin with full life and brilliance, and aids very materially in removing the dull and muffled effects so commonly associated with loud speaker reproduction.
Although the receiver has three radio frequency circuits the tuning controls have been reduced to two by placing the second and third variable condensers on the
same shaft. A small compensating condenser in parallel with the third variable condenser has been provided to compensate for the small difference in circuit capacity of the third tuning circuit chiefly due to the detector grid condenser. This compensator needs no adjustment after its setting has once been determined. A volume control has been provided which is exceptionally smooth and gradual, allowing the operator to adjust for a powerful local or a weak and distant station with equal facility.
The Magazine of the Hour
This volume control is a 10 ohm rheostat regulating the filament brilliancy of the two radio frequency amplifier tubes. To eliminate the possibility of applying more than the rated voltage to the filaments of these tubes, a 2 ohm resistance unit is used in series with the radio frequency tubes and rheostat. The filaments of the remaining tubes are held at their proper operating temperatures by separate automatic resistances.
Voltages induced in the antenna ground system are magnified by the action of the first stage of radio frequency amplification and passed on to the second radio frequency stage where their intensity is still further increased.
Since the second radio frequency stage and the detector stage are shielded unwanted signals are weeded out due to the filtering action of the three tuned circuits through which they would have to pass in order to reach the detector tube. This shielding also prevents direct pick-up by the second radio frequency and detector circuits.
The use of stage shielding also eliminates any interaction between circuits, thereby stabilizing the radio frequency amplifier and greatly increasing its over all efficiency.
O+a
Circuit diagram of the Hammarlund-Roberts receiver described by Mr. Biles