Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

Radio
Broadcast
Front view of the Roberts de luxe receiver.
FIG. 5
The meter is in the plate circuit of the audio amplifier
riodic primaries arc usually found on tuned
radio-frequency receivers, which have an
actual tuned or resonance point on the low
wa,ves which act as an additional absorption
circuit at these high frequencies, choking
the tendency to oscillate.
In a two-stage tuned r. f. amplifier, the
resonant absorption coils should be used in the
first and second stage, and in the detector circuit.
In receivers already completed, it may
be most convenient to wind these coils on
separate forms, mounting them in the ends
(grid or filament) of the r. f. transformers.
Another interesting, though slightly inferior,
method of differential stabilization is the
shunting of high resistances (R) across the
r. f.
tuning condenser (in Fig. 2, B). This
has the effect of increasing the "phase difference"
of the circuit. The phase angle (unfortunately
a rather technical consideration)
is a determinant of the power loss in any
circuit which increases with the frequency.
Hence, the loss will be greater on the higher
frequencies or lower wavelengths.
On two stages of tuned radio-frequency
amplification, the resistances across the first
and third (detector) condensers may be fixed
resistors, of the Daven type, having an ohmage
of fifty thousand. The resistor across the
middle condenser controlling, as we have demonstrated,
the least
stable of the three circuits,
should be a Bradleyohm, having a
variable resistance of from ten to one hundred
thousand ohms. This, again, will provide the
desired regeneration control.
Another very interesting and efficient
method of stabilization, though not strictly a
loss method, should be considered by the experimenter
studying these possibilities. For
several reasons, the tendency of a circuit to
oscillate, or to become unstable, increases with
the plate voltage. Hence, in any fairly stable
circuit, for instance, an average r. f. circuit, a
gradual reduction of the plate voltage to just
below the critical point provides an adequate
and efficient method of regeneration control.
This principle is
employed in the Deresnadyne
receiver and in others of similar design. This
idea is illustrated in Fig. 2, C.
The resistance R, a ten to one hundred thousand
ohm Bradleyohm, adjusts the plate potential
applied to the r. f. tubes and detector.
FIG. 6
Rear view showing construction