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

What Reflex Means 673
might be expected. Here, however, we are
dealing with a different problem. In the tickler
case we are interested in a very small percentage
of the total frequency, that of the incoming
signals, say one million cycles. The
tuning is so adjusted and the position of the
FIG. 2
One of the simplest forms of reflex and one of the
best known. The coil in the plate circuit is known
to all users of regenerative sets. Some of the radio
frequency energy is fed back again through the tickler
coil to the primary circuit
tickler so arranged that oscillations over this
comparatively narrow band can be controlled.
In audio-frequency amplifiers, however, we
are interested in the uniform amplification of
the whole band of frequencies from fifty to
several thousand cycles. If the tickler were
adjusted for one particular frequency, oscillations
would probably occur at another. For
code reception where all signals can be brought
to a single audible note, say 1000 cycles, the
audio amplifier may be made to regenerate in
this fashion with remarkable results.
In the case of the resistance-coupled amplifier,
as illustrated in Fig. 3, part of the output
energy of one tube is sent around the circuit
again. There are certain precautions that
must be taken before such a scheme works,
precautions with what the engineer would call
the "phase relations." In general such feed
back is undesirable because of the difficulty in
controlling the system.
WHEN AMPLIFIERS OSCILLATE
A UDIO-FREQUENCY amplifiers very
** fre-
quently do oscillate, a result of some undesirable
coupling between the output and the
A high pitched squeal may betray this
input.
state of affairs, but often the oscillations take
place at a frequency above audibility. Lack of
amplification and some distortion is the result.
It is for this reason that all grid and plate leads
in multi-stage amplifiers should be as short and
as far apart as possible.
In practice, then, a frequency changer follows
the amplifier that is to be reflexed and it
is the greatly changed frequencies that are
returned to a precceding part of the circuit to
go the rounds again. Fig. 4 shows the general
principle. Incoming signals are sent through
a radio-frequency amplifier where they are
boosted in amplitude. They are then changed
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