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

How to
Build a One-Dial Reflex
Receiver
A Sensitive Non-Radiating Receiver Using Crystal Detection and
Push-Pull Amplification which is Simple to Build and Operate The
Combination of Crystal and Push-Pull Produces Excellent Quality
BY JOHN C.
DAVIDSON
THE receiver described in this article may very well be called universal. It is,
in effect a one-, two, or four-tube layout, depending on just how much the
builder cares to do at a time. One may start with the single-tube layout and add
the remainder at will. The two last tubes are used in a push-pull audio amplifier
arrangement and are entirely unnecessary for loud speaker operation on stations
up to several hundred miles at night, of course.
In our tests of this receiver we operated a loud speaker on stations up to 300
miles distant with a single tube, up to 700 miles distant on two tubes, and on stations
1
500 miles away with all four tubes. This is not guaranteed operation, but is
not highly unusual operation. The selectivity is very good, though it does not
equal the two- and four-tube Knockout receivers employing the Roberts circuit.
There is but a single major tuning control. The receiver is very easy to build.
We suggest building the two-tube receiver first, because it
produces enough
volume for most purposes. If after trying two tubes, the music is not loud enough
for dancing, the push-pull amplifier may be added. THE EDITOR.
years study of reflex circuits audio-frequency amplification. This combination
gives plenty of
have brought out a number of interesting
points about this popular
TWO loud speaker volume with
local reception and good loud speaker volume
method of radio reception. It is for reception up to 1000 miles, it
being understood
that long range work is
claimed that a circuit with one tube reflexed
subject to seasonal
and atmospheric conditions. At the
through a crystal detector will equal on distance
reception, and at the same time give same time, particular attention has been paid
far more volume and better quality than the to selectivity, so that at least six or more distant
stations can be brought in through the
ordinary one-tube regenerative receiver. This
additional energy gain is due to the fact that locals of the large cities. This performance
one tube reflexed, gives a stage of radio-frequency
amplification, a certain amount of that it is obtained with a single tuning dial
is especially noteworthy in view of the fact
regeneration, which seems to be inherent in and one volume control dial, which makes the
the majority of reflex circuits, and a stage of operation about as simple as can be desired.
audio-frequency amplification. At the same Stations heard can be logged and returned to
time, ther~, ; s a limit to what can be expected on their own dial setting.
irom one iibe, and this makes the reflexing of
more than one tube a THE CIRCUIT
questionable procedure,
when using an antenna-ground system.
The circuit described here is a logical development
obtained from a study of the above radio-frequency tuning coil secondary
THE circuit in principle can be best understood
by referring to Fig. i. Part of the
is contained
in the first grid circuit and the remainder
analysis. It
comprises one stage of radiofrequency
amplification with a stage of audio of this coil is used in conjunction with the
frequency reflexed through it, a fixed crystal small balancing condenser "CB" to stabilize
detector, a second stage of audio-frequency the circuit, and control the volume. It is the
amplification and finally a stage of push-pull experience of the writer that the first audio