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

724
Radio Broadcast
.0005 OPTIONAL
HI-
FIG.
The schematic circuit of the reflex receiver. The peculiar connection to the grid and plate circuits are
especially to be noted. The constructional details are contained in the accompanying article. The radiofrequency
transformer, shown in the diagram is completely illustrated in Fig. 3
I
transformer should have a ratio of the order
of 10:1 to work best with the crystal. A small
fixed condenser, "3" should be placed
across the secondary of this transformer to
bypass the radio-frequency currents. Its capacity
should be .00025 or .0005 mfd. The
condenser "4"
is fixed, and may be as large
as .005 mfd., and is
placed across the two outside
terminals of the double circuit jack, as a
further by-pass. The second audio transformer
should be of about 4:1 ratio for good
quality.
The tuning unit used here is the result of
considerable experimental work. It has a
low distributed capacity and at the same time
the three coils have a high degree of coupling.
In designing this unit the following conditions
had to be met. The circuit must be
able to oscillate over the entire range of broadcast
wavelengths, with a fixed winding radiofrequency
transformer in the plate circuit of
the tube. By means of the balancing condenser
and part of the secondary coil, these
oscillations must be balanced out. At the
same time, a high degree of selectivity must be
maintained.
Thetuningcoilisofthefixedcouplertypewith
an untuned primary and a tuned secondary.
The primary has one tap to be used with a
long antenna. The tuned secondary has a
tap so placed that one part of the coil is included
in the grid circuit, while the other part
acts as a balancing-out or compensating
When coil.
used in conjunction with the balancing
condenser "CB", all regeneration can be
controlled.
The coil winding data for the tuning unit is
outlined below:
Size of Tubing:--2" long x if" diameter.
Coil No. i consists of 21 turns tapped at the i6th.
Inside turn connects to grid i6th to LA and 21 st
to SA.
Coil No. 2, 35 turns, outside lead to C2, inside
lead to A.
Coil No. 5, 25 turns, outside to A, inside to Cl.
All the coils are wound with No. 26 d. s. c. wire
on a form slightly greater in diameter than the tubing.
The form has 19 spokes f" in diameter producing
a coil about iV wide. The winding style is over
two and under two spokes.
BALANCING CONDENSER
A PHOTOGRAPH of the balancing condenser
is shown. It will be noted that
this condenser is
designed to have an extremely
small minimum capacity. Unless this minimum
capacity is very small, the circuit will be
of the
sensitive only over the middle portion
broadcast wavelengths.
The small commercial condensers available
were found to be unsuitable for this purpose,
so that a special condenser had to be designed.
Those who desire to make their own can do so
with old parts on hand. The only conditions
to be met are a low minimum capacity and a
maximum capacity of about fifty micromicrofarads.
"C" is a variable condenser
whose capacity may be as low as .00029 mfd.
and should be of fairly good design. This capacity
is equivalent to 13 to 17 plates depending
upon the make of the condenser.
A FIXED crystal
THE DETECTOR
is used for the detector.
Right here it may be advisable to point
out a great weakness of the variable crystal
detector.
A given setting of the detector introduces
a certain resistance into the circuit. If
the circuit is tuned and adjusted to a certain
wavelength and the variable detector is then