Improving the Receiver Using a Screen-Grid Coupling Stage
December 1931 QST Article
December of 1931, the discovery of
(aka 'heavy water) was announced by Harold Urey, Japan abandoned the
New York Metropolitan Opera broadcasted an entire opera over radio
for the first time (on Christmas day), and the ARRL's QST magazine published
an article about how to improve a receiver by using a screen-grid coupling
stage on vacuum tubes. A 'tickler coil' is introduced via a tuned circuit
to provide a small amount of positive feedback to the grid in order
to make the amplifier stage more sensitive in the band of interest.
Care needed to be taken to avoid so much feedback that oscillations
could occur. As with most of these old articles I post, while the exact
application might not be relevant in today's world of electronics, the
basic principles are certainly timeless.
December 1931 QST
These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
Improving the Receiver Using a Screen-Grid Coupling Stage
By Howard R. Cassler
There are certain disadvantages in
the use of an "untuned" r.f. stage in the high-frequency receiver, as
compared to a tuned r.f. stage, chief among these being loss in selectivity
(already none too great with one lonesome tuned circuit), "cross talk"
from local B.C. transmitters, high noise level, etc. Referring to Fig.
1 the usual method of tuned impedance coupling, we see also that the
r.f. tube's plate voltage is across the tuning capacity, that there
is possibility of d.c. leakage through the detector grid condenser,
a loading up of the minimum tuning capacity, etc. All these have previously
been pointed out. The logical method of improvement between the r.f.
stage and detector of course is the use of the inductive coupling; but
this ordinarily requires coils with three separate windings and coil
forms with six prongs, whereas most of us now have four or five-prong
Now the receiver circuit described here was developed with the following
in mind: First, to improve the selectivity and signal to noise ratio;
second, to provide" hard and fast" single-control tuning retaining calibration;
third, to use four-prong tube-base coils.
FIG 1. The usual method of tuned impedance coupling.
FIG. 2 - This circuit uses inductive coupling with a combined
primary-tickler coil in the detector screen circuit.
feature of this receiver is the method of regeneration, using inductive
coupling from a combined primary tickler coil, but with the tickler
coil removed from the plate circuit and put into the detector screen-grid
circuit and shunt fed (L2 in Fig.2). This leaves the tuned
secondary or grid circuit free from any direct connection to the r.f.
tube. (This is not exactly an original idea and due credit probably
belongs to the designers of the new Universal Super Wasp which uses
a somewhat similar arrangement.)
For all coils L1
is "close wound" with last few turns at top spaced to cover band. For
L2 spacing between turns is approximately the diameter of
the wire. The spacing between L1 and L2 is about
3/8 inch, except for 28,000 kc. where adjustment may be found necessary.
The older type (longer) Bakelite tube bases will be required for the
first three coils. All coils are made "hard and fast" with several coats
of quick drying lacquer.
Now to obtain a reasonable transfer
of energy the impedance of this primary tickler (L2) should
be somewhat greater than that provided by the usual size tickler coil.
This is accomplished by using a greater than usual number of turns spaced
somewhat further from L1 and of rather fine wire, space wound.
(See coil table.) It is important that as L2 is increased
the spacing from L1 should be increased to keep the point
of oscillation at the optimum value of about 22 volts on the detector
screen grid. Right here we might wish that tube bases were just a wee
bit longer and that the Type '24 tube did not oscillate quite so easily
- which it certainly does in this circuit, no trouble at all being had
in getting up to and above 30,000 kc. Further following this impedance
matching idea, a Type '35 tube was used in the r.f. stage. The plate
impedance of the '35 is about half that of the '24, and a noticeable
gain did result. A '35 tube in the detector stage seemed to make no
A word might be said about the by-pass C3.
Its value should not be greater than the 40μμfd. shown. Even smaller
values gave slightly better results, except, strangely enough, at 28
mc. In general its size should be kept down to where smooth regeneration
results over all the bands to be covered. It goes without saying that
both C3 and C6 should be high grade mica condensers;
for instance, a certain .01-μfd. paper condenser at C7
resulted in a bad dead spot somewhere about 6800 kc. A beautifully smooth
regeneration control resulted with a 100-μμfd midget variable
"throttle" at C3. It did affect the tuning, however, and
was not used.
Other experiments were tried with tuning the grid
circuit of the r.f. stage, but even though interlocking was at an absolute
minimum, the extra gain did not seem to justify an added control. In
this case the antenna was switched to the post "A," and L3-L4
wound identical to L1-L2.
FIG. 3 - Complete circuit of the receiver.
C1 - Remodeled 150-μμfd. S. L. F. variable split
stator. Large section has two plates, small section one plate. Rotor
has thee plates. All plates double spaced.
C2 - 100-μμfd.grid
C3 - 40-μμfd. blocking condenser.
C4 C5 C6 - .006 μfd.
C8 C9 - 1.0 μfd.
R1 - 3 megohms.
R2 - 1 megohm.
- 50,000-ohm potentiometer; regeneration control.
- 10,000 ohms.
R6 - 2000 ohms.
R7 - 1000
RFC - Choke.
AFT1 - Coupling impedance - audio
transformer with windings connected in series.
L3, L4 - Next
larger coil above L1-L2.
final receiver circuit is shown in Fig. 3. Photos are not available
but the mechanical details are much the same as most ham receivers.
The receiver is completely and heavily shielded using all-aluminum shields,
panel and sub-panel, with sub-panel tube sockets. As much of the r.f.
wiring as possible is kept above the sub-panel and all other wiring
beneath in the usual manner. The two r.f. chokes are mounted below the
sub-panel as close as possible to their respective socket terminals.
The resistor R5 across the primary of the series-connected
audio transformer was used to suppress fringe howl before the set was
changed over. Although now not essential for this purpose it was left
in because it improved the quality of phone signals. R6 and
R7 furnish grid bias for the '27 and '47 tubes respectively.
C8 and C9 have little effect on the output; no
a.c. hum is noticeable with either phones or loud speaker. Incidentally,
headphones are not used with a '47 tube in the output stage!
As for results, this circuit gave even better than anticipated.
The improvement in selectivity was very gratifying. (Try it out in the
3500-kc. 'phone band.) Signal to noise likewise improved and the biggest
surprise of all was the drop in tube noises, the detector hiss being
down fully 50%. We haven't figured it out in db.
This drop in
tube noises leads us to consider this circuit as the familiar separate
detector and regenerator with their common grid circuit. The plate corresponds
to the usual detector plate and the detector screen grid takes the place
of the oscillator or regenerator tube plate. Incidentally, the detector
plate voltage in this circuit can be dropped to as low as 22 1/2 volts
with practically no effect on the regeneration. For full signal strength,
however, the recommended 180 volts are used.