Improving the Receiver Using a Screen-Grid Coupling Stage
December 1931 QST Article
December of 1931, the discovery of
deuterium (aka 'heavy water) was announced by Harold Urey, Japan
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
December 1931 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Improving the Receiver Using a Screen-Grid Coupling
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 forms.
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.
The unusual 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
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 difference.
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
Fig. 3 - Complete circuit of the receiver.
- 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.
C7 C8 C9 - 1.0 μfd.
- 3 megohms.
R2 - 1 megohm.
R4 - 50,000-ohm potentiometer; regeneration
R5 - 10,000 ohms.
R6 - 2000
R7 - 1000 ohms.
RFC - Choke.
- Coupling impedance - audio transformer with windings connected
AFT2 - Audio-frequency transformer.
L3, L4 - Next larger coil above L1-L2.
The 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
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.