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Vintage Radio
By Rodney Champness, VK3UG
The National SW5 Battery-Powered
“Thrill Box” Receiver
A classic American shortwave radio from the 1930s
The SW5 with its matching
external speaker (above) and
reproduction power supply.
High-performance shortwave radios became popular in the USA in
the late 1920s and early 1930s, with quite a few manufacturers getting
involved. This began happening at a time when home-made receivers
were the norm for shortwave reception and manufacturers were just
getting to grips with the problems of sensitivity, selectivity, frequency
stability and ease of use for such sets.
T
HE PIONEERS OF shortwave
radio manufacture in the USA
were Hammerlund, Hallicrafters and
National Radio Company, all of which
subsequently disappeared from the
field. In particular, the National Radio
Company was based in Malden, Massachusetts and in 1928, it employed
James Millen as chief engineer. His
leadership and technical know-how
subsequently resulted in a number of
very good shortwave receivers being
produced, starting with the model
SW3 around 1928 and followed later
by the SW4 and SW5, the latter produced from around 1930-1932.
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This SW series of receivers were
all called “Thrill Box”. Perhaps that
was because they thrilled the user by
their ease of use, particularly when
compared to other receivers of the era.
The SW5 “Thrill Box”
As shown in the photos, the SW5
shortwave receiver was housed in
a metal coffin-style cabinet with a
crackle finish (although it looks more
like a crinkle finish). The dark brown
was very much the colour of choice
for most radios of the era.
The three controls are laid out neatly
across the front panel, with the regen-
eration control at left and the tuning
control at centre with a numbered
dial-scale immediately above it. Frequencies were not directly marked on
dial scales at this stage of radio development and so listeners usually made
a list of where the various stations appeared on the scale. With the “Thrill
Box”, several lists were necessary as
this is a multi-band receiver.
The final control (at right) is for
antenna peaking and this was simply
adjusted for best reception of an incoming signal.
As well as these controls, the set also
features an on-off switch. This switch
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is located inside the set, in the front
lefthand corner of the chassis.
Changing frequency bands
Despite tuning several bands, there
is no band-change switch on the SW5.
Instead, in order to change bands, it is
necessary to remove two plug-in tuning coils and swap them for another
pair that correspond to the wanted
frequency band. A selection of tuning
coils is shown in one of the photos.
Both the loudspeaker and the power
supply are external to the receiver.
The set uses battery valves, so power
is normally supplied by a bank of batteries supplying 2V for the filaments,
-3V and -22.5V for bias, and +67.5V
and +135V for the high-tension (HT)
rails. The receiver could also be used
with an external mains 110V 60Hz
AC power supply. This had to be an
external supply as unwanted 60Hz
hum would be induced into the detector and/or audio transformers if it was
mounted inside the cabinet.
It’s also worth noting that not everyone used a loudspeaker when listening
to shortwave back in the 1920s and
1930s. Instead, many people preferred
to use headphones as ship operators
commonly did, perhaps because it
looked more “professional”.
With the four pairs of plug-in coils
normally supplied, the receiver can
be tuned from 2609kHz to 19,355kHz.
Another three pairs of coils could also
be purchased which extended the tuning from 2729kHz down to 526kHz,
giving seven pairs of coils for a full set
(ie, 14 separate plug-in coils).
At least, that’s according to some
of the literature I have on this set.
Other literature indicates that the set
could tune from 1.5MHz to 33.3MHz
as standard and with additional coils,
could tune as low as 90kHz and as high
as 35MHz. Whether these are National
Radio Company figures or whether enterprising amateurs decided to extend
the tuning range is unknown. In my
opinion though, it would not be easy
to get it to operate reliably all the way
up to 35MHz.
Operating ease
Prior to the “Thrill Box” series of
shortwave receivers, most shortwave
sets were homemade and it usually
took considerable skill to get them to
perform well. Unfortunately, most had
significant deficiencies. There were
not many builders who fully grasped
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The lid of the SW5’s cabinet is hinged at the back, so that it can be opened to
change the plug-in coils for different bands. There are just three front-panel
controls: regeneration (left), tuning (centre) and antenna peaking (right).
and understood the intricacies of radio
design during those early days.
However, James Millen was one who
did understand and considerable time
was spent designing the “Thrill Box”
series of receivers to make them easy
to use. These sets are the forerunner
of the communications receivers that
January 2013 87
Fig.1: the SW5 receiver is a 5-valve regenerative design with plug-in coils in the front-end to tune various bands up to
about 33MHz. Two type 32 valves form the RF and detector stages, while a type 30 acts as an amplifier. The latter then
drives a transformer which feeds the grids of two type 31 output valves wired to operate in class-A push-pull mode.
This photo shows just some of the plug-in coils used with the receiver. Four pairs
of coils were supplied as standard and additional coils could also be purchased.
became well-known a few years later,
eg, the National HRO.
Circuit details
Let’s take a look now at the circuit
details for the National Shortwave
“Thrill Box” – see Fig.1.
In operation, the signal from the
antenna coil is inductively coupled
into a double-tuned input circuit.
One side is individually tuned to frequency while the tuning capacitor in
the second tuned circuit is mechanically coupled to the tuning capacitor
for the detector stage (few sets had
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ganged tuning capacitors at this stage
of receiver design, ie, around 1930).
In this set, the shaft coupling the
two tuning capacitors together goes
through a metal shield which separates the RF amplifier and detector
stages inside the case (see photo).
Note that relatively small-value tuning capacitors are used, ie, around
90pF maximum. This makes tuning
the high-end shortwave frequencies
easier than when using larger-value
tuning capacitors because it restricts
the tuning range to 1.75:1.
By contrast, most receivers have a
tuning range of 3:1 (ie, the ratio of the
highest to the lowest frequency) in any
selected band.
In addition, National developed a
new type of tuning capacitor to overcome some of the shortcomings in
earlier tuning capacitors.
As shown, the RF tuned circuit is
coupled via a 0.001µF (1nF) capacitor
to the grid of a type 32 sharp cut-off
tetrode valve. This amplifies the applied signal and feeds it to the next
plug-in coil assembly which forms
part of a regenerative detector grid
tuned circuit.
The grid leak resistor used here is
5MΩ and this is shunted with a 100pF
grid coupling capacitor. The regenerative detector is another type 32 tetrode
and the regeneration coupling coil is
in the plate circuit. The regeneration
level is set by varying the screen voltage of the type 32 from 0-48V using
a 50kΩ potentiometer which has its
wiper connected directly to the screen.
The output from the detector stage
then goes through an RF choke (RFC)
to remove most of the RF signal that’s
superimposed on the audio. It’s then
fed via a capacitor to the grid of a type
30 triode valve and amplified. The
plate circuit of the type 30 is connected
via a headphone jack to either a set of
headphones or, if no headphones are
plugged in, to the primary of an audio
push-pull driver transformer.
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This transformer is connected to the
grids of the two type 31 output valves
wired to operate in class-A push-pull
mode. The bias on these two valves
is -22.5V and their current drain is
around 16mA. This push-pull stage
then drives the loudspeaker via an
audio output transformer.
The nominal speaker impedance
specified for use with this transformer
is around 1000Ω. However, in order for
a normal low-impedance speaker to
be used, a 1000Ω:8Ω line transformer
has been added to the original circuit.
The audio gain of the type 31 valves is
quite low at a maximum of 3.8 times,
so the preceding type 30 stage has to
work fairly hard to drive these valves
to a reasonable output.
The valve filaments are rated at 2V
and these would normally be powered
by a 2V lead-acid cell capable of supplying around 0.45A. The high-tension
voltages (67.5V and 135V) are also supplied by batteries and the maximum
current is around 23mA.
The bias batteries have virtually no
current drawn from them and so lasted
many years. In fact, I have seen some
bias batteries from the 1930s that still
have a reasonable output voltage. The
on/off switch opens (and closes) the
filament return line to the chassis and
disconnects the voltage divider across
the 67.5V battery, so that no current is
drawn when the set is turned off
Physically, the chassis is laid out
in a logical fashion so that there is
little interaction between stages. In
addition, the various sections of the
receiver are shielded from each other
in the interests of stability.
Basically, the RF stage is to the rightfront of the chassis and is shielded
from the detector stage to the left-front.
Both RF valves are also individually
shielded. The audio amplifier section
is at the back of the chassis and is also
shielded.
Restoration
The receiver featured here was
obtained in very good mechanical
condition. Unfortunately, some plugin coil sets were missing but there are
enough to make for some interesting
shortwave listening.
A few quick checks of the receiver
with no power applied revealed that
the grid resistors attached to the two
32 valves were open circuit. These
were both replaced, along with the
20kΩ resistor attached to the 50kΩ
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This is the view underneath the chassis. The set is logically laid out and
the various stages on top of the chassis are separated by metal shields to
prevent interaction between them.
regeneration potentiometer.
Initially, no circuit was available and
it seemed that the “audio transformer”
(as marked in the set) at the output of
the detector had an open circuit secondary winding. Once the circuit was
obtained it was clearly obvious that
this so-called transformer was not a
transformer at all. Instead, it consisted
of an audio choke with a coupling
capacitor from the valve end of the
winding to the grid of the type 30 audio
valve. A grid resistor for this valve was
also enclosed in this assembly.
Just why the resistor and capacitor
were enclosed in the choke case is a
bit of a mystery. In any case, something
wasn’t quite right here so the unit was
carefully dismantled (see photograph).
Both the mica coupling capacitor and
the grid resistor had failed, so these
were promptly replaced. The values of
these two parts are not shown on the
circuit but both still had their values
marked on them.
Further checks revealed that all the
mica capacitors underneath the chassis were leaky. As a result, they were
disconnected from the circuit and new
ceramic capacitors wired in beneath
them to retain the original look. In fact,
all the fixed capacitors and resistors
had to be replaced. Fortunately, no
coils or transformers needed replacement as the original types would now
be difficult (if not impossible) to find.
A simple numbered dial-scale was
used in the SW5 receiver, so users
normally kept a list showing where
various stations appeared.
Many early radios used very few
fixed components (to keep costs down)
and the addition of one or two bypass
and/or decoupling circuits can often
improve the performance. In this set,
RF was present on the 135V HT rail
(as “seen” by an oscilloscope) and this
was eliminated by connecting a 47µF
capacitor and a parallel 10nF ceramic
capacitor from this rail to chassis. This
also provides extra HT filtering.
In addition, a new power supply
January 2013 89
The unit came with this signed card, listing two distant
shortwave stations that had been received during factory
tests.
Left: inside the so-called “audio transformer”. The enclosure
actually holds an audio choke at the output of the type 32
detector, along with an associated coupling capacitor and the
grid resistor for the type 30 valve. Both the capacitor and the
grid resistor had failed and were therefore replaced.
lead was made up. This exits from
the back edge of the chassis and runs
to a home-made mains power supply,
as batteries are either very expensive
or now unobtainable. This AC mains
power supply was built into a case
that resembles the original 110V AC
power supply, with all the necessary
output voltages catered for.
Testing
Having carried out these repairs, it
was time to test the receiver. This was
done by plugging in a set of coils, connecting a loudspeaker and an antenna
and using a signal generator to inject
a modulated sinewave signal into the
RF amplifier stage.
This simple test revealed that the
audio was of good quality at the headphone jack. However, when I subsequently tuned to a local radio station,
the audio from the speaker was atrocious. The high-impedance speaker
used with the set was perfectly OK, so
the problem obviously lay somewhere
in the push-pull output stage.
This fault was tracked down by
once again injecting a modulated sinewave signal from the signal generator.
Checking with an oscilloscope then
showed that a good sinewave signal
was present at the headphone jack.
However, when the scope probe was
connected to the plates of the two 31
valves, one showed the expected half
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sinewave while the other showed
almost nothing.
This indicated that that particular
type 31 valve was defective and replacing it immediately cured the fault.
It didn’t take long for the next problem to crop up. After tuning across the
band a few times, the dial cord broke
so that also had to be replaced.
This set has one of the early corddrive systems and the dial-cord used
was quite coarse, being much larger
in diameter than the dial-cord available today. However, for sets of this
vintage, nylon builder’s line makes
good dial-cord and so that was used
to restring the dial.
Performance
These receivers were very good
performers for their time. This set is
quite sensitive and is relatively easy
to tune but is subject to overload on
the broadcast band. In my case, a
strong local station could be heard in
the background right across the band.
The set can also be a bit fiddly to
operate, as there is a degree of interreaction between the various controls.
One thing that did puzzle me initially was that the set’s performance
varied from one band to the other.
However, after searching the internet,
I discovered that the coil formers used
in this set (and many others) up until
around 1931 were made of a material
that absorbed moisture. This adversely
affected the tuned circuits and degraded the set’s performance.
Summary
This is a well-built receiver which
used the best technology available at
the time. It’s a pity that the coil formers give problems, since coil formers
that didn’t absorb moisture became
available not long after the set was
manufactured.
As stated, the receiver is prone to
overload from strong local stations
and it has no volume control other
than the regeneration control. This
probably didn’t matter too much at
the time, as the receiver (in standard
form) only came with shortwave
coils and there weren’t many strong
shortwave stations around in 1930.
However, strong local broadcast-band
stations were springing up around
the country at this time, so overload
on the broadcast band would have
become an increasing problem (a set
of coils for the broadcast band and the
low shortwave band were sold as an
accessory pack).
Detuning the antenna tuned circuit
and reducing the size of the antenna
would have helped solve this overload
problem, as would placing a 50pF capacitor in series with the antenna or
installing a wave-trap in the antenna
SC
lead.
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