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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
Building the best
2-3 valve radio receiver
Almost every year, the Vintage Radio Club
of North East Victoria runs a competition
called the “Hellier Award”. The challenge
is to build an item of equipment, usually a
radio receiver, to see who can produce the
best result.
– if you could obtain them.
If the set was to be mains-operated,
then the rectifier valve was excluded
from the valve count. In addition,
the set could be a broadcast-band or
multi-band unit, it could be housed
in a suitable cabinet and it could have
either loudspeaker or headphone output or both. Both superhet and tuned
radio frequency (TRF) receivers were
eligible for the 2010 competition.
Unfortunately, a few members ran
out of time to produce a suitable entry,
including yours truly. In the end, four
members came up with workable sets,
all of them TRF receivers.
One receiver was a variant of the
“1955 Miniature DXer” while another was based on the “1958 Basic
Three”, both originally described in
“Radio & Hobbies” (the forerunner
of “Electronics Australia” magazine).
The other two sets were based on the
“1967 All-Wave-Two” from “Electronics Australia”.
Only one entrant (Dennis) built a
cabinet, which made his entry complete and he was also judged the winner. Dennis’s cabinet is based on the
“Aristocrat 3”, circa 1931.
As can be seen from the photographs, the construction techniques
used varied somewhat. We’ll look at
each set in turn a little later on.
The 1967 All-Wave-Two
Eric’s 1967 All-Wave-Two was built on a simple folded aluminium chassis
and is neatly laid out. The addition of a “band-spreading” capacitor in
parallel with the main tuning gang should make tuning a breeze.
T
HE PROJECTS FOR the Hellier
Award are designed to stretch
members’ abilities and this year’s
project was to produce a 2-valve radio
receiver. What made this a challenge
was that each valve could have more
than one active device in the one envelope. Valves such as the 6BL8, which
88 Silicon Chip
has both pentode and triode sections,
could be used or going even further,
a 12-pin Compactron valve such as a
6AF11 could be pressed into service.
The 6AF11 incorporates two triodes
and a pentode in the one envelope, so
just imagine the sort of set that could
be built using a couple of Compactrons
This little TRF receiver was originally published in “Electronics Australia”
in June 1967. It has some interesting
design features that overcome some of
the limitations of a regenerative TRF
receiver, with the first stage functioning as both an RF amplifier and a
detector.
A common problem in most TRF
sets occurs because antennas are
a complex combination of inductance, capacitance and resistance. In
combination with the RF coil, this
combination gives rise to a number of
resonances across the tuned frequency
range, especially in multi-band receivers which cover from 500kHz to
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Fig.1: the 1967 All-Wave-Two is a regenerative
TRF receiver with three switchable coils to
cover from 500kHz to 30MHz. The first triode
stage isolates the antenna from the RF coil
which results in good sensitivity right across
the band and reduces interference problems.
30MHz. The effect is to desensitise the
front-end circuitry at these resonant
frequencies.
This is due to the regenerated signal being “absorbed” by the antenna
(which acts as a tuned circuit at some
frequencies). As a result, it may not be
possible to adjust the receiver so that
it is just shy of going into oscillation,
thus significantly reducing the gain.
This problem was overcome in the
1967 All-Wave-Two by using a triode
to isolate the antenna – see Fig.1. This
triode stage very effectively isolates
the switched RF coil from the antenna
circuit (more on this shortly).
As a bonus, this feature also makes
life much easier for people listening
to sets nearby. When the receiver is
being used to listen to Morse code or
single sideband (SSB) transmissions
on shortwave, the detector circuit must
be oscillating. In most sets, this injects
a signal into the antenna which is then
radiated (ie, the set acts as a transmitter). This signal then interferes with
other receivers tuned to the same
frequency nearby.
However, because the antenna is
well isolated in the 1967 All-WaveTwo, that problem does not arise in
this design. As shown in Fig.1, the
first valve in this receiver is a 6BL8 or
a 6U8. Its triode section is connected
as a grounded-grid amplifier and the
antenna is connected to the cathode.
The output is taken from the plate as
normal.
siliconchip.com.au
Electronics Australia
June 1967
The three tuning coils in Eric’s set are quite close together, so there may be
some problems with mutual inductance upsetting the performance. All parts
are readily accessible for servicing.
This configuration doesn’t provide
much gain but what it does do is to
make the characteristics of the antenna
relatively unimportant. It effectively
smooths the operation of the regeneration on each band and reduces any
radiation from the detector when it is
oscillating.
By contrast, nearby receivers tuned
to the same frequency are likely to
receive interference if the regenerative
detector is the first stage of a receiver
February 2011 89
lytic provide the necessary filtering for
this HT rail, while a 6.3V winding on
the transformer feeds the valve heaters.
In summary, the 1967 All-Wave-Two
is a good choice for this project. It is
a simple design with band-switching
to cover from 500kHz to 30MHz. It
also has high gain and due to the
carefully-designed front-end, is much
more docile to use than many other
regenerative receivers.
Eric’s 1967 All-Wave-Two
This photo shows the above-chassis views of David’s version of the 1967 AllWave-Two. It closely resembles the original “Electronics Australia” design.
(as in most other TRF designs). However, after looking at this part of the
circuit, I wonder whether the gain
of the stage could be increased by
inserting a small RF choke in series
with the 330Ω cathode resistor. This
resistor places a fairly heavy load on
the antenna signals and isolating these
signals from ground with a small RF
choke could be worth a try.
The pentode section of the valve
is used as a regenerative detector.
Instead of having a tertiary winding
for regeneration, the tuned winding is
configured as a Hartley oscillator. The
screen voltage is varied to control the
gain of this stage and hence the point
at which oscillation occurs.
This method obviates the need
for a variable capacitor (eg, 100pF)
regeneration control. These are now
hard to come by and in any case, are
more expensive than a carbon-track
potentiometer.
The RF stage has three tuned coils
and these are switched by a 3-pole,
3-position switch. As stated in the
original article, these tuning coils must
be carefully positioned, otherwise
the mutual coupling between them
(if great enough) can create sensitivity problems in some sections of the
frequency band.
The audio amplifier is conventional
and uses a 6GW8 triode-pentode. This
stage then drives a loudspeaker via a
90 Silicon Chip
The coils are more widely spaced in
David’s set, leading to less interaction
between them. The set performs quite
well.
transformer and there is more than
enough gain for most stations to be
heard at good volume.
The power supply uses a mains
transformer with a 125V secondary.
This feeds two silicon diode rectifiers
which are wired as a simple voltage
doubler to derive a 225V HT (high tension) supply. Two 50µF 200V electro-
Unfortunately, Eric didn’t quite get
his version of this receiver finished, so
it can be considered a work in progress.
And like most of the other entrants,
he hasn’t yet built a cabinet to house
the chassis.
The chassis and front panel were
both made out of aluminium sheet.
The chassis was bent to suit and the
edges riveted, while the holes for the
valve sockets were made using a hole
punch. The cut-out for the power
transformer was made using a nibbling tool.
Most of the other holes in the chassis
were drilled and these are fitted with
rubber grommets where appropriate,
to protect the wiring insulation. In
a few cases though, the edges of the
holes were simply chamfered to make
sure no damage could be done.
Considerable care has been taken to
ensure that no mains wires or terminals are exposed within the receiver.
The cable is clamped and is sheathed
with heatshrink tubing on all terminations, including on the on-off switch/
volume control pot.
The layout of the coils is reasonably
critical to avoid mutual inductance
problems, as occurred to some extent
in the original receiver described in
“Electronics Australia”. What happens
is that the distributed capacitance of an
unused coil resonates on a frequency
that’s covered by the next coil up the
band. As a result, some of the energy
in the selected coil at this frequency is
coupled into the unused coil and this
significantly reduces the performance.
In this receiver, provided the coupling between the two coils is not excessive, the problem can be overcome
simply by advancing the regeneration
control further than normal at the affected frequencies. However, if the
detector cannot be brought into oscillation by the regeneration control, then
it’s necessary to modify the coil layout
to solve the problem.
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In fact, the original article offers
a few suggestions to reduce the coil
inter-coupling problem, including
metal shielding and orientating the
2-8MHz coil at right angles to the other
two coils. Taken together, these two
techniques should virtually eliminate
the problem.
In Eric’s set, the under-chassis wiring is laid out so as to provide easy
access to all valve pins. This makes it
easier to troubleshoot the circuit later
on, should it become necessary. However, the tuning coils are quite close
together, so he may experience some
of the problems referred to above. The
speaker is mounted externally, which
is different to the layout of the original.
Eric also added a low-value variable
capacitor in parallel with the main
tuning gang. This technique is called
“band spreading” and makes it easier
to tune single sideband (SSB) and
Morse code transmissions on shortwave. Band spreading was a common
technique in amateur radio receivers
during the home-built era.
Unfortunately, Eric ran out of time
with this set. The dial scales had
not been completed by the judging
deadline and the control shafts were
also still at full length. In addition,
on the day of judging, the set threw a
“hissy” fit and refused to work when
the speaker transformer decided it had
had enough and the primary winding
shorted to the frame.
Apparently the speaker had become
disconnected whilst the output was at
high volume. As a result, high voltages
were developed across the speaker
transformer primary and the insulation broke down because there was
no load on the transformer.
It’s a pity that Eric had not been
able to complete the set by judging
day, as its ability to easily tune SSB
would have been interesting. Tuning
SSB voice transmissions on shortwave
is not usually easy with simple TRF
receivers and Eric’s band spread modification should make a big difference
in this regard.
David’s 1967 All-Wave-Two
David’s receiver was also built on
an aluminium chassis. Like Eric, he
bent the chassis himself but instead of
riveting it together, it is secured with
self-tapping screws. The front panel
has been rubbed down with steel wool
and the finish looks good.
In fact, David’s set more closely
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David’s 1967 All-Wave-Two closely resembles the original “Electronics
Australia” design. A cabinet will be necessary to protect the user from high
voltages under the chassis
resembles the original set shown in
“Electronics Australia”. He hasn’t made
a cabinet for it but this will have to be
done at some stage in the near future, if
only to protect the user from dangerous
voltages under the chassis.
The power transformer is mounted
above the chassis and is secured in
place using four screws. The mains
cord is clamped with a through-hole
cordgrip grommet and the chassis
securely earthed.
The coils in David’s set are more
widely spaced than in Eric’s receiver
and so any interaction between them
should be inconsequential. Basically,
David has closely followed the original
design when it comes to the component layout. As a result, the parts are
a little crowded around the audio
amplifier.
Once the set was performing satisfactorily, the dial scale was calibrated.
The resulting receiver works quite
well.
Ray’s Basic Three 1958
“Radio & Hobbies” magazine (the
forerunner of “Electronics Australia”)
occasionally described receivers that
used valves that were older in vintage
than those commonly in use at the time
and the “Basic Three 1958” is one such
receiver. The circuit is basically the
same as that for the “Miniature DXer”
of 1955. However, Ray’s Basic Three
uses a 6SJ7GT and a 6V6GT instead
of the 6AU6 and 6BV7 valves used in
the “Miniature DXer”.
Most 2-valve regenerative receivers
with just two active stages are almost
identical to each other. In fact, it isn’t
hard to find your way around the
circuit without a circuit diagram, although a diagram does makes working
on a set somewhat easier.
As can be seen in the photographs,
Ray’s entry is not conventional in presentation. It is built more like a display
item, with the valves, controls, major
components, speaker and transformers all mounted on one flat sheet. No
part of the receiver is mounted on the
wooden surround. The oval-shaped
loudspeaker faces upwards and is
protected by perforated aluminium
mesh which covers two large round
holes directly in front of the cone.
Unlike the other entries, the mains
input to this receiver is via a male
IEC socket. All the exposed mains
terminations have been covered with
heatshrink tubing to ensure safety and
the mains wires have been secured
with cable ties. The wiring is neat,
with easy access to all the valve and
coil pins at their respective sockets.
The first stage uses five plug-in
tuned coils to cover from 500kHz to
32MHz, with generous overlapping
of each range. Each of these coils was
wound onto PVC tubing, which was
then glued onto an octal plug. The
coils are inserted as required into a
February 2011 91
transformer is quite close to the power
transformer. This is risky, since the
power transformer can induce hum
into the speaker transformer that no
amount of filtering will eliminate.
However, it looks as though Ray has
been fortunate in this set, as there is
no obvious hum.
One deviation from the original
design is that the valve rectifier has
been eliminated and replaced by a diode bridge arrangement. This is more
compact and efficient than the valve
rectifier used in the original design.
In practice, this set worked quite
well and many stations can be heard
on the broadcast band.
Dennis’s Miniature DX Set
Ray’s 1958 Basic Three is built like a display item, with the major parts clearly
labelled. It uses five plug-in coils to cover from 500kHz to 32MHz
The circuit used by Ray is basically the same as that for the “Miniature DXer” of
1955 (also used by Dennis for his set). However, Ray’s set uses a 6SJ7GT and a
6V6GT instead of the 6AU6 and 6BV7 valves shown here.
matching octal valve socket on the top
of the chassis.
There are three windings on each
coil assembly – the antenna coil, the
tuned winding and the regeneration
winding (also known as the reaction
or tickler winding). The 6SJ7 valve is
wired as a conventional regenerative
detector.
Normally, an RF choke is wired into
the HT feed to the plate of the detector to prevent RF from being passed
to the following stage. However, an
92 Silicon Chip
RF choke is not used in this instance
as any inductor will have a resonant
frequency or multiple resonant frequencies across the receiver’s tuning
range of 500kHz to 32MHz.
As a result, a 10kΩ resistor has been
used instead and on the broadcast
band, at around 530kHz, this has a
greater resistance to RF signals than a
2.5mH RF choke.
The power supply uses a transformer that Ray wound on a lathe. One
possible problem is that the speaker
Dennis’s Miniature DX Set from
1955 uses a similar circuit to Ray’s but
its execution is very different. That’s
because Dennis’s interest in vintage
radio leans heavily towards sets with
beautifully-finished timber cabinets.
Many years ago, Dennis came across
some information on a set called the
“Aristocrat Three”. During its heyday,
this set was advertised as the “1931
Wonder Receiver” and one of its main
features was its attractive tower-style
timber cabinet.
Dennis decided to reproduce this
cabinet for one of his own sets and
because no dimensions were available,
designed his replica just by looking at
a photograph of the original. The cabinet was made from timber that Dennis
recycled from some old furniture and
is a tribute to his woodworking skills.
As well as functioning as a radio,
the original set also had an “Eight Day
Jewelled Movement Luminous Clock”
and Dennis was able to obtain a clock
that looked very similar. The dial-scale
is also similar to the original but is
made by Healing.
When the Hellier Award competition for 2009/2010 came along, Dennis
decided that the Miniature DX Set
would be suitable receiver to fit into
his replica cabinet. The receiver is
built on an aluminium sheet that has
been cut, bent and riveted to make a
conventional chassis.
The geared dial-drive system used
by Dennis allows the Healing keyhole dial escutcheon to be used. The
loudspeaker is mounted at the top of
the cabinet facing upwards, as in the
original “Aristocrat”. In addition, the
inside of the cabinet has been painted
matte black – just like many sets of
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Dennis’s Miniature DX Set of 1955 also uses a set of five plug-in coils to cover
the broadcast and shortwave bands. The rubber band fitted to the dial drive
systems is a stop-gap measure only.
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Enclosed is my cheque/money order for
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These are the remaining four plug-in coils for Dennis’s Miniature DX Set (the
fifth coil is shown in the chassis view above). Each coil is clearly labelled.
the valve era were. There is plenty of
room for the chassis and there is also
ample ventilation, as the chassis sits
on raised wooden runners.
The circuit uses the valves originally
specified for the Miniature DX Set, ie,
a 6AU6 regenerative detector, a 6BV7
audio output (or alternatively a 6M5)
and a 6X4 rectifier. The coil formers
are made from old valve bases with
_________________________________
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electrical conduit glued to them. The
five coils were then wound onto the
conduits and terminated at the appropriate pins on the plugs.
During construction, Dennis fitted
a valve that he believed to be a 6BV7
even though part of its type number
had rubbed off. Unfortunately, the
completed set refused work and after
spending some time trying to locate the
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February 2011 93
The high point of Dennis’s set is its beautifully-crafted timber cabinet, complete with a clock and a keyhole dial
escutcheon. The loudspeaker is mounted inside the cabinet facing upwards.
fault, he eventually took the set to a
friend who pointed out the valve was
in fact a 6BM8. This valve is a triode
pentode and is quite a different beast.
Unfortunately, this can be a problem
with used valves which have missing
(or partially missing) type numbers.
If you aren’t certain, then it’s a good
idea to compare the unknown valve’s
internal structure with valves that
have their type numbers intact.
Having finally identified the mistake, Dennis then had to fix the
problem. He didn’t have a 6BV7, so
he rewired the valve socket to suit a
6M5. That proved successful – with a
6M5 installed and power applied, the
set burst into life.
The underside of the chassis is not
unduly crowded, although access to
the valve socket pins isn’t as easy as
it is in the other sets. In addition, one
electrolytic capacitor has heat-producing resistors mounted underneath
it. Fortunately, the heat produced by
these resistors is quite moderate but as
a general rule, it’s best to keep parts
like valves and high-wattage resistors
clear of other components to ensure
long-term reliability.
One problem with Dennis’s set is
that he has temporarily “anchored” the
mains cord by tying a knot in it, just
inside the chassis. This was common
practice back in the 1940s and 1950s
but it’s no longer acceptable and Dennis has promised to remedy this at the
earliest opportunity.
Despite this, his set was judged to be
the winner in other areas and it’s not
hard to see why, especially with that
SC
beautifully-crafted cabinet.
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