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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Building a vintage radio “replica”
Have you always wanted a 1920s or 1930s
“cathedral” style radio. They’re as scarce as
hens’ teeth these days – or are they? If you
can’t get an original, what about one of the
many replicas now coming onto the market?
From time to time, “replicas” of
early radio sets appear in catalog advertisements from various electronics
and electrical retailers. Consoles and
cathedral sets seem to be the favour-
ites but of course, they’re not true replicas. First, the cabinets are nothing
like the those from the 20s, 30s and
40s, usually being made from cheap
ply or particle board with a lacquer
finish of some sort. Second, a glance
at the front panel reveals that these
sets can receive FM transmissions as
well as AM. In reality, FM didn’t get
under way in Australia until well after
the era that the “replica” is supposed
to represent.
However, it’s not until you expect
the “insides” of such radios that
you realise just how far away they
are from being a true replica of the
era. Hidden inside the cabinet will
be a small transistor radio and that’s
hardly something that was around in
the 1920s or 1930s!
So these sets are in no way an accurate copy or replica of any early radio.
The fact is, there are very few
genuine 1920s (and not many more
1930s) sets now available on the
market. Many collectors will never
own radios of this vintage. But there
is nothing to stop you from building a
replica using the components (either
originals or reproductions), wiring
layouts and construction techniques
of the era. The resulting set will look
like a brand new 1920s or 1930s radio
(not a restored set), although it still
won’t be authentic.
Building a replica requires a lot
of work when it comes to sourcing
the parts, selecting a representative
circuit and planning the layout. You
then have to assemble it and get it to
work properly. It may also be necessary to vary the original circuitry or
layout somewhat, as some parts may
just not be available any more.
The Rice Neutrodyne
This view shows the
completed Rice Neutrodyne
with its loudspeaker.
74 Silicon Chip
I haven’t personally built a replica
of an early set but a few members of
the Vintage Radio Club of North East
Victoria have. In particular, I was
most impressed with Jim Birtchnell’s
replica of a 3-valve Rice Neutrodyne
(you’ve heard of Jim and his vintage
radios before in this column).
The design for the Rice Neutrodyne was originally published in the
7th October, 1927 issue of “Wireless
Weekly”. That article was quite detailed and covered more than four
pages. Jim started from scratch, either
sourcing or making the parts himself
so that his completed replica very
closely resembles the original.
In the original article, the
author began by discussing
the Rice neutralising system
as seen in an old publication of 1918, some
nine years before! This
publication discussed
the various neutralised
triode amplifiers that were
used at that time.
The problem is that triode valves,
when used in tuned radio frequency
amplifiers, are very prone to oscillate
at the frequency of one of the two
tuned circuits around the stage (ie,
the grid circuit or plate circuit). This
oscillation is due to the capacitance
between the grid and the plate of the
valve.
This capacitance is actually quite
small – only 4pF in the case of a
6SN7-GT. However, this value is more
than enough for this triode to oscillate
fiercely if used in a tuned plate and
tuned grid amplifying circuit. In fact,
this circuit is actually used as an oscillator in some transmitters!
By contrast, a 6BA6 has a gridto-plate capacity of only .0035pF,
which is over 1000 times less than
for a 6SN7-GT. However, this low
The completed Rice Neutrodyne replica closely resembles the original receiver
described in “Wireless Weekly” in 1927.
capacity is only realised when the
screen grid is earthed to RF signals.
Even so, the 6BA6 can still oscillate
in some circuits and so the IF stages
in quite a few HMV Little Nipper sets
are neutralised (we’ll explain what
neutralising is shortly). This was not
done because the stage was inherently
unstable but to make sure the stage
was unconditionally stable.
Of course, it is also important to
isolate the input and output circuitry
of an RF stage, to minimise any coupling between them.
Neutralisation
Radio experimenters of the early
1900s and into the 1920s did not
have tetrode and pentode valves to
amplify radio frequency signals, so
Fig.1: the circuit for the
Rice Neutrodyne
receiver. The neutralising
capacitor (N.C.) is
installed between the
plate of the RF valve
(V1) and one end of
tuned winding L1. This
capacitor effectively
cancelled out – or
neutralised – the grid-toplate capacitance of the
valve, thus making the
stage stable.
January 2000 75
This view shows how
the completed receiver
fits into the home-made
cabinet (it slides in
from the front). The
large socket in the top
panel is for the
external power supply
connections.
other ways had to be found to stabilise
triode RF amplifiers. Neutralising, in
the various forms that it took, was not
always particularly easy to accomplish although the results were quite
reasonable.
Rice Neutrodyne
The Rice Neutrodyne was one such
circuit that employed neutralising
(Fig.1). Essentially, this involved
installing an additional capacitor
between the plate of the RF valve and
one end of tuned winding L1. This
capacitor effectively cancelled out –
or “neutralised” – the grid-to-plate
capacitance of the valve, thus making
the stage stable.
The neutralising capacitor was
made adjustable in most cases and
is adjusted for optimum stability. It
worked because the RF signal at the
bottom end of coil L1 is 180° out of
phase with the signal at the other end
(ie, on the grid) and so the grid-plate
capacitance was effectively “eliminated”.
This system works well but having
the tuning gang “floating” above earth
can cause problems. Because the
aerial, which is connected to the top
of L1 via a 100pF capacitor, can vary
in length, the capacitances around the
circuit can also vary. For this reason,
the value of the neutralising capacitor sometimes required adjustment
which is why a variable type is used.
By the way, this circuit can be
slightly modified to make it much
easier to achieve good results. How76 Silicon Chip
ever, that is a story for another article
in the future.
Another common method of ensuring stability was to install a variable
resistor between the grid of the valve
and the tuned circuit. The value of
this resistor was then adjusted until
the set was stable (a value of around
850Ω was commonly used by Atwater
Kent, for example).
The big problem with this was that
the gain of the stage was dramatically
reduced. This meant that extra amplifying stages were required to make up
for the low gain of an “un-neutralised”
RF amplifier. It may seem surprising
that not all manufacturers used neutralised RF amplifiers, as valves at
that time were very expensive. They
didn’t because patents on neutralising
were held by Hazeltine and Rice and
they weren’t going to let anyone else
use this technique without paying a
considerable royalty.
Having talked about the neutralised
RF amplifier stage, it can be seen
that the rest of the radio is quite conventional, with a grid detector and
a transformer-coupled audio output
stage. The Rice Neutrodyne is really
only a headphone set but it can do a
credible job on strong stations, as Jim
can attest.
Jim’s replica
Jim built the set virtually as per the
article in “Wireless Weekly”, with
some slight modifications to suit the
connection of the power supply to
the set. Building a replica such as
this doesn’t require enormous skill
as the circuitry is quite simple and
the article included detailed layout
instructions. However, this is not
meant to detract from the obvious
skill Jim used in making this set and
getting it to operate.
Obtaining the parts to build such a
set is quite another story. Where does
the builder get 201A valves, or square
section copper wire, UX valve sockets,
5:1 audio interstage transformers or
the old style vernier dials? However,
as can be seen in the photographs, Jim
has succeeded in obtaining suitable
parts.
Jim told me that he imported most
of the bits from America because of
price and availability. However, if one
is prepared to hunt around, most if not
all of the bits can be found (or made)
in Australia.
For example, there are several advertisers in SILICON CHIP that cater for
the vintage radio buff and contacting
them should bring results. They have
extensive stocks of all sorts of bits
and pieces that are used to restore or
build receivers.
Members of the Historical Radio
Society of Australia or the New Zealand Vintage Radio Society are also
often able to assist when it comes to
obtaining that special part. And bits
and pieces can turn up in all sorts of
other places – garage sales, second
hand shops, deceased estates and “for
sale” columns in local newspapers,
to name but a few sources. Placing
adverts in magazines such as SILICON
CHIP, in local newspapers and on supermarket notice boards can also help
track down the bits required to make
an authentic replica of a bygone age.
Each vintage radio buff has his or
her particular area of expertise, so it
is quite reasonable to enlist the aid
of others to help in areas where you
are no expert. As can be seen from
the photographs, the cabinet and the
works of Jim’s set reflect the era that
the Rice Neutrodyne came from. Jim
is very good with cabinet work and
with circuit layout, although he admits to getting a friend to help with
any difficult electronic work on more
complex sets.
Other replicas
Replicas can be made of sets from
any era, from the very first sets made
(with coherers and the like) up to
transistorised radios of the 1960s.
However, an enthusiast is more likely
to build an early wireless set from the
1920s rather than a 1960s set. There
are still many of these later radios
around and if you can scrounge more
than one unit of a particular model,
it’s usually possible to make at least
one good working unit using parts
scrounged from the others.
For this reason, I always endeavour
to collect several sets of the model I
want to restore so that I can make one
“perfect” radio.
There is of course another type of
replica. This replica is not a slavish
copy of any particular set but rather a
copy of the style of set that was used
during a particular era. For example
I know of some enthusiasts who have
built typical 4-valve mantle sets using
octal valves and a wooden cabinet.
They look the part, are similar to
many commercially made sets of the
era depicted and work much the same.
Summary
Above: the top of the cabinet is hinged
to allow easy access to internal
components.
As we’ve seen, replicas
can be direct copies of
receivers from bygone times or can
simply be representative of sets of
a particular era
while not copying
any particular make
or model. And although they are not
true vintage radios,
they can be inter
esting and valuable
sets in their own
right. Other projects
that have been held
in recent times by
various groups include building a “Little General”, a “Little
Jim”, a “Hikers One”
or some other radio,
as described in popular radio magazines
of the time. All of
these are examples of
replicas.
Finally, my thanks
to Jim for sharing with
us the informa
t ion
on his replica Rice
Neutrodyne. The photographs in this article
clearly show what can
be achieved with attention to detail.
SC
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January 2000 77
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