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VINTAGE RADIO
By JOHN HILL
A simple regenerative receiver
Building simple regenerative receivers is a lot
of fun and, best of all, it won’t break the bank.
Here’s how to build a simple 1-transistor radio
receiver.
I can still remember the excitement
caused by my first one valve regenerative receiver, which was built when I
was a lad. It seemed to perform nearly
as well as the 5-valve Radiola in the
lounge room, the only difference
being that my little radio would only
drive headphones, not a loudspeaker.
Of course, such a statement is
strongly biased by youthful memories
of something that had been homebuilt with loving care. Naturally, a
1-valve regenerative receiver could
not compete with a 5-valve superhet
– although it seemed to at the time!
One station that was often received
at night was 2NZ. To hear “this is 2NZ
northern New South Wales” through
the headphones was nothing short of
amazing when one lived in Bendigo.
That little regenerative set could
really drag in those distant stations.
Regeneration or “reaction” is a form
of positive feedback whereby some of
the amplified radio frequency (RF)
energy is fed back in phase to the
tuning coil, boosting signal strength
and improving selectivity. Another
way of looking at this is to visualise
the signal being fed back as acting to
overcome the natural losses – mainly
resistive – in the tuned circuit. It was a
technique commonly used in early receiver designs before the superhet era.
Unfortunately, too much regeneration causes distortion and the
possibility of the set bursting into
oscillation.
Regeneration gave a simple receiver
such as a 1-valver a tremendous lift in
performance. In fact, when connected
to a good aerial and earth, a 1-valve
regenerative outfit is nothing short
of amazing. One gets so much from
so few parts.
Even today, I still enjoy building
and listening to simple 1 and 2-valve
regenerative sets and I know that I’m
not alone in this regard; many other
vintage radio enthusiasts do likewise.
It seems as though little boys never
really grow up.
A 1-transistor design
The original Trans-1 as built by David Waldron. The receiver was built into an
aluminium chassis and went through several experimental stages before this
unit was produced.
David, a young collector friend, is
also a keen devotee of regenerative
receivers and has built numerous
sets employing this simple circuitry.
He has built several AC-powered
short-wave sets with plug-in coils
and they really are good performers.
With careful regeneration control
manipulation, even single sideband
transmissions can be received reasonably well.
The latest regenerative set which
David has built is a departure from
normal and uses a single high gain
transistor and a ferrite rod aerial.
This month’s story is about David’s
one-transistor regenerative receiver
– the “Trans-1”.
The circuit shown is as supplied
and there have been no alterations to
it at all. The set went through several
January 1998 49
developmental stages before reaching
finality and involved quite a few hours
of trial and error experimentation.
The main problem encountered
with Trans-1 was with the regeneration control. It would operate reasonably well at the low-frequency end of
the dial but was a bit touchy and difficult to control at the high-frequency
end. The addition of a few resistors
at strategic places in the regeneration
circuit smoothed over this problem
and the reaction control is as good
as one could hope for in a receiver
of this type.
Regeneration is controlled by a 5kΩ
linear potentiometer. This was used
in preference to the small variable
capacitor often used in this type of
receiver.
Perhaps the most remarkable aspect
of this little 1-transistor radio is the
fact that it performs every bit as good
as a 2-valver. In fact, it outperforms
my “Junk Box 2” with its two type
30 triodes (detector plus a transformer-coupled stage of audio).
One reason for Trans-1’s better
performance is the higher gain available from a transistor, even a simple
low-cost device like the BC549 which
David used. A type 30 (1H4G) triode
valve has a theoretical gain (µ) of 9.3
(less in practice) whereas the BC549
has a minimum hfe of 200. On this
basis, it’s not hard to see why the
Trans-1 performs so well.
As set up in the regenerative receiver circuit (see Fig.1), the BC549 draws
2.5mA from a 9V battery. Using the
2-valver as a comparison again, the
filaments draw 120mA at 2V, while
the plates consume about 2mA from
the 45V “B” battery.
Trans-1 can be used with either
The author’s Trans-1 was built into an existing timber cabinet that had
previously housed other projects. The switches are for on/off and for selecting
between the 5 and 10-turn taps for the transistor base connection.
Fig.1: the circuit diagram for the Trans-1. Transistor Q1 acts as a detector
and amplifier stage, while VR1 sets the amount of regeneration.
This rear view of the author’s partially completed unit
shows all the major components in place. Note the two
tag strips for mounting the minor components.
50 Silicon Chip
This is what the unit looks like with all the minor parts
installed. A slightly larger cabinet would have made
construction easier.
An output transformer must be used if
low impedance headphones are to be
used. Shown here is the M1100 audio
line output transformer from Dick
Smith Electronics.
This end view shows the M1100 output transformer that’s used to drive a pair
of 8-ohm headphones. The 9V battery is attached to the top of the transformer
using double-sided masking tape.
high-impedance headphones or can
drive 8-ohm stereo headphones via
an output transformer. The latter
method is by far the better alternative
when it comes to comfort and fidelity.
A Dick Smith M1100 transformer or
equivalent works reasonably well as
an output transformer.
Practical details
David built his receiver on an aluminium chassis, whereas I built mine
into an existing wooden box which
had housed a few past projects. It
doesn’t matter how you build Trans-1;
the result will be much the same.
However, one advantage of David’s
metal chassis construction is that it
eliminates hand capacitance effects.
The bakelite front panel on my set
doesn’t do this and hand capacitance
can be noticeable when the receiver is
tuned to weak stations which require
maximum regeneration. But it’s not
much of a problem really.
The choice of components is not
critical and if a constructor doesn’t
want to use a ferrite rod aerial, then
he can do his own thing and wind a
coil on a cardboard former. However,
if a ferrite rod is not used, the coil
winding information will differ considerably from that specified in the
circuit. What’s more, the small 350pF
tuning capacitor shown on the circuit
may have insuf
ficient capacitance
range if used with an air-cored coil.
In the latter case, a 400-500pF tuning
capacitor should be used if the whole
of the broadcast band is to be covered.
It is interesting to note how few
turns there are on the reaction coil,
although the number can vary depending on where the transistor base
The ferrite rod antenna is easy to wind. The author used a
length of fibre tubing on which to wind the coils. Rubber
grommets hold the unit together and allow it to be
mounted on right-angle brackets secured to the baseboard. The wire diameter is 0.4mm, the rod diameter is
10mm and the reaction coil can be placed 2-3cm away
from the tuning coil (the exact location isn’t critical).
You can convert 8-ohm stereo headphones to 16-ohm mono by using the
tip and ring connections only. This
effectively connects the two 8-ohm
earpieces in series but note that the
they now operate in antiphase.
connection is placed on the tuning
coil. If the 10 or 15-turn taps are used,
there will be sufficient regeneration.
If the 5-turn tap is used there may
not be enough regeneration at the low
frequency end of the dial.
Increasing the value of the 100Ω
resistor or decreasing the 3.9kΩ resistor will increase the regeneration
response.
As the coil tap positions have a significant effect on the set’s selectivity
A rotary switch is used to select the desired antenna tap
and is mounted on the rear panel.
January 1998 51
This photo shows one of David Waldron’s mains-operated regenerative short
wave receivers. It drives a loudspeaker and is a good performer.
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52 Silicon Chip
and regeneration response, there may
be some need to experiment in this
regard. It’s all to do with the reception
conditions the set has to work in. In
some cases, a wave-trap may be used
to advantage to block out a strong
local station.
There is one aspect of Trans-1 that
needs to be brought to the attention
of would-be constructors and that is
the capacitor that couples the radio
frequency signal to the base of the
transistor.
In the circuit diagram this capacitor is shown to have a value of 1µF
or larger. This is important because
a value less than 1µF has an adverse
effect on tonal quality and will result
in a thin, raspy, unpleasant sound.
A small electrolytic will work OK in
this position.
Switchable taps
My set differs from David’s in that
I prefer switchable taps to wandering
leads and alligator clips.
On the back panel of my receiver
there is a 3-position switch in the
aerial circuit which connects the
aerial to either tap 2, 5 or 15, the
latter being used for short aerials. On
the front panel, a 2-position switch
connects the base of the transistor,
via the 1µF capacitor, to either tap 5
or 10. Constructors can do their own
thing regarding tap connections.
Building the Trans-1 is relatively
straightforward and does not require
detailed constructional information.
The circuit diagram, a few hints,
and the accompanying photographs
should be sufficient.
In conclusion, the good aspects of
Trans-1 are as follows: it is easy and
relatively cheap to build; it can be
built using mostly over-the-counter
parts; it works well on local stations
without an aerial or earth; it is neat
and compact; and it operates from a
single 9V battery.
The only unfavourable aspect is that
sound fidelity is not quite as good as
that from a similar valve receiver, particularly when receiving distant transmissions at maximum regeneration.
Although Trans-1 is based on modern components, the regeneration
circuit on which it is based dates back
to the early days of radio. It’s just a
SC
new version of an old idea.
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