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VINTAGE RADIO
By JOHN HILL
Anode bend to diode detection
During the early to mid-1930s era, the low
priced 5-valve superhet console radio was very
popular. Many employed anode bend detection
but they can be easily converted to diode
detection for improved audio performance.
The early ’30s were the tough times
of the Great Depression years, when
about 25% of the workforce was out
of work. And, of course, they were
without the back-up support that the
unemployed have today. This meant
that any radio manufacturer who wanted to stay in business had to produce a
range of receivers that were affordable.
The formula, in most cases, was to
keep things fairly basic.
The usual format for these cheaper
radios was the autodyne superhet – a
5-valve receiver with an autodyne mixer, IF stage, anode bend detector and
a single output stage. Some designs
used a 175kHz IF and this necessitated a pre-selector stage, using a 3-gang
Fig.1: the circuit of a typical anode bend detector. The
valve type shown is a 24A tetrode or similar sharp
cutoff tetrode. Later circuits used a type 57 pentode,
although the basic arrangement remained the same.
Fig.2: this is what the circuit looked like after conversion to diode
detection. The original type 24A tetrode was replaced by a type 27
triode valve.
86 Silicon Chip
tuning capacitor, in order to control
the double spot problem created by
of such a low value.
Other makers chose a 455kHz IF,
which was rapidly gaining popularity, and which solved the double spot
problem automatically. This allowed
the use of a cheaper 2-gang capacitor.
Either way, this broad design concept was a compromise between price
and quality and while these sets work
ed reasonably well, they had several
disadvantages.
Design drawbacks
One problem was the lack of automatic gain control. This circuit innovation came into existence in the early
1930s but was only found on the more
up-market receivers.
Another difficulty with the auto
dyne setup was that, while it worked
OK on broadcast band frequencies,
its performance on short
wave was
not so good.
And finally, the anode bend detector
used in these sets created a level of
audio distortion that left something
to be desired. While this distortion
may have been acceptable in the 1930s,
by today’s standards it is not very good
and can be quite distracting. It may
have been fairly distracting in the
1930s too, because by the middle of
that decade most manufacturers had
changed to diode detection.
Some of those old receivers with
anode bend detection sound better
than others and in many instances
the loudspeaker must play a part. The
moving coil loudspeaker had been in
existence for only a few years at that
stage of radio development and there
were still things to learn and manufacturing techniques to master. While
an early ’30s moving coil loudspeaker
was a remarkable improvement on
a ’20s horn speaker, there was still
quite a lot of developmental work
ahead of it.
Basic circuit
Fig.1 shows the circuit of a typical
anode bend detector. The valve type
shown is a 24A or similar sharp cutoff
tetrode. When the type 57 valve (a pentode) was developed, it replaced the
radio frequency tetrode, although the
circuit arrangements for anode bend
detection were still the same.
The main aspect of the anode bend
detection method is the very high
cathode bias resistor, which operates
the valve at close to cutoff. The term
“cutoff” simply means that the anode
or plate current will be at or near zero
when no signal is being received.
When a modulated radio frequency
(RF) signal is applied to the control
grid, there will be pulses of anode
current during the positive half cycles and little or no anode current
during the negative half cycles.
Therefore, the anode current is a rectified version of the signal waveform
at the grid.
Filtering of the RF component after
detection is achieved by a small plate
bypass capacitor (typically around
250pF) to chassis and an RF choke in
series with the plate load.
Anode bend detection has some
odd characteristics and the distortion it produces can be minimised
by varying the value of the cathode
bias resistor.
However, if the cathode bias is
selected to give good low distortion
sound with a strong signal at the
control grid, then the performance is
not as good on a weak signal and vice
versa. So, after much experimenting,
the cathode circuit is often returned
to its original form, as the manufacturer’s setup was probably a reasonable
compromise.
Detector conversion
I have quite a number of old auto
dyne/anode bend console radios and
I find some of them quite irritating
due to their high levels of distortion.
There are times I like to listen to my
radios for hours on end and if they
sound crook, there is no listening
pleasure at all.
The last of these receivers to come
off the restoration assembly line was
an old 1932 Darelle (see June 1995).
While the Darelle was no more annoying to listen to than any of the
Shown here is the Darelle 5-valve superhet cabinet. It is affectionately known
as the “tea chest on legs”. The Darelle’s chassis was converted from anode bend
detection to diode detection and this simple modification gave a significant
improvement in sound quality.
others, it was the one I selected to see
if the sound reproduction could be improved by converting the set to diode
detection. The experiment produced a
surprisingly good result, so allow me
to fill you in on the details.
There are several choices when it
comes to converting a set to diode
detection. One can use either a valve
with diodes in it, a triode connected
as a diode, or do the unforgivable and
use a germanium signal diode.
As the old Darelle used tetrode
valves, there was no applicable diode
type valve apart from the 55 duo-diode
triode. The use of this valve would
require a valve socket change from
5-pin to 6-pin.
Using a triode connected as a diode
was not an option either because there
was insufficient room to accommodate
it. So that left the unthinkable – a germanium signal diode.
Not being a modern electronics man,
I was not really sure how to incorporate a solid state diode into a valve
circuit. I mentioned what I planned to
do to young David (a collector friend)
and he drew up a circuit of what he
thought I needed to make a solid state
diode detector work in a valve receiver
– see Fig.2. I might add that David’s
January 1996 87
24A audio amplifier. It was tried as a
tetrode, a triode, with high and low
plate voltages, and with a variety of
cathode bias setups. None proved to be
really satisfactory, although the triode
connection wasn’t too bad except for
a drop in overall volume. It had to
be considered unsatisfactory for that
reason alone.
Valve replacement
The original anode bend detector valve was a 24A, as shown at left. This was
replaced with a 27 triode (right) and this worked well as an audio amplifier,
something that the 24A could not do.
circuit was a little more involved than
what I had in mind.
Another aspect of my conversion
was to retain the existing 24A anode
bend detector valve and use it as an
audio amplifier – if that was at all
possible. David was not confident that
this could be done but as I wanted
to keep the original valve line-up, I
would try to do it anyway. Whether
or not it would be successful was in
doubt at that stage.
Many radio frequency valves (the 57
and the 6J7 for example) can be used
as audio valves when connected as
either pentodes or triodes. Hopefully,
the 24A would perform likewise, although there is no mention of audio
frequency application in the valve
manual. (Editorial comment: the 24A,
being a tetrode – as distinct from the
above mentioned pentodes – is less
suitable for use as a resistance/capacitor coupled audio amplifier. When
it was used as an audio amplifier, it
was usually in the choke/capacitor
coupling mode. This permits a much
greater plate voltage signal swing
without distortion).
The detector circuit was made up
on a small piece of tagstrip to form a
compact detector module (see photo). This module was then bolted to
a convenient part of the chassis and
wired to the second IF transformer
and the control grid of what was the
anode bend detector. But while the set
worked, one could not say that it was
working well.
Actually, the sound quality was
really good at moderate volume levels, but distorted badly as the volume
increased.
Various alterations were made to the
The diode detector
module was built from
miscellaneous compon
ents mounted on a
tagstrip. The small size
of the module allows it
to be mounted in some
out-of-sight location if
so desired.
88 Silicon Chip
It was time to do what should have
been done in the first place and that
is fit a valve that was more suitable
for audio frequency work than a 24A.
A 27 was a logical choice as its 5-pin
base was compatible with the existing
valve socket. Rewiring the socket to
suit the triode valve required a couple
of alterations, as the 27 has no top-cap
grid connection.
After fitting the 27, all the previous
problems associated with the diode
detection modification suddenly
disappeared. Triode audio amplifiers
were all the go in the early 1930s
and a triode also proved to be most
successful with this particular circuit
arrangement.
Once everything was working OK,
it was time to experiment a little. The
detection module was disconnected
and another signal diode substituted.
This setup used no grid leak, no coupling capacitor or anything else – just
the diode between the IF transformer
and the grid of the valve. It made little
difference apart from an ever so slight
increase in volume.
So it would appear as though there
are many ways to incorporate a signal
diode into a valve circuit – and they
will all probably work. However, if
one decides to do this modification,
remember that the second IF transformer will require realignment. That
would be about the only inconvenience incurred.
After this little experiment, the original detector module was reconnected
into the circuit.
Practicality vs originality
No doubt some readers will have
difficulty in understanding why I
would want to modify an existing
circuit and ruin the set’s originality!
Well, in this case, I want to listen to
the radio and not be annoyed by it. It
is as simple as that! What’s more, if a
receiver can be significantly improved
by implementing such a simple modification, then why not do it? In this
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The missing top-cap and connector may look a bit odd but so be it! Removing
the anode bend detector and replacing it with diode detection was an
experiment that paid off with a cleaner audio output.
instance, the improvement was well
worth the effort.
Should a future owner wish to
convert the receiver back to original,
it can easily be returned to its anode
bend state. Why someone would want
to do this I don’t know, but if they
did, they may not be happy with the
distortion that this detection method
produces.
The diode detector described
here can be a completely invisible
modification if so desired. Although
I chose to mount the diode and accompanying components on a small
tag strip underneath the chassis, there
is no reason why it cannot be housed
inside the second IF transformer
shield can or positioned in some
other out-of-the-way place where it
is out of sight.
As far as I’m concerned, if everything
looks OK then that’s all that matters.
A few devious modifications here
and there don’t upset me in the least,
especially if they improve the set’s
performance. The fact that the old
Darelle sounds a bit better than most
radios from that era must be worth
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January 1996 89
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