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VINTAGE RADIO
By JOHN HILL
How good are TRF receivers?
In the early days of broadcasting, the TRF or
tuned radio frequency receiver reigned supreme.
Although there were odd superhets around from
about 1924 onwards, they did not become really
popular until a decade later.
The reluctance of buyers to go the
way of the superhet has always puzzled me as there is little doubt that
the superhet was by far the better
receiver. But price often dictates terms
and it was perhaps for this reason
that the TRF remained popular for so
long. Another factor may have been
that selectivity – the superhet’s main
claim to fame – was less important
This stylish looking Radiola 45E
console is a 5-valve TRF receiver of
1930 vintage. Most 5-valve TRFs were
reasonably selective because of their
three tuned circuits but the cheaper
4-valve types had selectivity problems.
86 Silicon Chip
while there were only a few stations
on the air. As the number of stations
in
creased, better selectivity became
more and more important.
Nevertheless, in recent months I
have restored a number of TRF receivers and, as a result, I have come to look
upon them more favourably than I had
in the past. Compared to superhets of
the same era, some TRFs were very
good receivers – and still are!
Back in those distant days of the
early 1930s, the TRF receiver had
reached the peak of its development,
whereas the superhet was still in the
developmental stage.
Those early superhet designs were
unduly complex and expensive, and
there were problems with double
spotting and the choice of a suitable
IF. It also needed an extra valve for
the local oscillator which, according
to superhet opponents, “didn’t do anything”. This initial criticism created
a marketing problem and it wasn’t
until the mid to late 1930s that an
acceptable design compromise was
reached and the superhet came into
its own.
Let’s take a look at some of those old
TRF receivers and try to ascertain just
how good (or bad) they really were.
The TRF receiver
A TRF receiver must have at least
one stage of radio frequency amplification ahead of the detector, typically
a leaky grid or anode bend type. Those
two stages alone constitute a TRF re-
ceiver and a 2-valve set of this type is
practical although it would be suitable
for headphone use only.
However, such a simple receiver can
be greatly improved on. More valves
and tuned circuits can be added to
the front end to increase amplification
and selectivity, while extra valves can
be added after the detector to give increased amplification and more power
output for the audio signal.
These additions have their limitations, however, and three RF stages
and three AF stages was about as far as
most manufacturers were prepared to
go. Exceeding these limits could lead
to instability unless special precautions were followed.
Some of the cheaper TRFs had
Another 5-valve TRF receiver. This
set is typical of many early 1930s
receivers that were made for a price.
While the front looks good with its
attractive walnut veneers, the sides
were just very plain plywood.
A 4-valve “el-cheapo” TRF receiver.
This unit has been left unrestored and
does not inspire much enthusiasm.
It lacks aesthetic appeal and its
performance is poor to say the least.
only 4-valves, including the rectifier.
With just two tuned circuits, these
simple budget-priced receivers were
not very selective or sensitive. They
did not perform as well as a 4-valve
receiver with a regenerative detector,
for example.
Regeneration was, however, incorporated into some of the low priced
TRFs which was perhaps a mixed
blessing in a radio of this type. Positive feedback (regeneration) improves
both sensitivity and selectivity quite
dramatically but it can also introduce
distortion and alter the tuning of the
detector stage. Most TRF receivers did
not use regeneration.
still another tuned circuit.
In fact, up to five tuned stages were
used in a few of the really up-market
receivers such as some of the American Majestics. Short wiring and well
shielded stages allowed such receivers to be quite stable. They were very
selective, extremely powerful and
boasted a huge complement of valves.
They also had a loudspeaker that
could handle the power. The speaker alone in an old Majestic receiver
weighs close to 10kg and the fully
assembled sets were big and heavy
to say the least.
Perhaps one problem with some
early TRF receivers was the fact that
the ganged tuning capacitors used in
the late 1920s and early 1930s were
not manufactured to the precision
standards that were to follow in later
years. The same can be said for the RF
coils used in these receivers.
Component variations like this
make perfect multi-stage alignment
a difficult, if not impossible, process
because, unless the tuned stages track
together in near perfect unison, the
set’s performance will be only mediocre. TRF receivers need to be well
aligned.
Valve limitations
TRF receivers were first developed
in the days when the triode valve was
the only type available. However, there
are two distinct disadvantages when
using triodes as RF amplifiers.
First, a triode valve does not have
This mediocre 4-valve TRF receiver
at least looks a bit different from the
usual console. Actually, this Radiola
34E is a large table model that was
sold with optional legs (circa 1931).
a very high amplification factor and
many valves are needed if high gain
is to be obtained. Second, the internal
capacitance between the grid and
plate of a triode valve provides an
unwanted positive RF feedback path
between the plate circuit and the grid
circuit. In an RF amplifier stage, with
the plate circuit and the grid circuit
The 5-valve TRF
The standard 5-valve TRF was
a better compromise, as it allowed
three tuned circuits which gave more
selective tuning. Even so, if such a
set is operated in close proximity to a
strong local broadcasting station, then
that station will occupy a considerable portion of the dial. This clearly
indicates the lack of selectivity of the
basic TRF design.
However, where the average TRF
had a 3-gang tuning capacitor and limited selectivity, some of the better sets
had 4-gang capacitors which added
This 8-valve Apex receiver with its push-pull output stage performs rather
well for an old TRF. Many budget-priced TRF receivers from the late 1920s
were housed in pressed steel cabinets.
November 1995 87
TRF receivers were at their peak when this unit was made. With its three
24As and 47 output pentode, it is quite a reasonable radio set. The chassis
cleaned up quite well.
both tuned to the same frequency,
this feedback will cause instability,
whereby the set bursts into uncontrollable oscillation.
The triode’s feedback problem was
overcome by a process known as neutralization and receivers using this
technique were known as “Neutro
dynes”, a registered trade name at the
time. Neutrodynes have very stable
RF amplifiers when the neutralizing
capacitors are correctly adjusted.
Unfortunately, the adjustment can be
quite critical.
The RF tetrode
Neutralizing suddenly became
history with the advent of the radio
frequency tetrode, or screen grid
valve. The tetrode valve’s screen grid,
between the control grid and the plate,
eliminated the positive feedback problem of the old triode.
The screen grid valve had another
advantage apart from better RF stability. It had a much higher amplification
factor than the triode and this provided
a significant boost to the performance
of TRF receivers using screen grid
valves.
Speaking from my own experience,
88 Silicon Chip
I believe that a tetrode TRF with two
RF stages is roughly equivalent, in
gain, to a triode TRF with three RF
stages.
The last of the TRFs went one better
and used the first generation radio frequency pentodes. A TRF using these
valves and using diode detection and
automatic gain control could be quite
an interesting receiver – if such a thing
actually exists.
(It most unlikely that such a commercial set was ever made, if only because there was no real mass demand.
There was also a technical problem
in that the tuned circuit feeding the
detector had to be earthy on one side,
which does not suit a conventional
diode detector circuit. This problem
could be overcome, with some difficulty, and home construction designs
were published. Ed.)
All of the mains-powered TRF
receivers I have encountered use
American-designed valves. The triodes are nearly always type 27, while
the tetrode types have been 24, 24A
and 35. No doubt there are a lot of
sets around with other valves in them
(26s for example, as well as European
types) but the majority are these old
faithfuls from the early AC era.
Speaking of old faithfuls, the output
valves seem to be either 71As, 45s or
47s. These old warriors are direct heated types with a rather heavy filament
for thermal stability.
Many of the better TRF receivers
had twin output valves in push-pull.
Such a setup can produce quite a few
watts of output power and a set of this
type can sound surprisingly healthy
for such an ancient radio receiver. I
have a 1929 Apex, an 8-valve set with
two 45s in push-pull, and it really can
make that speaker cone rattle back
and forth.
Interestingly, only a few years earlier, around 1926-27, nearly all radio
receivers were battery powered with
outputs that were considerably less
than half a watt. What was the latest
thing in 1927 was completely obsolete
by 1930. The radio scene changed
rapidly during that period.
Collecting TRF receivers
From a collector’s point of view,
any TRF receiver is a good find but
they are few and far between. As far
as mains-powered receivers are concerned, I have found only two in 10
years of collecting. On the other hand,
I know a collector who has located
about 10 in the past 12 months, so I
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This old Apex chassis has an impressive line-up of valves. The rectifier is out of
sight behind the transformer cover. The old style valves really look the part on
this chassis.
guess I must be looking in the wrong
places.
I do have a few battery-powered
TRFs which I have yet to restore. Some
are multi-dial types in which the tuning capacitors are not ganged but are
individually controlled by separate
dials. There is also another old battery operated Neutrodyne in the shed
which has single knob tuning and it
should make an interesting story one
day. All I need is a little more time!
Radio collectors are a funny lot
with some specialising in receivers
of specific types. Personally, I like
to diversify and have a little bit of
everything and that includes a few
TRF receivers to maintain some kind
of balance in my collection. As I stated
earlier, I have come to look upon them
more favourably than I had previously.
So how good were those early TRF
radios of the pre-superhet days? Well,
they varied from poor to very good,
with several categories in between.
Then, as now, price dictated the
quality of an item and if you paid out
enough of that crinkly folding stuff,
then you bought yourself a good radio
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November 1995 89
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