4 Valve TRF Receiver.
On/Off Volume is at left, with radio/external input switch at right.
This set was built back in 1997 to determine the performance of a TRF set with only one RF amplifier.
I had seen from reading Wireless Weekly issues from the late 1920's to early 1930's that this was not an unusual arrangement in good signal
areas. Most TRF sets used a three or four gang tuning condenser, along with the same number of extra valves and coils. By omitting all but one
stage, a much cheaper receiver could be built. At this point I departed from the 1920's designs by using TV valves and an infinite impedance
detector. As with many of my projects I used TV valves here because they're common, cheap, and I have a lifetime supply of them.
The power supply was also novel, using a modern power transformer I had on hand.
RF Amplifier
A conventional tuned circuit formed by the aerial coil and one gang of the tuning condenser feeds the input of the RF amplifier valve in the usual
way. The valve is a 6BX6, or as Europeans would know it, EF80. This is an RF pentode developed in the late 1940's for use in TV IF amplifiers, but
like others ended up being used for many different applications.
As such, it is of fairly high gain. For a 7Mc/s channel bandwidth, three 6BX6's are required in a typical television IF stage. It can be imagined that
down on 1Mc/s with say 10Kc/s bandwidth, the gain will be very high indeed. To put things into perspective, the 6BX6/EF80 has a
transconductance of 7.4ma/V, whereas a typical RF pentode normally used in a MW set, such as a 6K7 has a transconductance of 1.5mA/V. Later
valves such as 6BA6 have around 4.4mA/V.
Your eyes might light up at the thought of replacing the IF and RF valves in your MW sets with TV IF pentodes in view of this tremendous increase
in gain. Imagine all the distant stations you'd be able to receive! Unfortunately, it doesn't work out this way. The gain is actually too high and due to
stray capacitances and other couplings you end up with an unstable whistling mess. So, when used for narrow band RF amplification on medium
waves, it is necessary to reduce the gain.
One obvious way to reduce gain is to leave the cathode resistor unbypassed. However, I still found the gain too high. By reducing the B+ supplying
this stage, stability and good performance was achieved. It may surprise those who can only understand valves working at 250V to find the RF
stage in this set working at 21V. The gain is still very high at this voltage, but not so much to cause instability. Not surprisingly, the 6BX6 is another
valve useful for low voltage designs.
The source of 21V was simply taken from the output valve cathode.
Detector
Back in the 20's and 30's, the detector would usually be an anode bend type, or if regeneration was added, a grid leak type. However, I decided to
use an infinite impedance detector instead. This type of detector appeared much later in the scene and was usually associated with hi-fi receivers
due to its very low distortion.
Anode bend detectors are essentially a sharp cut off pentode biassed almost to cut off. It is connected as a normal resistance coupled audio
amplifier, but with a higher than normal cathode resistor. Incoming RF will cause the valve to cut off on negative excursions, and conduct much
more heavily on positive excursions. Hence, it detects AM. This type of detector was standard in TRF and superhet receivers up until diode
detectors took their place in the mid 1930's, when valves such as 6H6,75, and 6B7 appeared. An advantage of this detector is that it does not load
the circuit feeding it as grid current never flows. This means good selectivity as the Q of the preceding tuned circuit is not reduced. It also can
handle large signals without overloading. However, it suffers from distortion as the detector valve is also functioning as an audio amplifier which
has too much negative bias.
Another common detector of the time, which is even older, is the grid leak circuit. It is still the standard today with regenerative receivers, and can
be used with just about any triode or pentode valve. Here, the grid and cathode form a diode. The grid leak condenser and resistor simply form the
diode load. Because the plate of the detector valve is bypassed at RF, only the audio component developed across the grid-cathode diode is
passed on. Such a detector must not be operated with any negative bias. To do so would desensitise the detector as the incoming RF voltage
would have to overcome the bias voltage before the diode could conduct. In fact, in many battery circuits, the grid resistor is actually returned to
the positive filament supply to do the opposite and make the detector more sensitive.
As a grid leak detector operates with no, or slightly positive bias, it is essential that excessive plate current cannot flow as the valve will be
damaged. Hence, such detectors operate at only 22.5 or 45V if feeding headphones or audio transformers, or through a resistor of 100K or more
when operating from higher voltages.
While this type of detector has good sensitivity, it does load the tuned circuit, thus reducing the Q and selectivity. When used in a regenerative
circuit these losses are made up and become irrelevant. Likewise, in a TRF set with no regeneration, a couple of preceding tuned stages will
improve this. The other disadvantage is that, as the valve is again also an audio amplifier, it is this time operated with no bias. So, the signal
handling is limited before it starts distorting. Grid leak detectors are thus best used with weak signals. A few grid leak detector circuits, such as this
are available elsewhere on this site.
Another kind of detector eliminates the loading and distortion problem. It is called the "Infinite Impedance Detector". The grid is never driven
positive and thus does not load the tuned circuit, allowing full selectivity and gain to be obtained. In terms of the audio signal, it is operating with
100% negative feedback due to the cathode follower configuration. As can be imagined, audio gain is not as high as the other types and needs to
be made up in successive stages.The type of valve used is not critical and most triodes can be used.
This kind of detector was often used in various TRF and wideband superhet circuits throughout the 1940's and 50's and was used in many Radio &
Hobbies circuits. In view of its advantages, this is the detector I chose for this receiver.
As can be seen, it uses another 6BX6/EF80. This time the valve is triode connected as it is operating as a cathode follower. The 100K and 270uuF
form the diode load, with additional filtering by the 47K and 100uuF. A line level signal is available at this point.
Terminals from left to right are: Aerial, Earth, Speaker. RCA socket is for external audio input. Note the shielding between the two stages. Trimmer capacitors
adjacent to the coils are for alignment at the top end of the band. The top cap of the 21A6 is live at about 180V.
Audio Stages
Yet another 6BX6 is used as the first audio amplifier and is used in the conventional way. I provided an audio input socket for use with a CD player
or FM converter as an added convenience.
For the output stage, things became unconventional as I'm using a 21A6 (PL81) line output pentode. This is not a hugely common valve in
Australia as series heater TV sets were in the vast minority. However, it was used in some Ekco models and a few others. The 6CJ6 (EL81) which
is the 6.3V heater version was used in a number of Astor models. However, I do have some in my collection, and as I was tied to using a 300mA
series heater circuit, it was a perfect choice. As I've discussed at length here, TV line output pentodes generally are a poor choice for single ended
audio, at least when used in conventional circuits. Not all are as bad as the 6CM5/EL36 in this application, and I found the 21A6 to provide a
couple of watts of good quality audio. In view of the heater current, it would be wasteful to use this circuit on a 6.3V heater supply with the 6CJ6
when much more efficient audio output valves are available.
The output transformer is an Altronics M1120 100V line transformer with the 4K primary tapping used. See here for more details on this.
Negative feedback with a slight amount of bass boost is achieved by the 820K and .001uF network. The RC network across the primary of the
output transformer is for this particular type and helps improve frequency response.
Power Supply
I have a number of Ferguson PL80/60VA transformers available. These are rated at 60VA with two 40V secondaries. I had often thought about how
they would go for powering a valve radio by connecting the secondaries in series and then following with a voltage doubling rectifier. For the
heaters, they would have to be in a series circuit powered from 40V. This receiver was the test for this, and it worked perfectly.
As it happened, I didn't need a dropping resistor as all the heater voltages added up to 40. It is because of the 6BX6's that the heater current had
to be 300mA. This meant the output valve also had to be 300mA and was one reason I chose the 21A6. I could have used a 16A8/PCL82 with a
dropping resistor instead. Note the order in which the heaters are connected. It is important to have the valves more susceptible to hum at the
earthy end of the heater string. While I could have used a full wave voltage doubler, it would have meant the whole heater chain floating at about
90V above earth which is not a good thing.
Once again, we see just how easy it is to use valves with modern transformers.
As with some of my other MW receivers, I've made the coil formers to plug into octal sockets. Power supply circuit is to the right, RF to the left, and audio at the
top.
Performance
Sensitivity is surprising given there's only one RF stage. I can easily get 2LT from Lithgow and 2BS from Bathurst in the day time, even though
their signals are directed away from where I live. All Sydney stations come in with good strength and there are no problems with selectivity. At
night, the usual interstate stations are receivable.
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