In many country areas of Australia and New Zealand, 240V AC
mains power didn’t become available until the 1950s. Before that, all sorts of
voltages were used in country towns, while those living on farms may not have
had any source of power other than batteries for their radios.
Where power did exist, voltages such as 12, 32, 50, 110, and
250V DC were common. Of course, some places had their own 110V or 240V AC
supplies, although in general these sources only covered a small area and were
rather limited in output, with frequent interruptions to the supply. Are you old
enough to remember having to pay the "electric light bill"? Before the war,
electricity was almost exclusively used for lighting with few or no power points
in the home, hence the bayonet adaptor that went into the light bulb socket so
that the radio could be powered.
This is the view inside the cabinet of the converted HMV 268 receiver. The conversion involved replacing the valve line-up and replacing the 6V vibrator circuitry with a mains-operated power supply.
To cater for areas where there was no mains supply, radio sets
were specifically designed to run off a 1.5V or 2V battery for the filaments and
90-135V dry cell batteries for the HT supply. Unfortunately, these were
expensive to operate relative to the cost of running sets off the 240V AC mains.
As a result, to keep costs down, many sets that used battery valves were
designed to operate from a 4V or 6V wet cell battery via a vibrator power
supply. The valve filaments were usually wired in a series-parallel
configuration to minimise current drain.
Suddenly, these sets became obsolete when mains reticulated
power came to an area and homes were connected to it. As a result, many old sets
were either stored in the garage or thrown onto the local garbage tip – vintage
radio collection and restoration was not even thought of in the 1940s, 50s and
60s. This was a shame because these sets were generally very good performers as
they were designed for rural areas where signals weren’t all that strong.
I hate seeing things that are still in good working order go to
waste and, along with many others during that era, converted many of those
otherwise obsolete sets to 240V AC mains operation. Of course, you wouldn’t do
that today as there are few of these receivers still around in original
condition. At that time however, it was much better to convert the sets rather
than have them go to the rubbish tip.
It’s worth noting that "Radio & Hobbies" (later "Radio, TV
& Hobbies") ran articles on converting many of the common types of sets used
in country areas to 240V AC mains operation. It was cheaper to convert than to
buy a new set and, what’s more, the conversion was usually very successful.
Often, a converted set worked better than before and was cheaper to run into the
Ease of conversion
Some sets were easily converted to mains operation, these being
the 32V sets with vibrator power supplies and using "mains-type" valves, eg,
6AQ5, etc. All that was necessary with this type of set was to remove the
vibrator power supply, replace it with a mains supply and rewire the heaters and
dial lamps for 6V operation. And because they were designed for remote country
areas, these sets usually outperformed the newer AC mains sets which invariably
lacked an RF stage (as used in the vibrator-powered designs).
The HMV 268 came in a stylish wood veneer cabinet and featured both shortwave and medium-wave AM bands.
Unfortunately, sets using "battery-type" valves were much more
difficult to convert. This applied regardless as to whether the set used
batteries to supply all the necessary voltages or whether it used a vibrator
power supply to derive the necessary voltages from one battery.
It really was much more of a challenge with the battery sets.
First, it was necessary to change all the valves and this involved finding out
which valves in the AC range had similar characteristics to the battery valves
being replaced. Second, AC valves usually work on higher supply voltages
(usually 200-250V), whereas the battery sets usually ran on 135V and some on
only 90V. This meant that many of the paper capacitors had to be replaced with
higher voltage types.
Third, quite a bit of redesign was necessary in order to obtain
good performance from the new valve line-ups. However, many servicemen in
country areas rose to the challenge and many fine conversions were made to
radios otherwise destined for the local rubbish tip.
Converting 32V sets that used only 32V of high tension was a
challenge too. That’s because the valves, although AC types, run at very low
voltage and have low gain. For starters, it was necessary to replace the 25L6
valves with 6V6GTs or similar but because of the large increase in gain with the
increased supply voltage, considerable redesign was necessary – even to the
point of removing some stages.
The RF and IF sections were usually left running off 30-45V
which meant that no modifications were necessary to their operating conditions.
This usually achieved a satisfactory result – after all it was only the audio
section that needed beefing up.
Converting an HMV 268 6V vibrator Receiver to 240V AC
My parents owned an HMV 268 dual-wave table model, a 6V
vibrator receiver using five 2V battery valves. The circuit is shown on page 193
of Volume 7 of the Australian Official Radio Service Manual.
In its original format, the old HMV 268 was an excellent set.
The shortwave band extended from 6-18MHz but where I lived, the local emergency
fire service used a frequency of 2.836MHz and we had no radio capable of
listening to important fire calls.
This end view shows the location of the "new" power transformer and audio output valve. The new supply was much simpler than the vibrator supply it replaced.
As a young and relatively new devotee to radio, I decided that
I would modify the shortwave coils so that it tuned from around 1.7-5MHz, so
that the fire calls and amateurs on the 1.8MHz and 3.5MHz bands could be heard.
The receiver was duly modified and I had a lot of fun listening to these
stations – when my parents weren’t using the set of course.
Unfortunately, recharging the 6V battery that powered the
receiver was something of a chore, requiring a trip to the local garage.
However, my parents had a smart, young son who reckoned he could save them the
trouble of this ritual.
I decided that I could charge the unit directly from our 32V
lighting plant by putting two 12V globes in series with the battery. The total
calculated voltage added up to 30V, so the globes weren’t going to be
drastically overloaded. What’s more, by only charging the battery at night, the
two globes would become part of our home lighting system – waste not want
I found out after I had installed the system that it was very
effective, provided the 32V batteries were off charge. However, it was a
different story when they were on charge, the 12V globes glowing brilliantly for
a short while until they blew!
The real test of my radio prowess came at the end of the 1950s,
when we got 240V AC power after having had 32V DC for about 15 years. The set
would either have to be converted to 240V AC operation or thrown out, as we no
longer had a ready source to charge the battery. Fortunately, there was an
article on converting receivers to mains operation in the December 1953 issue of
"Radio & Hobbies" and this steered me in the right direction.
Using the article as a guide, I started by checking out which
AC valves had similar characteristics to those that were being replaced. I
decided on a 12AH8 converter to replace the 1C7G but a 6J8G or a 6K8G may have
been a better choice, as I wouldn’t have needed to change the valve socket. In
addition, a couple of 6K7GT valves were wired in place of the 1M5G and 1K7G
valves in the IF amplifier, as their mutual conductance is similar to the valves
they replaced and I didn’t want any problems with instability.
A 6B6G was used in place of a 1K7G for the second detector and
first audio stage, the gain of a pentode being considered unnecessary in this
position as the overall gain of the set would be higher with AC valves anyway.
The audio output stage became a 6AM5 instead of a 1L5G. I would have liked to
have used an octal output valve but I didn’t have one with a similar output
impedance and the 6AM5 nearly matched the 1L5G.
It was then necessary to look at the voltage ratings of the
capacitors. The set ran on 135V but now it would run on about 250V. Most of the
capacitors had a 200V rating and were replaced with 400V units where
At this stage, the 6V vibrator power supply was taken out of
the set and consigned to the junk box. A metal sheet was then bolted across
where the vibrator supply had been and a power transformer and a selenium block
rectifier fitted in its place. The electrolytic filter capacitors were wired
into position under the chassis.
The retrofitted power supply can be seen in the photographs.
The new AC supply was certainly much simpler than the vibrator supply it
Wiring the heaters of the new valves was straightforward, since
it was no longer necessary to use a series-parallel arrangement. However, it was
necessary to fit a resistor and capacitor between each cathode and earth to give
the bias required and rewire the valve sockets to suit the new valves.
In my enthusiasm to stabilise the screen voltages, I also wired
in a VR105 105V gaseous regulator. This was really an overkill and quite
unnecessary (at that time, I wasn’t as competent as I thought I was).
Anyway, it all worked reasonably well and the old HMV once
again took pride of place in the lounge room. Eventually, my sister took
possession of it and it continued to work satisfactorily until a brush with
lightning caused the shortwave aerial coil to go open circuit. After that, she
didn’t want it any more so I got custody of it and decided to get it operating
Being more knowledgeable now than I was then, I soon found a
few problems with my original conversion which caused the set to be slightly
unstable. After some investigation, I found that the automatic gain control
(AGC) line was radiating a signal at the intermediate frequency (IF) and this
was being picked up by the IF front end – hence the instability.
Don’t assume that the AGC line is always "cold" with no signals
on it –some have quite a lot of IF signal on them. The original valves in the
old HMV didn’t have as much gain as their replacements, so this problem didn’t
occur with the original circuit.
Carefully re-routing the AGC lead and adding some extra
bypassing solved the instability problem and the set now goes extremely well. It
is one of the favourites in my collection and has quite a lot of sentimental
Converting an AWA 532MF 32V receiver to 240V AC
A number of these radios were going to be thrown out as the
reticulated power mains snaked around the country area in which I lived. These
sets used a 6BA6 RF amplifier, a 6BE6 converter, a 6BA6 in the IF stage, a 6AV6
detector and a 6AQ5 audio output stage. They also used a synchronous vibrator
power supply which ran from a 32V DC supply.
This valve line-up is the same as used in many high-performance
AC sets, so they were well worth converting. And the conversion was even simpler
than for the HMV 6V vibrator set described above.
In brief, the vibrator power supply was removed from the set
and the dial lamps and valves heaters all wired in parallel to run off 6.3V. A
power transformer was also installed and solid-state diodes used to rectify the
high-tension voltages. The electrolytic capacitors were reused since they were
quite adequate for the job.
This "under-chassis" view of the converted HMV 268 shows the wiring layout. The electrolytic capacitors for the new power supply are at left, adjacent to the socket for the audio output valve.
These sets and similar 32V sets that had AC valves and a
vibrator supply were very easy to convert and the sets performed better than
before. That’s because there was no longer any residual vibrator hash.
Should we convert sets now?
My personal belief is no, we shouldn’t convert any more
vibrator sets to mains operation. There are several reasons for this: these
radios are now quite scarce, they are a part of our radio heritage and they are
interesting receivers in their own right.
Some collectors wrongly believe that these battery or
vibrator-powered radios are useless because there is no easy way of powering
them. However, suitable AC-operated power supplies are available to operate
these sets and occasional advertisements can be seen in electronics magazines.
Articles on making your own power supplies have featured in the magazines too,
so there is no reason why these radios cannot be made fully operational.
So why did I do conversions on these sets if I now believe that
they shouldn’t be done? Well, it was a different era and the conversions were
done to save good high-performance sets from the rubbish tip during the period
that the 240V reticulated mains spread throughout the countryside. In the period
from the mid-50s into the early 60s, these conversions were commonplace and made
good economic sense.
And even though those converted receivers are no longer
‘standard’ they are an example of what happened in that era. It was a short but
interesting period in the history of radio in Australia.