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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
The Diason P.P. 32/6 DC receiver
Designed for rural communities, the Diason
P.P.32/6 employed six valves and ran off 32V DC.
It was a good performer with some interesting
design features.
Many will not know the Diason
brand, as Diason Radio was a small
family company and was not a major
player in the 1950s domestic marketplace. They produced a couple of
portable receiv
ers, one exclusively
battery operated and another a battery/
AC model in 1950. In addition, they
produced three DC-powered receivers
– the 32/6, 50/6 and 110/6 – in the
same year.
80 Silicon Chip
The 32/6 and 50/6 were virtually
identical except for the way the heaters
were wired in these 32V and 50V DC
receivers. The 110V (model 110/6)
receiver used the same heater wiring
circuitry as the 50/6 but employed
12V heater series valves instead of
the 6V series. For example, a 12SK7
was used in lieu of a 6SK7, while the
audio output valve was a 50L6 instead
of a 25L6. The high tension (HT) to the
plates and screens came directly from
the supply voltage, as was common
with some Astor 32V receivers.
Just how well these near identical
receivers worked would be interesting
to know. The gain of each stage with
32V HT would not be very high, so
even with a radio frequency (RF) stage
and two intermediate frequency (IF)
stages, the set would have been quite
stable despite the minimal use of decoupling throughout the set.
With higher voltages, the gain
would have increased and I suspect
that the higher voltage sets could be
quite unstable. Additionally, the bias
on the 25L6 (50L6 in 110V set) was
only -1.5V, as supplied by a dry cell.
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Fig.1: the Diason P.P. 32/6 is a 6-valve receiver that runs off 32V DC. The
first stage of the 6SL7 operates as an audio amplifier, while the second
stage functions as a phase splitter. This in turn feeds two 35L6 valves
operating as a push-pull audio output stage.
On 32V, this would have been quite
adequate but with 110V on the plate
and screen of the 50L6, the bias should
have been -7.5V.
In the standard format that is shown
in the Australian Official Radio Service Manual (AORSM) Volume 9 for
1950, I believe the higher voltage sets
would have been absolute disasters.
For this reason, I suspect that either
no 50V or 110V sets were produced or
if they were, the circuit was altered to
overcome the problems of the higher
voltage supply.
A worthy successor
The successor to the 32/6 (50/6 and
110/6) receiver was the interesting 32V
DC-operated P.P.32/6 model of 1953.
This set overcame the very real faults
that were evident in the previous models. Some time ago, I had an opportunity to examine one of these later sets,
which belonged to Ralph Robertson,
a fellow member of the Vintage Radio
Club in northeast Victoria.
It is noticeably better in design than
the earlier models and there is no sign
of instability – just good performance.
It uses a 6SK7 RF stage, a 6A8G converter and a 6AR7 IF amplifier and
detector. This then drives a 6SL7 audio
amplifier and phase splitter, which in
turn feeds two 35L6 valves operating
as a push-pull audio output stage.
This general circuit layout is inherently quite stable with a low HT
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The chassis is easily removed from the cabinet for service, although the loudspeaker also has to be removed since its leads are too short.
supply, as the gain of each individual
stage is relatively low. There is some
gain at RF frequencies, some at the
converter and some at the IF frequency.
However, the receiver does not have a
lot of gain within any one frequency
range, although the total gain throughout the receiver when the gain of all
stages is totalled up is quite significant.
The low-level audio stages have
quite reasonable gain and feed the
push-pull 35L6 valves. A single
class-A 35L6 would be struggling to
provide enough audio output to satisfy
listeners, since the output would only
be in the order of 100-150mW. With
two push-pull valves, the audio output
is more than doubled.
Radio Corporation’s Astor receivers
also used two 25L6 valves (similar to
the 35L6) operating in push-pull in
February 2002 81
This under-chassis view shows just how few components there are. This made
for a reliable set that’s very easy to work on.
the audio output stages of their 32V
HT receivers.
Restoring the PP 32/6
When Ralph obtained his radio, it
was in rather a sorry state. The 6A8G
and 6SL7 valves were missing and
the loudspeaker had been damaged
when someone had carelessly put the
chassis back in the cabinet, putting
the volume control spindle through
the cone in the process. Admittedly,
the set’s physical layout is at fault in
allowing this to occur.
The loudspeaker is attached to the
cabinet and its leads are too short to
allow the chassis to be withdrawn
easily. Additionally, it is not practical
to work on the set with the speaker in
the cabinet. The leads can be extended or the speaker removed from the
set to allow service as shown in the
accompanying photograph (extending
the leads is my preference).
After removing the chassis, both
it and the inside of the cabinet were
cleaned. Fortunately, this was not a
particularly difficult job as there had
been no rodents as house guests.
Inspection of the underside of
the chassis didn’t reveal any paper
capacitors sufficiently leaky to warrant replacement. They must have
been a good batch – either that or the
low HT doesn’t stress the capacitors
as much as in a set with higher HT
82 Silicon Chip
voltages. And of course, there would
not be much heat generated under the
chassis.
Even so, I would still replace the
most critical capacitors, these being
C9 and C13 in this set.
The twin power lead was fitted with
a 2-pin polarised plug. It’s interesting
to note that many DC sets were fitted
initially with the normal 3-pin mains
plug and, as a result, many 32V sets
have been plugged into the 240V AC
mains with disastrous results. People
forget that voltages other than 240V
AC were often used in earlier times.
Typically, the owners of secondhand shops, eager to see if the sets
work, plug them into the 240V AC.
Hopefully, a fuse blows and no further
damage is done.
No other components proved defective in Ralph’s set so it was time to
give it a test run. It was connected to
a 32V DC supply but it soon became
evident that the set was not well.
Checking revealed that one 35L6 had
no emission and a replacement soon
had the audio section up and running.
The voltages around the receiver
were then checked and found to be
in order. So what voltages do you expect in a radio with a 32V HT supply?
The plates and screens of all valves
other than the 6SL7 should be 32V;
the cathodes of the 6SK7 and 6A8G
should be about 1V; and the cathodes
of the 35L6 valves should be around
1.5V with respect to chassis. Pin 2 of
the 6SL7 should be around 15-20V, pin
6 about 8V and pin 5 approximately
24V. There are no high voltages here!
Aligning the receiver
The alignment procedure for this
type of set is quite straightforward.
First, the IF is aligned with a signal
from a generator on 455kHz. A digital multimeter (DMM) set to the 20V
DC range and connected across the
volume control or C9 will read higher
voltages as the IF alignment is peaked.
The oscillator, RF and aerial coil cores
are adjusted for peak output at around
600kHz and the trimmers (not shown
on the circuit diagram) are adjusted
at around 1450 kHz or, in each case,
on a known station near to these
frequencies. Stations 3RN (3AR) on
621kHz and 2QN on 1521kHz can be
used where I live.
One problem with the alignment
is that the dial scale remains in the
cabinet when the chassis is removed.
In cases like that, I tend to close the
gang and adjust the oscillator so that
the receiver will tune 530kHz and then
adjust it at the other end of the dial so
that it will tune to 1620kHz. The dial
scale itself will give you some idea of
what the tuning range actually is.
To get the oscillator tracking accurately across the band, it will be
necessary to put the chassis in and
take it out of the cabinet a few times
while doing the adjustments – a tediwww.siliconchip.com.au
There’s plenty of room for the chassis inside the cabinet. In this case, the twin
power lead was fitted with a 2-pin polarised plug but many early DC sets were
initially fitted with a 3-pin mains plug. As a result, many 32V sets have been
plugged into the 240V AC mains with disastrous results.
ous job. The aerial and RF circuits can
be adjusted out of the cabinet as their
adjustments do not affect the accuracy
of the oscillator tuning.
By the way, the latter job is made
more difficult because the dial pointer
is about 15mm behind the dial scale
so there can be considerable parallax
error.
Having completed the restoration,
the set proved to be quite a good performer and is well-suited for use in
country areas, away from broadcast
stations.
Shortcomings and features
The set is designed to have the negative side of the 32V supply attached
to the chassis. However, the fuse is in
the negative line so, under some circumstances, the fuse could blow and
the set would still be supplied via the
set’s earth system (assuming that one
side of the 32V supply was earthed). It
would have been a better idea to have
placed the fuse in the positive line.
Having said that, one interesting
feature is the use of C1. This capacitor would prevent “burn out” of the
aerial coil if the antenna fell across the
live +32V line from the lighting plant
(most 32V supplies on farms used bare
overhead cables).
It is odd that incorrect component
values are often used in receivers. The
series dropping resistor for the dial
lamps (R15) is 100Ω but for use on
32V, this should be 127Ω, otherwise
the dials lamps will each have more
than 6.3V across them. What’s more,
K&W HEATSINK EXTRUSION. SEE OUR WEBSITE FOR
THE COMPLETE OFF THE SHELF RANGE.
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February 2002 83
Photo Gallery: Astor Model JN
Manufactured by Radio Corporation (Melbourne) in 1948, the Astor Model
JN is a 6-valve dual-wave superhet housed in a substantial Bakelite cabinet and fitted with an 8-inch (200mm) loudspeaker. The combination of the
large loudspeaker and an effective loudness compensation circuit around
the volume control resulted in good audio performance, for which Astor
sets were renowned. The set was fitted with the following valves: 6U7-G
RF amplifier; 6J8-G frequency changer; 6U7-G IF amplifier; 6B6-G first
audio/detector/AVC rectifier; 6V6-G output; and 5Y3-F rectifier. (Photo and
information courtesy of Historical Radio Society of Australia).
a 32V set of batteries on charge can
reach 40V, so even 127Ω is too low .
In fact, R15 should be around 180Ω.
And although a 20W resistor is used,
a 10W unit would be quite adequate
for the job.
Conversely, R16 should be lower at
around 23Ω for use off 32V. However,
most better quality 32V sets used a
3-position power switch instead of
the 2-position switch used in this set.
The first position was “off, while the
second position was “charge”. In the
latter case, a resistor was switched
in series with the supply to drop the
voltage to the valve filaments to around
32V when the batteries were on charge.
However, with the set connected to
a set of charging batteries it doesn’t
matter if the valve plates and screens
do get 40V – they’ll just perform more
effectively.
The third position of the power
switch was “on”, where the 32V was
connected directly to the valves.
There are no trimmer capacitors
shown on the circuit but they are fitted
nonetheless. The fixed capacitors inside the IF transformers are not shown
either and neither are the adjustment
cores for the IF transformers or the
front-end coils.
Another oddity is that the circuit
shows the set as having one 35L6 valve
in the output and one 35LG. The latter
is simply a drafting error – the valve is
in reality a 35L6. The valves probably
all had the suffix GT.
At this stage, the cabinet has not
been restored although it has been
cleaned. The cabinet itself is quite
light, being made of relatively thin
veneered plywood.
Summary
AWA TEACHING AID
Made by AWA during the 1960s, this device was designed to teach radio technology students how valves worked. The valve envelopes were large so that
students could easily see the various parts that made up the two valves (one a
triode, the other a CRT) on top of the chassis.
84 Silicon Chip
The set is a good performer and was
quite suitable for use by the farming
community. However, I suspect that
it would have been prone to picking
up commutator whine from the 32V
lighting plant when it was charging
batteries or, for that matter, from any
small motor attached to the 32V supply. That’s because there is absolutely
no filtering of the 32V rail before it is
applied to the valve plates and screens,
other than for the 6SL7.
However, the set works well when
powered from a 32V DC filtered power supply (eg, as described on p88 of
the January 2001 issue). If you like
collecting 32V sets, this one deserves
SC
a place in your collection.
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