Moreton Cheyney Amplifier
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This very rusty chassis
was carried in the same wheelbarrow as the accompanying T1154
and Moreton Cheyney receiver. It needed a wheelbarrow because
of its weight. I checked but it only managed 40 pounds on our
scales, but as it isn't very large it seemed to weigh more than
this. |
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When I saw this thing
I almost turned it down, but it was saved because of a KT66 peeking
out of the wheelbarrow. I placed it out of the way in the workshop
until, after examining the Moreton Cheyney receiver and finding
that its circuit did not seem to match its description in the
very sparse information available, I looked again under the chassis
and noticed it used exactly the same tagboards and tagstrips,
plus some Sprague condensers and a yellow electrolytic and some
wirewound resistors whose mates were under the receiver chassis.
The real clue however is the
pair of cables terminated in 5-pin plugs which exactly match
the sockets on the rear of the receiver chassis. There's little
doubt that these are original and no doubt that they are back-to-front.
What I mean is that the exposed plug pins are carrying high current
LT and the HT voltage so that careless handling will result in
some danger and the risk of blowing a fuse. A safe design would
have the cables fitted to the receiver and not the amplifier/PSU.
The mystery of the missing output
transformer in the receiver is now solved. The thing is on the
amplifier chassis above... and the audio output isn't supplied
by two 6J5 valves, but a pair of KT66 beam tetrodes. Obviously
these are much more able to deliver the 10 Watts in the Wireless
World write up. But.. are these KT66s original, or did the owner
change whatever was originally fitted? The Wireless World description
mentions "triodes" not triode connected tetrodes, which
in itself is clear enough evidence, but to help answer this question
the wiring and components provide a clue. The first thing I spotted
are the white ceramic valveholders held in place by spring clips.
Valveholder fixing holes are visible and quite ordinary composite
holders could have been fitted as 10 Watts is not an enormous
output requiring very large voltages and the designers would
surely wish not to waste cash and use bog standard valveholders?
Visible on several of the parts
under the chassis is the trade name "Radiospares".
They started business in 1937 but only sold components generally
by 1954 so these parts could have been fitted in the late 1940s,
but may well have been used in the 1950s to carry out modifications.
Clearly visible is a spare heater supply from the large mains
transformer and that being so the output valves could have been
PX4s... that may be so, but bearing in mind the government surplus
market after WW2 a couple of EL32s or 6V6s might have been a
lot cheaper... but no, because back in 1945, when this receiver
was designed, a pair of PX4 valves was £1:3:2d and this
was a few shillings less than 6V6s. A far cry from 2019 prices.
The immediate postwar period predates the glut of surplus components
and equipment so even things like the R1155 is seen in one 1946
advertisement as the "V55" "all British"
9 valve receiver.. because supply was limited and more money
was to be made by selling modified stuff. So.. the amplifier
might have used the PX4 and later, when better amplifying valves
appeared on the market, our owner could have dispensed with the
directly heated PX4s and fitted hum-free KT66s which also promised
lots more output power.
The rat's nest of parts around
the octal holders, some dangling in the air (see that bias pot!)
is in sharp contrast to the tag board/tagstrip layout of the
Moreton Cheyney designer. Then that re-used aluminium chassis
added at the rear is a dead-giveaway of amateur "improvements".
Now prepare yourselves for
some ghastly pictures of what is quite probably the only surviving
Moreton Cheyney amplifier.
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As you can see above the
amplifier has been more than a little damp for the best part
of its life. Unlike the companion aluminium receiver the amplifier
has a steel chassis, and probably of wartime quality so rusting
would have been quick to start. Still, the chassis is fairly
heavy gauge and may scrub up well. Lots of dry solder joints
and some scruffy wiring tells me the last owner wasn't too happy
with the original design (and wasn't too good at soldering as
many joints have failed). Supporting the suggested valve change
I notice also that the KT66s are not triode connected and the
way the components are fixed the amplifier has been redesigned,
not for 10W output but for somewhat more (that output transformer
surely is rated at a lot more than 10 Watts). There are two EF37A
valves at the end of the chassis which makes the total receiver
valve count of 18 plus rectifier, not the 16 quoted in Wireless
World. Why do I believe it's an original Moreton Cheyney? Well,
as I said previously, there are several parts that are identical
to parts used in the receiver. Compare the tag boards, one of
the tag strips, that yellow condenser, the Sprague condensers
and the green ceramic resistors... also those odd 4BA locknuts
are used extensively on both chassis, and of course that cabling
with matching plugs. |
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Above is more clear evidence
of modifications.. an outrigger chassis carrying an extra choke
and smoothing condenser.
Below, I've identified the main
parts. Whether damp conditions have ruined the wound components
I can't tell until power is applied, but at least the vital mains
transformer is shrouded and therefore the windings are mostly
protected. The block condensers are marked with dual ratings
in terms of working voltage and temperature and I've quoted the
max temperature ratings, hence the lower working voltages.. |
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I suppose we must ask
the question as to whether these two EF37A valves were added
by the last owner? Their positioning looks very symmetrical which
suggests to me that they were part of the original design, although
the position of the output transformer in between the KT66s and
the EF37s is slightly odd and is very close to the left hand
valve (see the layout above)? Was this transformer fitted to
replace a smaller one?
That "TV" coax feeding
the valve grid caps is not original. A slim possibility is the
amplifier has been modified as a general purpose equipment, but
as there are no controls, unless a pre-amp was intended, the
design could be original, with the receiver tone and volume controlling
the amplifier.
Floating in the wiring under
the chassis is a small potentiometer which must have been added
for adjusting the balance of the KT66s, at the time the valve
configuration was changed to tetrode from triode push-pull.
Moreton Cheyney apparently brought
out a stand-alone version of their amplifier. It seems they did
if you look at the "For Sale" ad from June 1948 Wireless
World. Note that this ad predates the NHS so maybe the urgent
sale was needed finance the owners hospital visit? |
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I looked at the EF37A
valveholders. These are identical to that used for the missing
rectifier valve and are held in place by those same 4BA locknuts
used extensively in the receiver. I also spotted scrape marks
around the fixing holes adjacent to the KT66 ceramic holders
indicating their previous use, but perhaps by B4 bases? It's
impossible to tell. Will I restore this amplifier? Possibly,
but I'm dubious about going back to original triodes, especially
if they were (now dramatically over-priced) PX4s. I have a box
full of KT66 valves so I'd leave the later mods in place as part
of it's history, but actually getting it going within a realistic
budget might be an impossible task if the transformers and chokes
are U/S. |
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I've now spent lots of time tracing
the receiver circuitry. Initially I had thought the two output
valves in the receiver were stand-alone push-pull audio output
valves, then I thought they were push-pull drivers, but after
further thought I'm of the opinion that V12 and V13 are used
for tone control and their outputs are passed to the external
amplifier (but is there a wiring error?). Before looking at the
amplifier chassis I thought V12 and V13 outputs were sent to
the main amplifier where they were added at an EF37A which amplifies
the combined audio signal before passing it to a phase splitter.
The phase splitter anode and cathode would produce a pair of
audio drive signals with 180 degrees of phase difference to drive
the pair of push-pull KT66 output valves. However this turned
out to be incorrect... after tracing the amplifier circuit I
discovered that the condensers at the anodes of the two final
triodes should be connected together and these then connect to
one of the EF37A valves used to drive the KT66s. The second EF37A
driver apparently is driven from the 0.001uF condenser at V10
anode in series with the 240K resistor. Although at first sight
these values of capacitance and the series resistor look wrong.
The first step will be to clean up and
de-rust the steel chassis and the surface of the major components.
Once this is done I'll trace the circuitry then tidy up the wiring
and confirm the wound components are serviceable.
***************
I brushed away loose material then painted
the chassis with a rust remover. Whilst this was taking effect
I traced the circuit and found it was relatively straightforward.
Each KT66 is driven by an EF37A using resistance/capacity coupling.
The EF37As are each driven via a screened lead from one of two
5-way cables which connect to the receiver (Socket P2). This
means that the output from the receiver must have a pair of audio
feeds already differing in phase by 180 degrees. The EF37As share
a screen grid supply and a cathode bias resistor. The KT66s are
balanced by a small 250 ohm potentiometer which can be adjusted
to give the same standing current in the two valves.
The power supply uses a large mains
transformer providing a number of heater supplies plus a tapped
HT supply full-wave rectified by something like a 5R4 or 5U4
(missing). The HT is smoothed via three low frequency chokes
decoupled by large block condensers and connected to the receiver
via one of the 5-way cables terminating at Socket P1 which also
carries a pair of 6.3 volt supplies. Mains tappings and a fuse
are provided, but there isn't a mains switch fitted on this chassis.
A second fuse is fitted in the ground return lead from the mains
transformer HT winding centre-tap. |
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Below is the circuit diagram
of the amplifier |
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RESISTORS
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R1 |
100 x 5W |
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R7 |
330K x 0.5W |
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R13 |
10K x 1W |
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R19 |
100K x 1W |
R2 |
100 x 5W |
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R8 |
330K x 0.5W |
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R14 |
10K x 1W |
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R20 |
240K x 1W |
R3 |
10K x 1W |
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R9 |
500 x 10W |
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R15 |
33K x 5W |
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R21 |
120K x 0.5W |
R4 |
10K x 1W |
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R10 |
51 x 1W |
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R16 |
47K x 5W |
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R22 |
100K x 5W |
R5 |
240K x 0.5W |
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R11 |
51 x 0.5W |
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R17 |
250 x 1W |
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R23 |
2.2K x 1W |
R6 |
240K x 0.5W |
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R12 |
500 x 10W |
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R18 |
120K x 0.5W |
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R24 |
6.2K x 1W |
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CONDENSERS
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C1 |
0.001uF mica |
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C7 |
0.01uF |
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C13 |
16uF x 350V |
C2 |
Block 8uF x 600V |
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C8 |
0.01uF |
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C14 |
50uF x 25V |
C3 |
Block 8uF x 600V |
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C9 |
0.01uF |
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C15 |
0.1uF x 350V wax |
C4 |
Block 8uF x 400V |
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C10 |
0.01uF |
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C16 |
- |
C5 |
0.1uF Sprague |
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C11 |
Block 8uF x 400V |
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C17 |
- |
C6 |
0.1uF Sprague |
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C12 |
8uF x 500V |
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C18 |
- |
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Above and below, the amplifier
chassis after cleaning and rust treatment. The workmanship is
so poor that the best way forward is to remove everything from
the chassis and start again. All the rubber covering to cables
and wiring is badly perished and has to be replaced.
Future progress will be dependent
on the condition of the transformers and chokes as replacement
will not be a worthwhile proposition. |
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These are the cables carrying
power to the receiver (below) and screened audio to the amplifier.
You can see the poor condition of the rubber insulation.
On the left is the power cable P1 and
on the right the audio output cable P2. Pin numbers as shown
below. T1 and T10 are the nearest tagstrips on the amplifier
chassis. ETH connects to the ampifier chassis.
P1 |
P1 |
P1 |
P1 |
P1 |
1 |
2 |
3 |
4 |
5 |
6.3V 5A |
6.3V 1A |
HT+ |
6.3V 1A |
6.3V 5A, ETH |
T1-E |
T1-B |
T1-D |
T1-C |
T1-F |
P2 |
P2 |
P2 |
P2 |
P2 |
1 |
2 |
3 |
4 |
5 |
not used |
not used |
V2 grid |
V1 grid |
ETH |
- |
- |
T10-C |
T10-D |
T10-B |
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Whilst tracing the circuit
of the amplifier and power supply it was clear that several changes
had been made over the lifetime of the equipment and many components
had been left soldered to the various tagstrips but unconnected
to the active circuitry. Several wires were not connected athough
some of these had probably not been properly soldered and had
just come adrift. |
Looking for a new project
since repairing my Hewlett Packard signal
generator, I decided to tackle this amplifier.
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