Moreton Cheyney Amplifier

 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.

 

 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.

 

 

 

 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.

 

 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..

 
 
 

 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?

 

 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.

*****

 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 90 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 90 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.

 

 

Below is the circuit diagram of the amplifier 

 

 

RESISTORS

 

 R1

 100 x 5W

 R7

 330K x 0.5W

 R13

 10K x 1W

 R19

 100K x 1W

 R2

 100 x 5W

 R8

 330K x 0.5W

 R14

 10K x 1W

 R20

 240K x 1W

 R3

 10K x 1W

 R9

 500 x 10W

 R15

 33K x 5W

 R21

 120K x 0.5W

 R4

 10K x 1W

 R10

 51 x 1W

 R16

 47K x 5W

 R22

 100K x 5W

 R5

 240K x 0.5W

 R11

 51 x 0.5W

 R17

 250 x 1W

 R23

 2.2K x 1W

 R6

 240K x 0.5W

 R12

 500 x 10W

 R18

 120K x 0.5W

 R24

 6.2K x 1W

 

CONDENSERS

 

 C1

 0.001uF mica

 C7

0.01uF

 C13

16uF x 350V

 C2

Block 8uF x 600V

 C8

0.01uF

 C14

50uF x 25V

 C3

Block 8uF x 600V

 C9

0.01uF

 C15

0.1uF x 350V wax

 C4

Block 8uF x 400V

C10

0.01uF

 C16

-

 C5

0.1uF Sprague

C11

Block 8uF x 400V

 C17

-

 C6

0.1uF Sprague

 C12

8uF x 500V

 C18

 -

 

 

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.
 

 
 

 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

 

 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.
 

 pending

 See the matching receiver

 See the receiver circuit analysis

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