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 mystery of the missing output transformer is now solved. The thing is on the amplifier chassis above... and the audio output isn't (now) 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 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 should 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 to save a little 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. Someone pointed out that there's 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 personally, bearing in mind the government surplus market after WW2 I might have opted for a couple of EL32s or 6V6s... but I'm wrong 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. That immediate postwar period predates the glut of surplus components and equipment so even things like the R1155 is seen in one advert 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.

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 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 amplifer.

 

 

 

 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. Supporting the suggested valve change I notice also that the KT66s are not triode connected and the way the components are fixed it has been redesigned, not for 10W output but for somewhat more (that output transformer surely is rated at a lot more than 10 Watts). To further support this hypothesis there are two (extra?) EF37 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.

 

 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 valves were added by the last owner? Their positioning looks very symmetrical possibly suggesting 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 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 thing 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, maybe for adjusting the AB1 bias voltage, at the time the valve configuration was changed to tetrode from triode push-pull.

Maybe Moreton Cheyney 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 his hospital visit?

 
 I looked at the EF37 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 ceramic holders indicating their previous use, but 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.

  pending

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