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Old 15th Aug 2020, 10:47 am   #1
allan
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Default Moreton Cheyney Amplifier

Some time ago I restored my Moreton Cheyney receiver to working order and then decided to tackle the matching amplifier which was in a horrendous condition. I've now discovered that the main components seem to be serviceable so I'm embarking on its restoration.
Not a straightforward task because the amplifier was rebuilt by its last owner.
There are several design considerations to be made. Because the amplifier was designed to match precisely the strange receiver audio output I'd like to change the circuitry to make it generally more useful but at the same time let me hear the audio from the receiver much as the Moreton Cheyney designers envisaged.
I'm at the stage where I need to work out a circuit for the amplifier. Should it be KT66 pentodes, or KT66s wired as triodes to echo the original PX4s and what should be the input circuitry which currently does not include a phase-splitter.
Any comments welcomed...
http://www.radiomuseum.co.uk/MCamplifier.html
Allan G3PIY
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Old 15th Aug 2020, 11:40 am   #2
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Default Re: Moreton Cheyney Amplifier

470-0-470v is a very high HT level for a capacitor input filter. The power supply capacitor voltages must be surge stressed at power on unless there are some hidden large bleed resistors.

I can see the screen dropper is 47k with a 350V cap - which is a lot of unregulated voltage drop for pentode mode. Given no global feedback, the high B+, and the RC plate snubbers (do they really go to HT2?), it seems like a PA style amp made for raw output power with no distortion finesse.

It seems like you have tested the output transformer for insulation resistance, but not for turns ratios, and hence had a guess at impedance levels?
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Old 15th Aug 2020, 12:47 pm   #3
allan
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Default Re: Moreton Cheyney Amplifier

I agree that the mains transformer HT voltage is a trifle high.
I think the transformer is original so presumably worked back in 1950 or whenever the receiver/amp were new. The 8uF input condenser rating is marked 700 volts at 60C (600 volts at 71C) and the two 8uF smoothing condensers 450 volts at 60C (400 volts at 71C). The rectifier was missing but I guess a 5U4 or a 5R4 or similar. That will drop the voltage somewhat unlike a semiconductor diode.
As yet I haven't tested the KT66 output transformer. I have a new one of those so if the old one is duff I can replace it.

I can add the +10 volt tapping on the mains transformer primary to ease things down a little while in a testing phase.
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Old 15th Aug 2020, 12:56 pm   #4
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Default Re: Moreton Cheyney Amplifier

And you have the bizarre circuitry of V11, V12 & V13 to try to decipher!
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Old 15th Aug 2020, 2:48 pm   #5
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Default Re: Moreton Cheyney Amplifier

Does the amp also power the receiver?

The receiver may have bleed loading that keeps all the cap voltages within limits (well at least 110% surge type limits) for the short period after power on. Do the caps have a surge voltage rating - some old caps have a surge rating that is higher than the defined 110% for modern electrolytics.

I end up confirming all part ratings are ok for a vintage restoration - so many decades where a quick fix could have been applied - and many parts need to be swapped out (or bypassed, such as paper/oil/can caps).
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Old 15th Aug 2020, 4:13 pm   #6
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Default Re: Moreton Cheyney Amplifier

Yes, the PSU also feeds the receiver which has 14 valves and some bleed current.
http://www.radiomuseum.co.uk/Morton.html
I measured the Rx current as something like 50mA but with a higher voltage than my test HT it's going to be around 60-75mA. HT is supplied from LFC2/C4.
All the small condensers will be replaced with modern ones. I'll use decent vintage carbon and wirewound resistors where possible.
Whatever V11/12/13 were supposed to do, they sounded quite interesting but the way the amplifier is fed is most peculiar (that includes the 2 EF37As as well)
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Old 16th Aug 2020, 7:39 am   #7
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Default Re: Moreton Cheyney Amplifier

Blimey, you have a stout heart, I'd have chucked it or ran away, well done. I have some NOS caps similar or the same as some of those big PIO caps, tested for leakage, capacitance etc if you need some.

Re the HT, you could leave out C2 making it a LCLCLC filter, that'd drop HT quite a bit. Looking at the size of the OPT you won't want a large OP, so can change things quite a bit, there's room for two valve so room for a standard triode IP stage and PS, but if the receiver dictates what you can do, your limited.

With valve amp design you start at the OP stage and design everything from there, including the HT supply, therefore you'd be better fixing the OP stage and then making everything else fit it. The receiver AF OP is just nuts, overly complex with very odd design choices, if originality isn't important best to start fresh, then all you need to know are receiver OP, line in V levels etc and then OP stage sensitivity.

Andy.
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Old 16th Aug 2020, 10:01 am   #8
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Default Re: Moreton Cheyney Amplifier

Hi Andy
The receiver output was rather odd but it did actually seem to work once I'd tidied it up. I used a rough and ready amplifier to hear the results and sure enough fidelity of medium and long wave broadcasts was impressive. There are four controls needing to be adjusted, volume, tone control depth, bass and treble boost. I can't admit to understanding the weird circuitry or the fact that the amplifier is driven from a straight signal plus a processed signal. There's also a few places where feedback is arranged through either common anode or common cathode resistors.
My idea is to faithfully arrange for the amplifier to follow the original design and to add circuitry to give me a traditional amplifier (including a phase splitter) except to add a double triode to save cutting the chassis for another valve.
Hopefully I can use a rotary switch to give me the choice of input: Moreton Cheyney or general purpose.
I'm moving towards triode connected KT66s as the original design appears to have used a pair of PX4s except the last owner decided to use the KT66s as tetrodes.
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Old 16th Aug 2020, 12:29 pm   #9
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Default Re: Moreton Cheyney Amplifier

Wow! Allan this is right up my street, restoring this is going to be a challenge, good luck with it, keep us informed on your progress.

Mick.
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Old 16th Aug 2020, 2:44 pm   #10
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Default Re: Moreton Cheyney Amplifier

Hi!

In a circuit like yours, you can't be certain of the characteristics of the o/p transformer without expert lab equipment to measure the leakage inductance, etc., so I'd play it safe and take a few circuit precautions!

I would personally use the KT66s in triode mode, but add additionally 1k 1W "screen–grid" stopping resistors between pins 3 and pins 4 of each output valve holder, plus a 10k series "grid–stopper" from pin 5 to pin 6 (pin 6 on the KT66 is spare so can be used as a "tag" point) with the wiring connected to pin 6!

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Old 16th Aug 2020, 3:58 pm   #11
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Default Re: Moreton Cheyney Amplifier

If it were mine I would consider re designing the phase- splitter cct the original is a very strange cct indeed. Interesting though, very nice restoration up to now.
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Old 16th Aug 2020, 8:18 pm   #12
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Default Re: Moreton Cheyney Amplifier

On the face of it, the push-pull stage is asymmetrically driven, one half by the unmodified signal, and the other half by the modified signal. The latter may be varied in frequency response by the treble and bass controls, and in level relative to the unmodified signal by the tone control depth control. Whilst asymmetric drive appears to have been strange, it was for example an inherent property of the orthogonal amplifier, although that was a 1950s development and so probably unknown in the late 1940s.

However, a closer inspection shows that the two EF37A driver stages share a common cathode resistor. Thus they look like a cathode-coupled phase-splitter, with the difference that both grids are signal-driven, one by the unmodified and the other by the modified signal. That I think would even things out, in that each input would appear in antiphase on the opposite channel. It would also mean that the amplifier should respond to an input signal applied to just one of the EF37A grids, with zero signal on the other. So perhaps no modification is needed to allow conventional operation with a single input?

An inference is that Morton Cheney used the pentode-input cathode-coupled (long-tailed pair) phase-splitter about a decade or so before Bailey invented it, but the latter was likely the first to delineate its specific advantages over the triode-input type.


Cheers,
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Old 17th Aug 2020, 12:08 am   #13
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Default Re: Moreton Cheyney Amplifier

I had noticed the common cathode resistor.
I suppose I could reconstruct the original amplifier and test it to see exactly how it works. If the pair of EF37As do work as a phase splitter then grounding the input for unprocessed audio may provide standard amplification as you say.
I'll be fitting a pot to provide volume control to make things easier and to protect my loudspeaker... and operating the KT66s as triodes. I was also considering making a negative bias supply from spare heater windings.
The output transformer has loads of winding taps so I'll need to figure out the optimum set for best matching.
I bought a transformer in Jan 2007 that was labelled KT66/KT88 so I might compare that with the Moreton Cheyney example. The latter could be the original designed for PX4s though?

The design of the audio section of the receiver is very interesting. Given the timescales involved their design engineer must have been very talented. I once met Baxandall.. I think he was working at RSRE Malvern when I was working on Project Linesman (UK air defence) around 1965 on a touch input display... They must have both had about the same knowhow? In fact could the two have been one and the same person??

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Old 17th Aug 2020, 12:09 am   #14
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Default Re: Moreton Cheyney Amplifier

I think it highly likely it left Morton Cheney looking very different. The amp chassis uses different valve sockets so I am not convinced it was ever a matching unit. Is it possible V11 was originally a phase-splitter and V12/V13 were a push pull pair?

It is only a minor rewire to run the EF37's as an LTP phase-splitter. It's probably worth testing the output transformer to get the turns ratio?
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Old 17th Aug 2020, 9:13 am   #15
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Default Re: Moreton Cheyney Amplifier

Although it is unknown just to what extent these units have been modified, as it stands the combination does seem to make some sense.

These were designed in the days when there was little standardization in respect of tone controls, and the rather elegant Baxandall circuit was still in the future. In that regard, a triple-path system with electronic addition may have appeared logical for what was heading towards being a valve-count-no-object unit. Thus the audio path was split into direct and tone control paths. And the tone control path was further split into treble and bass paths, following the tone control depth potentiometer. The latter two were recombined electronically within the tuner/control unit section. That recombination involved a phase inversion relative to the direct path, so perhaps rather craftily, the designers saw the opportunity for recombination of the direct and tone control paths in the phase-splitter stage of the power amplifier. Speculation, certainly, but it does fit the observations.

On the other hand, it is possible that an individual user conceived of that path splitting and recombination possibility (or maybe was aware that others, such as Sound Sales, had used it), and wanted to try it out by modifying a unit on hand or otherwise readily available, and one that had enough valves available for repurposing to do the job.

I think though that Occam’s Razor points more towards original design than subsequent modification.


Cheers,
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Old 17th Aug 2020, 9:22 am   #16
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Default Re: Moreton Cheyney Amplifier

The receiver and amplifier are definitely a matching pair.
The basic hardware used is identical. For instance all the original 4BA nuts and bolts are the same (using locknuts), the valveholders for the EF37As were cheap paxolin types and the power and audio cables are 5-pin RAF plugs and sockets.
The amplifier must have been modified quite early on in its life. The valveholders for the power amplifier valves were changed to ceramic octals but the original fixing holes are not for the same paxolin holders used elsewhere. I guess these were B4 types, maybe ceramic? Either the owner didn't care for hum due to the PX4s being directly heated or one or both needed replacing and KT66s were chosen. PX4s were really cheap in the 50s but at some point in time, either these became a lot dearer than the KT66 or the owner wanted more power. I say that because he changed from triodes to tetrodes. Loss of power could have been a fault having arisen in the receiver because the circuitry in that had been messed up (looking for the fault... which he'd failed to discover!). Now that the receiver has been sorted out the power level from it has increased dramatically. The adverts from the late 40s describe the amplifier as having "10 watts output with less than 2% distortion from a pair of triodes in push pull".
There are a some puzzling factors though... the mains transformer HT output and filter is too high for the PX4 so was the transformer swapped when the KT66s were fitted? A second puzzle is the heater supply to the first audio amplifier in the receiver (V9). This is taken to a different pin on the power cable. The cable connects to a winding on the transformer which has an output lower than 6 volts. Was this because the transformer was swapped or could V9 originally have been a special low noise low microphony type with a 4 volt heater? If the mains transformer is a new one then the old one could have had a second 6.3 volt winding and the KTZ63 was correct for V9. Another possibilty is that running a KTZ63 with reduced heater voltage may have been deliberate?
Allan G3PIY
PS Could the original output valves have been DO24s or PX25s rather than PX4s?

Last edited by allan; 17th Aug 2020 at 9:41 am.
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Old 18th Aug 2020, 12:55 am   #17
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Default Re: Moreton Cheyney Amplifier

Quote:
Originally Posted by allan View Post
PS Could the original output valves have been DO24s or PX25s rather than PX4s?
It seems possible. Conceivably a PX25 pair operating at 10 watts might produce lower distortion than a PX4 pair at the same power output.

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Old 18th Aug 2020, 10:27 am   #18
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Default Re: Moreton Cheyney Amplifier

An interesting advert- in the text both KT61 and KT66 are described as eminently suitable for triode connection but in the table, KT61 is "not recommended".
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Old 18th Aug 2020, 12:34 pm   #19
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Default Re: Moreton Cheyney Amplifier

Quote:
Originally Posted by Herald1360 View Post
An interesting advert- in the text both KT61 and KT66 are described as eminently suitable for triode connection but in the table, KT61 is "not recommended".
I guess the copywriter or proof reader lost their half-day holiday over this.
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Old 19th Aug 2020, 6:31 pm   #20
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Default Re: Moreton Cheyney Amplifier

I spent the day wiring up the amplifier.
I decided to base the circuit on the Williamson amp using 2xKT66 + a couple of double triodes.
If I'm slack tomorrow I might get as far as testing it.
If all goes well I might add a changeover relay so I can optionally drive it from the M.C. receiver which has its own phase splitter.
http://www.radiomuseum.co.uk/MCamplifier.html
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