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Old 12th Aug 2021, 1:38 am   #1
Synchrodyne
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Default Single-Ended Partial Cathode Loading Output Stage?

Mention of the Ekco C273 receiver in the thread “In Memory of the CONSOLE RADIO” (https://www.vintage-radio.net/forum/...d.php?t=182661) prompted another look at its circuitry.

Previously I had observed that the amplifier main feedback loop went from the secondary of the output transformer back to the grid of the output valve (EL84) rather than to the grid or cathode of the driver stage, but had thought that it might have been done to ensure that the main loop did not wrap around the tone control feedback loop between the EL84 anode and grid. (There was also another loop, EL84 anode to driver anode, on the A274 and A277 table models only, to provide some bass boost.)

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This time though it struck me that the Ekco circuit had some similarities with Pye HF10 Mozart amplifier output stage. In that case the cathode of the EL34 was connected to the secondary of the output transformer.

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The Pye Mozart was generally regarded as having a single-ended distributed load output stage of the partial cathode loading type. In fact it was said to have been inspired by the Quad partial cathode loading push-pull output stage (which dated from 1945); apparently a Pye staffer involved in the Mozart development had previously worked for Quad. I am not sure that Pye ever used the “single-ended ultra-linear” descriptor in respect of the Mozart, but it did for the “Super Black Box”, which had essentially the same output stage.

Thus the question is – did the Ekco C273 (and A274, A277) in fact have a distributed loading output stage of the partial cathode loading type?

The Ekco circuit seems to have been about a year ahead of the Pye Mozart. The A277 and C273 were mentioned as new in Wireless World 1956 September, the Mozart in 1957 October. The latter issue also mentioned a new Baird tape recorder that had a single-ended ultralinear output stage, although of the conventional type.


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Old 12th Aug 2021, 8:17 am   #2
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

In the book "Laagfrequentieversterkingstechniek" (Philips, 1949) this type of cathode feedback is described as a form of negative voltage feedback. Diagram 162 shows the effect (the interrupted curves show the distortion without the feedback, the non-interrupted curves show the distortion with the feedback; the feedback factor is 5).

In the Philips EL3516 tape recorder from 1957 it was implemented by using a secondary with a tap (like the Pye Mozart from 1958). I used two of these output transformers for an amplifier with EL3N's.

I saw more examples of this type of cathode feedback, but mostly with only the cathode capacitor connected to the secondary of the output transformer while the cathode resistor is connected to ground in the usual way (see for instance some of the schematics of single ended amplifiers in Brimar Valve Manuals).

I thought that the first Quad amplifier (QA12) was introduced in 1948 (source: Wikipedia) but I'm not familiar with their history.
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Old 12th Aug 2021, 8:40 am   #3
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Cossor seemed to have adopted this circuit in the early/mid 50's not sure quite when but it's certainly used in the 522 and 540 1954 and 1955 respectively. Snips attached.

Sets such as the 500/501 used feedback across the cathode resistor of the triode stage.

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Old 12th Aug 2021, 8:46 am   #4
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Thanks for that background information. It would seem that output circuits with some form of cathode feedback, howsoever described, were not so rare.

Quad (or Acoustical as it then was) first used its partial cathode-loading circuit on its M31 PA amplifier of 1945. The same basic circuit, albeit without transformer drive, was then used on the QA12, Q.U.A.D. (retroactively Quad I) and Quad II amplifiers.

Anyway, it does look as if Ekco had made an effort to improve performance beyond that of the “standard” EL84 output circuit.



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Old 12th Aug 2021, 12:09 pm   #5
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

It's a case of, 'what's in a name?'

I came up with this circuit years ago, tried it with a PCL82, it worked... only to find out that it was already in use! Another example (I'm almost sure) is the Ferguson 373RG radiogram. Never mind.

It could be called partial cathode loading, but the degree of loading of the cathode is very low (typically just 2% of the total load voltage appears at the cathode), the output power actually delivered by the cathode is correspondingly low. Whereas cathode-loaded amplifiers tend to have a signal-voltage on the cathode which is comparable with that at the anode.

So really, it's just a way of getting negative feedback around the single stage.

An interesting question of course is, what mode is the valve working as? It's not a pentode, because the cathode-screen voltage is not constant - screen is bypassed to 0V rather than to cathode. It's not a triode either, obviously. It could be called, common-screen ultralinear, with the UL tap at 2%... Thoughts?
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Old 12th Aug 2021, 1:12 pm   #6
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Is there any disadvantage to having the output valve standing DC cathode current flowing through the output transformer secondary winding and at least to some extent the attached loudspeaker(s)?
The Quad II arrangement with separate cathode windings seems more elegant to me, but more expensive to implement.

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Old 12th Aug 2021, 1:55 pm   #7
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

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Originally Posted by jjl View Post
Is there any disadvantage to having the output valve standing DC cathode current flowing through the output transformer secondary winding and at least to some extent the attached loudspeaker(s)?
It will add a few milliamp-turns to the core magnetisation, but that's negligible really given how few turns are on the secondary compared with the primary. DC offset across the speaker will also be tiny since the secondary winding resistance will be very small.
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Old 12th Aug 2021, 1:57 pm   #8
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

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Originally Posted by kalee20 View Post
It could be called partial cathode loading, but the degree of loading of the cathode is very low (typically just 2% of the total load voltage appears at the cathode), the output power actually delivered by the cathode is correspondingly low. Whereas cathode-loaded amplifiers tend to have a signal-voltage on the cathode which is comparable with that at the anode.
I just look at it like voltage feedback working 'against' the input voltage of the power valve, so not like it is a form of loading of the output transformer. But maybe I don't understand it good enough.

Attached a kind of summary of the ways you can connect valves to output transformers (and in this paper to one and the same 'universal' output transformer), both single ended and push-pull.
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Old 12th Aug 2021, 2:01 pm   #9
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

You don't get full transformer isolation from the HT related stuff to the external loudspeaker connections. If a chassis connection goes missing and some things leak, the speakers could go high(-ish). It takes multiple faults, but you have lost that cosy certainty.

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Old 12th Aug 2021, 3:38 pm   #10
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

The Ekco and Cossor models mentioned above that had feedback to the cathode of the output valve....EABC80, grid current bias and cathode's to ground so that feedback path was an easy option.

Lawrence.

Last edited by ms660; 12th Aug 2021 at 3:43 pm. Reason: word change
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Old 12th Aug 2021, 7:07 pm   #11
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Quote:
Originally Posted by jjl View Post
Is there any disadvantage to having the output valve standing DC cathode current flowing through the output transformer secondary winding and at least to some extent the attached loudspeaker(s)?
If you did it right - the cathode-current flowing through the output-transformer's secondary could serve to partially cancel-out the magnetisation caused by the anode current flowing through the primary.

You'd need to use a high-capacitance low-DCV-rated capacitor to stop the DC-secondary component escaping through the speaker-coil though.
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Old 12th Aug 2021, 8:49 pm   #12
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

No unfortunately - if you connected the secondary winding such that the cathode current CANCELLED partially the anode current, you'd find that feedback to the cathode was positive, not negative.

It's a nice thought, unfortunately if it worked it would be in violation of Sod's law!
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Old 12th Aug 2021, 9:21 pm   #13
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

IMHO a touch of positive-feedback at audio can be *positively* beneficial - to boost the audio response between 1 and 2KHz where most of the intelligence of speech lies.

Think of it as an AF version of the "Q-multiplier" traditionally used at intermediate-frequencies to improve selectivity. A number of radios have used this approach over the years; the military R209 has an audio positive-feedback filter in its AF stages to give a bit of a peak when receiving CW.
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Old 13th Aug 2021, 2:57 am   #14
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Thanks for all of the comments.

A difference between the Pye and the Ekco circuits was that in the former case, the cathode feedback was taken from the 15R speaker tap, whereas in the latter case it was taken from the only speaker tap, intended for a load of around 2R. So the proportion of the load attributable to the cathode would appear to have been somewhat higher in the Pye case.

Thus we might infer that in the Pye case, there was deliberate intent to have partial cathode loading, this also providing useful feedback. In the Ekco, and probably the Cossor cases, however, the primary intent appears to have been to use the cathode as the return point for the main feedback loop, with the partial loading thereby obtained being incidental. Nonetheless, it becomes a question of degree, and as kalee20 said, “what’s in a name?”

Regarding positive feedback in power amplifiers, the use of this had some currency in the mid-1950s, particularly in the US, where its use, in conjunction with the customary negative feedback, allowed variable damping. I think it was less used in the UK, where Pye was the main exponent, using it on several amplifiers including the HF10 Mozart. As well as the cathode feedback, the latter had an additional pair of loops from the OPT secondary back to each of the cathodes of the two preceding triode stages, amplifier and driver. That to the amplifier was a variable blend of both positive and negative via a pot, whereas that to the driver was a fixed amount of negative. This feature was not though carried over to the HFS20 stereo version, in which both of those loops were fixed NFB. Perhaps it had been found to have limited utility in practice.

Returning to the Ekco and Cossor cases, it does look as if the respective designers were making efforts to obtain some improvement from the standard radio receiver output stage without undue added complexity. One wonders if they had at all been influenced by the comment made in the well-known Williamson and Walker Wireless World article on ultralinear amplifiers, in which it was stated that cathode feedback was of the most desirable kind, not requiring that gain be thrown away or increasing the loading on the previous stage.


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Old 13th Aug 2021, 6:48 am   #15
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Taking the feedback from the speaker winding of the output transformer to the cathode of the output valve, introduces the prospect of the speaker impedance interacting with the transformer strays (leakage inductance, copper resistance etc) and affecting the phase of that feedback loop.

So I consider the Quad approach, with separate feedback windings for the cathodes, to be the best realisition of this approach.

Nowadays we have a number of people who really want to have valve amplifiers but can't justify the expense of the currently available boutique jobs, or of the respected classics. They are tied in what they can build for themselves by the availability of output transformers and valves.

Power triodes are priced out of reach due to demand from a cult following, who won't accept triode-connected tetrodes and pentodes.

Simple output transformers are available, but good ones with fine laminations and no corner-cutting in the winding design aren't cheap.

Ones for the standard 'Ultra-linear' arrangement are available with a few choices for the percentage tapping for G2 available. Again, good quality ones are expensive because of the issues of getting suitable laminations and the sheer amount of labour involved.

But nothing seems available with cathode windings. The EKCO and PYE approaches at least can be tried with ordinary transformers, but note that the transformer lives within the feedback loop, and its high frequency behaviour will have effects on the loop.

For the full-monty cathode feedback system with a separate winding, it would mean either buying a Quad II replica transformer from someone who does re-winds, or doing/commissioning a full design of a new transformer.

A long time ago... ooh, the mid seventies, I reckoned that valve amplifier design progress had come to a halt. It had got about as far as it could go, limited by output transformers as well as valves having gone out of mainstream production. Out of curiosity I designed one last one, transformerless and based on an awful lot of TV line output bottles to see what could really be done. It worked, it was quite free of 'valve sound' frequency response and distortion characteristics. It had problems, though. Matching issues were the worst trying to prevent one device in a parallel group taking more current than its neighbours and its anode going red hot. The overall heat dissipation was not good in a domestic setting except in Scotland, in winter. I'd need something else for use in summer, anyway.

So I went with transistors (FX: thundercrack in the distance fading into long silence)

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Old 14th Aug 2021, 6:09 am   #16
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Quote:
Originally Posted by Radio Wrangler View Post
Taking the feedback from the speaker winding of the output transformer to the cathode of the output valve, introduces the prospect of the speaker impedance interacting with the transformer strays (leakage inductance, copper resistance etc) and affecting the phase of that feedback loop.

So I consider the Quad approach, with separate feedback windings for the cathodes, to be the best realisition of this approach.
No doubt it was, and it stayed the distance for 25 years. But considering simple single-ended output stages in domestic radio receivers, then when more conventional forms of feedback were used, such as back to the output stage grid, or to the driver stage grid or cathode, the source was usually the speaker winding of the OPT. And in the case of the Quad II, the global feedback was taken from a tapping on the speaker winding – that approach actually went back to the QA12 of 1948. I think that using the speaker winding as the global feedback source was not unusual for hi-fi amplifiers in general. So the Ekco, Cossor and Pye circuits were not completely in left-field when it came to using the OPT speaker windings for the output valve cathode connection, although perhaps short of best practice, probably for cost reasons.

With a push-pull amplifier with partial cathode loading, I imagine that it could be more difficult to use the OPT speaker winding for the cathodes, since they need to be supplied in antiphase whereas a single-ended output, usually with one side grounded, is required for the speaker. On the other hand, for the single-ended case, use of the speaker winding for the cathode connection was straightforward.

All of that said, Baxandall did make the case for using a separate (tertiary?) OPT winding for feedback supply in his Wireless World 1948 January amplifier design. In the domestic receiver realm, Murphy used separate feedback windings for its early-1950s A188C and TA160 models, both of which had push-pull outputs.


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Old 25th Aug 2021, 5:20 am   #17
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

It occurs to me that even though there appear to have been only a small number of British receivers, amplifiers and record players fitted with single-ended outputs with cathode feedback/partial cathode loading, they may still have outnumbered those units fitted with single-ended ultralinear outputs (SEUL), that is with the screen grid taken to a tap on the OPT primary. In fact the only “definitive sighting” I know of is in Wireless World 1957 November, in which very brief mention is made of a Baird tape recorder (model number not given) fitted with an SEUL output.

WW 195710 p.472 Baird Tape Recorder SEUL.pdf


The Baird unit referred to by WW was likely the TR1, the subject of a current thread, https://www.vintage-radio.net/forum/...d.php?t=182242. On the face of it, a conventional domestic tape recorder seems to have been an unlikely candidate for going beyond the standard output circuit, but then the Murphy TR1, in the same general category, had a cascode input stage. So surprises abound. In the Baird TR1 case, the output valve also served as the output cathode follower when recording. There was no conventional feedback loop; such might have been awkward to include given the multifunctional nature of the stage. Possibly then the SEUL configuration was seen as another way to provide some form of feedback to linearize the stage. Or in other words, the reason for using it was essentially extrinsic, and not primarily for its intrinsic benefits, although once available, the latter were probably taken advantage of. But the same might have applied to Ekco’s use of cathode feedback, as that avoided enclosing the tone control within (but not being a part of) the main feedback loop, probably not such a good idea.


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Old 25th Aug 2021, 6:29 am   #18
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Some posts back, the possibility of standing current in a dedicated cathode winding, or in a speaker winding partially cancelling the DC magnetisation due to the standing current in the anode winding was raised.

Of course, the cathode current is rather similar to the anode current, and there has to be a significant turns ratio (anode turns/cathode turns) in order for there to be some useful gain left. So there won't be much cancellation.

Kalee20 pointed out that the sense of the cathode winding menat that unless you wanted a bit of positive feedback, the cancellation won't be cancellation. It will be net addition. So the paragraph above means that it's good that there isn't going to be much of it.

There is another factor about such current cancellation, if you could do it. In a single-ended amplifier, the DC magnetic flux represents stored energy, half times L times (I sqyared) joules. We need this. This is where the energy for half the output cycle comes from.

Unfortunately no-one has seen fit to invent complementary valves whose anode currents run the opposite way to normal and anodes are negative WRT cathodes. Dr Asimov's positronics never materialised, which at least saved us a lot of trouble with unruly robots. So we have to get the reverse current part od our waveforms some other way. Push pull valves driving a centre tapped transformer seems the best evasion, but for simpler circuitry, then using the transformer inductance as an energy store to run down in lieu of negative anode current, is what we're left with. In criminology, they say 'follow the money'. With physics, keep a beady eye on the energy. It tells you what's really going on.

In view of the larger transformers needed per output watt to handle the DC flux of a single ended amplifier than what would be needed for a push-pull design of the same power, I suspect the difference in transformer cost will offset some of the difference in cost of valves, holders and fiddly little components. The difference in efficiency will offset some of the extra heater power. I think this is what led to the modest power push-pull valves, the ELL ones.

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Old 26th Aug 2021, 6:44 am   #19
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

That’s an intriguing thought, that for a given performance/quality level, a push-pull output based upon say the ELL80 might have been more economical, due to lower transformer cost, than one based upon say the EL84. A little more powerful as well. The ELL80 had two 2 x 6 W anode dissipation, so in push-pull one might have expected around 4.5 to 5 W AF output, as compared with the 3 to 3.5 W typical of the EL84. Of course, where a piece of equipment was labelled as having a push-pull output, it was implied, or at least often would have been inferred, that it was towards the upper end of the “setmaker” quality range. With the single-ended output, liberties could be taken at the “down and dirty” end of the market without major repercussion.

The ELL80 arrived in 1959. I think, so was too late for the equipment mentioned upthread. Push-pull with two output valves would probably have been out-of-court for many setmaker applications, so for items intended to be better-than-average, but not stratospherically so, the designers had to find ways of tweaking the standard single-ended output. Cathode feedback/cathode loading from the secondary of the output transformer was on the face of it simpler than SEUL, in that there was no requirement for a tapped primary winding in the OPT. On the other hand, it would appear that the setmakers had no problem with tapping the primary for hum cancellation purposes, so perhaps that was a non-issue.

It would be interesting to know Pye’s rationale for choosing a single-ended EL34 output for its Mozart amplifier, rather than say an EL84 UL push-pull pair. Given that it was fitted into what was quite a small case, I wonder if space constraints had something to do with it. The same type of output as in the Mozart was used for the Super Black Box, and advertised as SEUL. Considering that the preceding “regular” Black Box models had push-pull outputs, latterly of the UL type, the change to SEUL was something of a sidestep that might have raised a few eyebrows. In hindsight, it is surprising that it did not invoke some discussion in the letters pages of WW. But hey, trying to find rational explanations for the “why”, after the “what” and the “how” have been discovered, does provide some fun 60-odd years on.


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Old 26th Aug 2021, 9:15 am   #20
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Default Re: Single-Ended Partial Cathode Loading Output Stage?

Hi, I think Pye initially improved the BB by going from PP to ULPP using EL42 valves.
It looks as if this coincided with then removing the shrouds from the transformers, so presumably the extra cost of the taps was negative in comparison with the saving on the shrouds/ pitch dip. It is also likely that they reduced the OP stack by 20% at the same time. As far as I know the total Pri and Sec turns remained the same on the PP and the PPUL versions.

The whole amp was then made cheaper (and nastier) when they changed to PCL83's and really reduced the core for the OPT, as well as now having 3 feedback loops and very marginal ultrasonic stability that could "hoot" at 50KHz or so and over dissapate valves and transformers.

Any thoughts on how well the ELL80 coped with the extra heat in the envelope over the EL84, which was noted for running hot?

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