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Old 6th Aug 2022, 4:52 pm   #1
thermion
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Default Williamson amplifier. Winding an output transformer.

It’s been a while, 7 years in fact ! Since I was last on here.
A lot’s happened in the meantime, sadly radiophile is no more, I was a regular!
At gnosall.. after several vicissitudes & a bad run in with Covid, a house move, I’ve set up a little shack in a spare bedroom & am playing around with valve radios & building amps again.
I’m building a Williamson amp to match the one I made years ago, got the chassis all machined, & mains transformer & chokes etc, but am struggling with the output transformer! My god there expensive! Is there anybody does coil winding nowadays? I’ve got a lovely big c core transformer all stripped down ready for rewinding as an output transformer. I think it ought to be suitable for use as an output transformer, as it has thin lams. It’s a bit bigger at 2”X 11/2” centre limb bobbin size by 3 1/4” wide across the cheeks. Williamson core size is 1 1/2” X 11/4” x 3” originally. I’d have a bash at winding it on the lathe, but this is a bit complicated for a first attempt at coil winding.
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Old 6th Aug 2022, 5:36 pm   #2
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Default Re: I’m kind of back..

Mike Barker (Murphymad) does rewinds, try him?
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Old 6th Aug 2022, 6:38 pm   #3
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Default Re: I’m kind of back..

Yes, I’ll look into it, I’m at the build stage where I need to decide on the output transformer position. As I say I would like to re use the c core transformer as it’s physically similar to the one on my other amp.
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Old 6th Aug 2022, 8:19 pm   #4
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Default Re: I’m kind of back..

Maybe this ought to be in the audio hifi section, I was sort of combining the two
Posts here.
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Old 6th Aug 2022, 9:29 pm   #5
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Default Re: I’m kind of back..

Thread moved.
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Old 7th Aug 2022, 8:59 am   #6
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Default Re: I’m kind of back..

Output transformers are critical parts of valve amplifiers, and through their limitations set what everything else in the circuit can do. Their phase lag at high frequencies sets the amount and bandwidth of feedback which can be employed. Even if got stable, this still affects ringing and the damping rate of ringing. So, if you're building a second amplifier to match an existing one for a stereo pair, you/ll need a matching output transformer.

If you know the winding recipe of your existing one, and you can get the same laminations, then one can be wound. As a one-off, with no series production and economies of scale, it won't be cheap.

If you go looking for a period transformer of the same make as in your existing amplifier then you will run into competition, lots of other people are also looking. Worse, these other people include cultist nutters who have to have fashionable amplifiers at any cost. This means they either have deep pockets, or spend all they have on their passion.

You'd think the price of period transformers would be limited by the price tag of having a new one wound from scratch. But in the eyes of the cultists, any new one lacks the magic fairy dust that only the originals have, and they can hear the difference. So they have to compete for any originals available.

Unless you are very lucky, having one wound is probably the way to go. If you have any uncertainty of your existing one, getting a pair wound will keep your amplifiers similar.

Williamson's designs were an important way-point in audio amplifier design, and you're lucky enough to have one. But mono was also the dominant format of that era, so you're already there as far as historical artefacts go. If you want to listen in stereo, you could treat it as a separate system and that gives you the freedom to explore some of the progress made since Williamson.

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Old 7th Aug 2022, 9:10 am   #7
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Default Re: I’m kind of back..

Sowter transformers make o/p tx's for the Williamson amp.
John.
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Old 8th Aug 2022, 12:07 am   #8
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Default Re: Williamson amplifier. Winding an output transformer.

thermion, do you have much in the way of technical details on your existing output transformer, as David points to ?

I'd also suggest making sure your existing amp is technically ok. Apart from the original Williamson (ie. WW 1947 and 1949), there are many many commercial variants and even more diy attempts. Given a little birdie said your existing OPT was UL, then you may have cloned a commercial amp or tweaked your own - which imho may be quite risky unless you were able to adequately test the amp's performance?

My concern would initially be if your existing amp wasn't 'bullet-proof' for stability or over-current fault protection - as both of those technical issues reflect back on the OPT design and survivability.

Another comment would be that if you don't have very intimate details on the construction of your existing OPT (right down to the type and thickness and lay-up of the insulation between windings) then the feedback design of each amp could end up being different, and you may not know that until testing time. One thought would be to make two new identical OPT's and go from there, but that is just from a purist's point of view and likely not very practical for you!
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Old 8th Aug 2022, 6:34 am   #9
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Default Re: Williamson amplifier. Winding an output transformer.

Exact details of Williamsons OPT's are easy to find on the net. Winding one exactly the same way, with a winding machine and LOTS of experience is a no-no unless you have limitless patience and time.
I have wound ONE pair of traffos to Williamson specs, at the same time. By that I mean I completed one bobbin then the next bobbin to make them as close as I could to "excatly " the same.
REmember the origional Williamson is NOT an Ultra linera amp, but a strapped tetrode amp ( making them triodes ). Williamsons origional traffos were made using two identical bobbins with exactly the same turns and insulation and terminations, BUT on assembley one bobbin is fitted " upside down " so that its in phase with the first. REmemeber there are five primaries and four secondaries on each bobbin, that equates to 32 "ends " of the wire to terminate correctly and without short circuits. My set of transformers were pretty good but NOT identical!!!
N.B. Note Well !!! NOT identical. I wound my first transformer about age 10 or 11 under my fathers guidance and he WAS a master engineer in regards to transformers. I have six months to go till I am 70. I have made many hundreds if not thousands of transformers. Origional Willamsons are VERY difficult if you follow the plan exactly. In fact, I would not attempt it at my age OR experience.

Ultra Linear versions were mostly from Hafler and Keroes fame who did make them "similar " to Williamsons, but they discovered that two bobbins means big mobs of leakage !!, so they devised a method of interleaving that did away with twin bobbins and the leakage and actually improved the origional design a great deal.

If you are very serious about making a stereo Williamson, I would start with buying the correct laminations. That in itself is not that easy. I "believe " but cant remember exactly, but were known as SuperSilcor lams. The predecessor to modern high quality laminations.
Today, the iron I use for output transformers is made in India, John Sankey, the Australian producer of SuperSilcor stopped making iron stampings about 25 years ago.
So, the iron I buy is called "Grain oriented 5% silicon iron of 14 thou or .35 mm thick.
Even to buy that my minimum order was 50 kilograms. A far better transformer could be constructed similar in every way EXCEPT the thickness of laminations. The thinner a lamination is, the better it is for an output transformer.

Problem number two is the copper wire used. Yes they make some pretty good wire these days, but the origional varnish is no longer used. Today its almost ALL poly urethane plastic solder thru insulation. Its probably better for pin holes ( tiny holes that are made in EVERY type of insulation ) But I find it makes for bigger interwinding capacitance. ( something to be avoided ).

VERY nice bobbins are available today made from super high quality fibreglass. You can do what I did too, which is etch the copper off copper clad circuit board material, and cut up the fibreglass to make the bobbin size you require. To fit the centre leg if you will, which is the width of the centre limb of the iron and in the other direction, the height of the stack of laminations.

Next we need the interwinding insulation. Traditionally that is specially manufactered paper that is varnished and rolled to very precise thickness. It was called "onion paper ". Today I cant buy it in thicknesses I ideally require so I do what "most " other winders do, and that is to use yellow polymer plastic tape. It works, but doesnt come close the the accuracy of onion paper. Or the sound, but that COULD be due to my audiophoolery.

When you finish the winding ( from memory again so I may be corrected ) its something like 4800 turns per bobbin. Thats primaries and secondaries. Times four!!!.
You then need to "finish " the transformer properly. Cheaper transformers used high purity wax. Today we use baking varnish. Why baking ?, because it needs baking for 24 hours at about 100 degrees C in an oven.

Thats after you have pumped all the air out of the constructed transformer. If you service airconditioning, thats easy!!. Just hook up your vacuum pump to you handy ACME vacuum chamber and take it down at LEAST ten atmospheres. The better quality vacuum you can achieve, the better the varnish impregnation will be, and the better insulated transformer will be.

When the baking is finsihed you terminate all the wire ends remembering of course EXACTLY which one is which ( or your transfomer will be a total failure ) and add either flying leads or soldereing to a transfomer tagstrip, which you will have to make, to suit the style of transfomer you have built, as well as some form of outer protection such as end bells.

There are a few of my handwound output transformers floating around this forum, and so far I havent had any feedback. That means they at least work!!. If they didnt I would have been smeared from here to eternity by now.

I hope my slight tongue in cheek lecture in transfomer construction was of some help.

My best wishes

Joe
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Old 8th Aug 2022, 8:26 am   #10
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Default Re: Williamson amplifier. Winding an output transformer.

And that, is why good output transformers are expensive.

David
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Old 8th Aug 2022, 2:43 pm   #11
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Default Re: Williamson amplifier. Winding an output transformer.

Interesting and comprehensive past from Joe!

If I may comment:

Quote:
Originally Posted by joebog1 View Post
The thinner a lamination is, the better it is for an output transformer.
To a point, yes... HF losses are reduced. Though as the turns number has to be adequate for the lowest bass frequencies, the flux density at HF is really small, so core losses are minute anyway. In fact, if you use super-thin lams, you end up with less iron because stacking factor is reduced, so performance starts to fall off anyway!

Quote:
Originally Posted by joebog1 View Post
Next we need the interwinding insulation. Traditionally that is specially manufactered paper that is varnished and rolled to very precise thickness. It was called "onion paper ". Today I cant buy it in thicknesses I ideally require so I do what "most " other winders do, and that is to use yellow polymer plastic tape.
I've come across Kraft paper at 0.75mil thick (18μm) but I generally use 2mil (0.05mm) Nomex which is slightly conformable, very tough, accepts impregnation, and has super electrical insulation and temperature withstand properties. And it's chemically inert.

Quote:
Originally Posted by joebog1 View Post
Just hook up your vacuum pump to you handy ACME vacuum chamber and take it down at LEAST ten atmospheres.
Hmm... The best I can ever expect to achieve is ONE atmosphere of vacuum!
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Old 8th Aug 2022, 2:59 pm   #12
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Default Re: Williamson amplifier. Winding an output transformer.

Perhaps a modest 1/10 of an atmosphere was meant.

I suspect air conditioning engineers don't need to work to cryogenic vacuum levels of maybe 10exp-9 atmospheres
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Old 8th Aug 2022, 8:26 pm   #13
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Default Re: Williamson amplifier. Winding an output transformer.

Quote:
Originally Posted by kalee20 View Post
Nomex which is slightly conformable, very tough, accepts impregnation, and has super electrical insulation and temperature withstand properties. And it's chemically inert.
Used in fireproof suits. That good!

David
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Old 9th Aug 2022, 12:51 am   #14
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Default Re: Williamson amplifier. Winding an output transformer.

Well, thanks for all that, I think I’m sorted now, I found a similar, but slightly bigger c core mains transformer among my pile, stripped it down & it’s off to be rewound as a Williamson.
It ought to do the job, big chunky core & nice thin lams.
Actually I have 2 Williamson amps, one looks homespun with elstone transformers,
But the one I’m trying to copy is basically a rebuild of a goodsell Williamson I think it was originally, I machined up a new chassis for it on my Bridgeport & remounted everything on a nice thick machined aluminum chassis. The transformer does have screen grid taps, yes. I’ll see if I can attach a pic of it.
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Old 9th Aug 2022, 12:54 am   #15
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Default Re: Williamson amplifier. Winding an output transformer.

Oh dear, it’s come out all sideways!
Anyway, this is the transformer I’ve sent away to be rewound.
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Old 9th Aug 2022, 1:18 am   #16
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Default Re: Williamson amplifier. Winding an output transformer.

Maybe I ought to qualify all this by saying I’m not an audiophile in any sense of the word, but I do enjoy collecting & making different amps.
I’m not looking for precision matching of my existing amp, just trying to make it as near as possible with what’s to hand. I’ve scrounged the aluminum for the chassis, & machined it as per the original I made about 10 years ago. I’ve got some nice oil filled mains transformers & chokes, paper condensers etc, all I was short of was the output transformer. So seeing as this is something of a recycled affair anyway, rewinding an old transformer is rather in keeping. That & im Not lashing out mega money on an original partridge/ parmeko transformer. If it sounds good when it’s done i’m happy
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Old 9th Aug 2022, 2:05 am   #17
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Default Re: Williamson amplifier. Winding an output transformer.

Right way up.

Joe
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Old 9th Aug 2022, 3:34 am   #18
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Default Re: Williamson amplifier. Winding an output transformer.

Thermion, do you think your Goodsell was a GW-18 ? Are you saying the OPT in that amp is original, and has UL taps, and that is the OPT you want to clone for your new amp?

It may be worthwhile to determine what your 'Goodsell' amp has in it. Do you know if it uses exactly the same part values as the 1947 W circuit, or are there some modifications identifiable?

Williamson started to walk the path towards unconditional stability by adding a first stage step network, and it was sad that so many early amps exhibited poor performance and even killed speakers. Nowadays we have ready access to tools that can confirm unconditional stability - have you been down that path?
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Old 9th Aug 2022, 10:56 am   #19
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Default Re: Williamson amplifier. Winding an output transformer.

I’m saying it was a goodsell, as that’s the closest visually I could find. As I got it, it was on a very tatty chassis beyond tarting up, & smothered in gold paint. It used KT 66’ & has all 8 secondaries brought out to a tag strip. Upon rebuilding on the new chassis the only problem I had was a very low frequency oscillation of about 2-3 hz. It will still do that if severely overloaded but at sensible output it’s just fine. I added the top cut components to V1 & that seemed to tame it nicely.
Strangely, I’ve not measured it unto death, it being so heavy, I must put a scope on it one day, & sweep the output. It just sounded right from word go. It’s by far the best sounding amp I have when driving a goodmans Sherwood speaker. Circuit wise, it’s built exactly to the Williamson circuit with all the balance pots etc. the only difference is the output transformer had SG taps. If I get time today I’ll put a few volts ac in the primary & see what we get at the anodes & SG taps. See what the impedance is. & try & work out the SG tapping point
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Old 9th Aug 2022, 10:59 am   #20
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Default Re: Williamson amplifier. Winding an output transformer.

Sorry. I meant a few volts in the secondary
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