Transformer rewinding

[Mick Bailey]

The increasing failure rate of modern transformers and the increasing replacement cost has brought me round to thinking about the feasibility of setting up for rewinds. The commercial rewind cost very often exceeds the price of a new replacement, though sometimes the rewound transformer can accommodate a higher spec for the winding - particularly OPT primaries and PT HT secondaries - and work out better value in the long run.

It could be that the investment of time and money doesn't work out economically, but a £170 transformer sitting by my feet with just a shorted HT secondary to CT, plus boxes of OPTs with shorted turns seems a compelling reason to at least investigate the subject. I'm not talking about toroidals though.

Who's currently doing this? I'd like to hear about the equipment you're using and techniques, and just get some basis on which to either further my interest, or to abandon the idea.

[BLS Transformers]

The question is, will you be reusing the parts from the old transformer or making new ones? It's a lot more economical for a transformer maker to use new parts, unless it's an old or obsolete pattern of core laminations and/or bobbin and I much prefer 10 minutes of calculator time to 2 weeks soaking in solvent, followed by half a day of stripping out laminations, unwrapping windings and trying to count turns (particularly if it's a "scramble" wind and the wire keeps snapping, like most HT windings). Rewinding a transformer in that way does lead to a case of "it's not the parts, it's the labour, guv". (Even making a one-off from a design's quite labour intensive, hence price breaks).

I would say, if you're going down that route, that either cultivate your friendly local transformer maker for parts, or be prepared to spend quite a bit at the component supplier of your choice, building up some decent stocks. Also, if you do rewind, will you be using a machine or a hand winder? Winding a 700V CT Ht winding with a hand-cranked machine can get awfully tedious, awfully quickly.

Hope I've been of some help.

Simon

[Mick Bailey]

The reasons for rewinding would be the high replacement cost of an identical transformer due to an unusual or custom sizing, or if it's obsolete. The trade pricing is still good with some mainstream amp companies where stock is still available, mainly due to the sheer number they had wound in each batch. Sometimes Hammond transformers can work out OK. I have to look at the whole economics of the process to determine if it could ever be a justifiable cost model. It may be like making my own jam and then thinking about making the jars as well.

When repairs get expensive because of prime-cost items, I'm faced with the customer not wanting to undertake the repair, or having to cut my labour rate right down to subsidise the whole job - often to the point where I would have been better off not doing it at all, or better off getting a paper-round. It could be that doing rewinds becomes just another form of subsidy and my hourly rate simply gets squeezed in a different area to make the job come out at a sensible price.

Machine winding would be a necessity. I already have a hand-cranked winder for radio coils and pickups and it's no fun at all.

[R.G.]

I have been involved with transformer design and building since about 1973 off and on. I never worked in a transformer factory, but did design them to be built elsewhere. Making one of a transformer is labor intensive.

Rewinding a transformer is easy enough once you've done a time or two, but it's a lot of work. It can easily take eight work-hours for a skilled hand-winder to make one fist-sized E-I output transformer. A split-bobbin power trannie is much quicker, but it's also not the kind of thing you'd rewind.

Anything much below $50 an hour is a non-starter for bench labor, so you're looking at having to charge $200-$400 for rewinding to avoid giving subsidies, as you mention.

I'd say that you're right - rewinding is something that only makes sense where there's a customer willing to spend the money for a hand-rewind of something that even MORE expensive to get an original of, or something that's simply not available.

That being said, I'm something of a nut about Thomas Organ Vox amplifiers, and I'm in the process of writing up a how-to on building a driver transformer suitable for the Buckingham/Viscount, Royal Guardsman, and Beatle amps from a small power transformer. As part of the process I reverse engineered the originals from the outside without de-winding one to find out the insides, and developed a winding scheme for re-making a workalike.

In doing that, I had to come up with a manual coil winder that your garage hacker could use to actually do the windings. I think I have one that is suitable. But that doesn't help you if you demand a winding machine.

If you're interested in a semi-manual winder, there are some decent looking ones on ebay right now for $60 -- $90 bucks, but they are hand cranked. You'd have to rig a drive motor to turn the crank; not too tough, but necessary.

Perhaps the biggest issue is access to materials. You'll need high temp tape, high-temp layer insulation, wire sleeves and short-oil non-urethane varnish, as well as a way to pull a vacuum on the trannie while it's being impregnated.

[bob p]

> "it's not the parts, it's the labour, guv".

reading this thread is giving me flashbacks about the time that I hand-wound a replacement stator for a Buell 1125. It was pure drudgery. I wouldn't wish it upon anyone.

having done a handwound stator, i'm cringing at the thought of hand-winding transformers.

[Mick Bailey]

I've rewound interstage transformers, solenoids, pickups, reverb transducers and other small-scale stuff. I've also made a few small specialised transformers from bobbin/laminate sets. Very often the time involved is either excessive because it's for myself, or only worthwhile for a customer because the equipment is rare and the parts almost impossible to obtain. I can pull a decent enough vacuum to impregnate the windings - I've built a few 100kv oil-filled capacitors that needed pumping down and have a good range of workshop machinery and tools to do most jobs, including building other machines.

The British transformers from the late 50s/early 60s don't look too bad to get apart - they appear to be impregnated with a shellac-type material as it dissolves readily in alcohol. Perhaps those could be a realistic prospect. I would expect some of the more recent materials are tougher to soften to a degree where the transformer could be stripped. How many spools of wire, how much varnish, how much tape? Seems like it could be a large stockholding to buy enough to get the material cost down. Then we come back to time, developing skills, learning from mistakes, understanding the material behaviour and performance, product liability, HiPot testing. The more I consider it, the more 'gotcha's' come to the fore.

It all points to the Time/Quality/Cost model - you can optimise any two elements at the expense of the third.

[BLS Transformers]

Older transformers can also be impregnated with wax, which is even easier to remove. If the old transformer's burnt out, the job's half done for you. Newer transformers with polyester varnishes ideally need to be soaked in a bucket of solvent for a few days and even then they can put up a bit of a struggle. The xylene solvent's not terribly nice either.

As for stockholding, put it this way, I have 150,000 transformer bobbins in stock. I know, I counted them all 2 years ago, plus laminations from 20mm at the widest to 200mm. And fixings, and flexes, and copper foil for the screens and hum bands, plus tape and eli-meli for interwinding insulation. Ideally an oven for drying/baking, dependent upon the impregnation method. This is of course for industrial as well as amplifier work. Usually, if there's something odd about the transformer, I can find a workaround.

Yes, time/quality/cost is a complicated equation. Sometimes it boils down to "Do you want it cheap, or do you want it good?"

[R.G.]

eli-meli for interwinding insulation.

I'm not familiar with the term "eli-meli". What's that?

Yes, time/quality/cost is a complicated equation. Sometimes it boils down to "Do you want it cheap, or do you want it good?"

The way I always heard it was "Fast, good, cheap: pick any two."

[Tom Phillips]

... duplicate post...

Why not just delete it entirely?

[R.G.]

Dang! You're right, I CAN!!
I'm used to forums where you can never delete posts because of some radically antisocial activities in the past.
Thanks!

[Ted]

If I absolutely have to get a transformer rewound I try to meet my transformer winder half way - stripping the transformer and separating and cleaning the laminations. There's not much point doing this if you can't get off-the shelf bobbins for your rewind. A lot of older lams and their associated bobbins are not available anymore. Most transformer manufacturers will make you a custom bobbin for an unusual lamination, but that's yet more money.

Transformer manufacturers are usually capable of designing you a transformer to spec. This is often the cheapest and most painless route. If you have a customer who sincerely believes that not using original 1950s laminations will completely alter the tone of his/her amp this may not be possible. I do notice that transformers designed this way are often significantly smaller than the originals. I'm happy to believe that this is due to improvements in insulation materials, varnish and laminations in the last 50 years. I've never had a new design transformer fail, but given the small sample size that proves almost nothing

[Mick Bailey]

I get plenty of newer transformers failing - more so than the old ones. They tend to be in small amps that have minute transformers driven hard - mainly OPT failures with shorted turns. I used to discard transformers but have been saving them for the last year or so just in case they ever become economical to rewind. The Marshall 20/20 collection I have looks like a candidate for experimentation as they have such a small bobbin and a fingernail can part the laminations without any effort.

I like the idea of meeting the winder half-way. If it's a choice between watching TV (electronic income reduction) or sitting at a bench, I'd always rather be doing something worthwhile such as stripping a transformer.

Mr Fahey has posted quite a bit of information on modern insulation materials in transformers - the contrary to what you may expect. I have to wonder why a wax/paper insulated transformer from a 50's Tweed Deluxe can survive for 55 years, yet a MM transformer from a reissue, with modern insulation, materials, process and quality control, shorts out with no other fault on the amp.

Mainly the customer won't have an issue with a replacement transformer, but some amps would need the chassis drilling or cutting to take the revised hole pitch or drop-through cutout and this could affect the value of an old or rare amp so it's something I don't do. Many new standardised transformers are either just too big, or just too small, or don't have voltage selectors in the end-bell, or something else different.

I'm getting a clearer picture of why a rewind is so expensive, but the idea of a 12lb transformer (and all the others) uselessly sitting there with such a 'minor' fault as an internal short on one winding is frustrating.

[bob p]

> I have to wonder why a wax/paper insulated transformer from a 50's Tweed Deluxe can survive for 55 years,
> yet a MM transformer from a reissue, with modern insulation, materials, process and quality control, shorts
> out with no other fault on the amp.

It's tempting at first glance to take a simple approach, and say that the new MM transformers just aren't all that good. Maybe they are, maybe they aren't. I've had my share of QC control problems with MM products. They did a great fill for me on my first order of 24+ units to capture my business, but my second order of a similar quantity looked like it got filled with all of the rejects/seconds that they had lying around the shop. The result has been that I haven't gone near that supplier for well over a decade. Their transition to a boutique marketing scheme that was accompanied by insane price hikes has made it easy for me to continue to avoid them.

but to be fair, it's probably not fair to blame MM when new production transformers fail when they're being abused. There may be a selection bias in how these OTs are being treated by end users. Since the advent of the attenuator, end users are a lot harder on amps today than they were decades ago. If someone has an original 1950s Tweed Deluxe, they're likely to be easy on it because of it's collectible value. OTOH if someone buys a new production MM transformer, it's viewed as a disposable item that can easily be replaced, so there is little impediment for the user to flog a new production OT to the point of failure. So maybe it's not a problem with inferior materials or construction. Maybe it's just that people are intentionally abusing the new iron because they're not worried about not being able to replace it.

[Mick Bailey]

I do know the history of that particular amp and it's been cosseted from new due to the price in the UK and only used for a limited periods. It still has it's tags, original cardboard box and invoice. The owner is over 60 and it never gets turned above 3. Maybe it's just an isolated case - there aren't many reissue Tweed Deluxes around over here so I don't have any other comparisions.

In the UK a MM transformer is a high-ticket item (price but not necessarily quality). 5E3 transformers appear to attract a premium and I wish UK manufacturers would build dimensionally-accurate transformer sets that appear to be commonplace in the US.

I'm puzzled why so many OPTs would fail with shorted turns; to my understanding the voltage relationship between adjacent windings or layers is derrived from the total voltage across the winding divided by the number of turns. Now, even accepting the flyback effect of a PP amp effectively doubling the B+, this still makes the volts/turn fairly low on an OPT primary - certainly within the limits imposed by the enamel insulation and any paper insulation between layers. I'm making an assumption here that the transformer was designed with an effective safety margin and that it can operate continually at its rated output. Perhaps manufacturers are squeezing designs by reducing wire gauge and core sizes, or the wire is getting stretched due to high-speed winding techniques and this is introducing cracks in the insulation.

[Steve Conner]

First of all, there are several standards for transformer sizes and mounting hole locations that don't quite interchange. I got my first inkling of this when I called Sowter looking for a replacement PT for my 1960 vintage Crown stereo amp. When I gave him the measurements, he said it was "metric" and he didn't do those sizes. But a Hammond replacement part for a Marshall JCM-something dropped right in.

Secondly, shorted turns. The old transformer winding technique was about building the windings up layer by layer, with layers of insulation in between. Modern transformers use a chambered plastic bobbin and in many cases the wire is just wound randomly into the chamber. It is quite possible for the odd turn to slip down the side of the general mass of wire, and see a much higher voltage stress than the total voltage divided by the number of layers would imply. A good output transformer should still be wound by the layered method, if only because it needs interleaving. But not all guitar amp OTs are "good".

I've never heard of Eli-Meli either, but I'll hazard a guess at Electrical grade Melinex, a brand of polyester film similar to Mylar.

A parting thought: surely an authentic British 5E3 would use an 18-watt Marshall transformer set?