WINDING PRACTICES, OUTPUT TRANSFORMERS

Over the last 4 years many people have emailed me for my advice about how they could wind their own OPT
for their own DIY amplifiers. I have always give what advice I can, and a few actually succeeded
although many just gave up because transformer winding is a trade needing learned and practiced skills, and
only knowing all about what makes a good transformer as described in text books or elsewhere at this website
will never lead to any really good transformers unless someone is at ease
with basic workshop practices, has the workspace for production, ie, the workshop,
and has a good tool set, and has the time and patience to devote to a time consuming activity.

But to keep my repeated advising time to a minimum, I now devote some time to exactly how to wind a transformer.

The first thing required is a powered lathe. For those without electricity, a pedal powered job will do
since the amount of power is similar to a treadle powered sewing machine that grandma may have used
in 1920.

The lathe speed need only rotate at a maximum of 5 turns per second,
or 300rpm, and one does not need a huge amount of torque, ( hence the pedal powered lathe will suffice. )

However, I have built my own powered lathe. This will then give you both hands free to handle the wire.
My lathe uses some 100mm x 50mm and 100mm x 100mm timber offcuts from building work used to make a
chassis on which to mount a motor box and lathe shaft with pulley belt drives.

Pictures below show some idea of the lathe with a bobbin being wound.

I bought a cheap electric drill for the motor, and clamp mounted it in a box for more silent operation.
It couples with a flex drive to a 40mm dia fan belt pulley wheel mounted on a 12mm dia DRIVE shaft running in two
12mm ball bearings and trunnions bolted to the woodwork frame.

The shaft is plain bright steel bar of 1/2" dia and was very cheap from an engineering supplier.
The ball bearings were also quite cheap from a tractor spare parts supplier.

A second 12mm LATHE shaft about 200mm long also mounted in two firmly bolted bearings has a  200mm dia pulley
for auto fan belt at one end.

The timber chassis frame is like a giant U shaped frame, turned 90degrees so each leg runs east-west
in front of you when you look at it. The motor and drive shaft is mounted on the rear furthermost chassis leg,
the lathe shaft mounted on the chassis leg closest.

The lathe shaft end away from the pulley has a 150mm x 40mm plate about 5mm thick welded on to the shaft
and exactly square to the shaft. The pulley belt isn't too tight, and allows the belt to be sprung off the pulleys if needed
when much unwinding from a bobbin is attempted because there is no reverse direction possible
on the motor. There is a speed reduction between the drill motor and lathe shaft of  about 5 to1.
I have a manual turn handle bolted to the 200mm pulley to allow slow hand turning which is sometimes required to force wires
to be where I want them at ends of layers, because, as you will find, wire tends to lay up irregularly at
the beginning and ends of built up concentric layers.
 
A 100mm long x 10mm dia threaded rod is also welded to the shaft beyond the plate so that bobbins can be
clamped in place between specially cut out plates of plywood to allow wires to be brought out of the bobbin
as the layers are wound on.
I use a few screws in the ply to hold the long free ends of wires to prevent them being tangled and yanked
which would ruin a wound bobbin.
When I began I counted the turns at the end of a layer to confirm I had the right number wound on.
This is quite tedious, difficult and prone to mistake, especially when wire sizes are less than 0.5mm dia
so I made a mechanical turn counter using an old automotive odometer mechanism powered by a copper clad wooden wheel attached
that is exactly 10 times the shaft dia so that for 1 turn of the bobbin shaft I get a reading of 1 turn, with a turn being equal to what was 1/10 of a mile.
By carefully filing down the thick copper wire outside cladding of the wooden odometer drive wheel I was able to
ensure that for 300 turns of the lathe shaft the number 300.0 appears with error less than a fraction of a turn.

Before commencing the layered winding with accurately counted turns, you will have got your turn counter arranged and checked for accuracy by counting the turns from one to 300, and reading what the turn counter says. It should say 300.0
If you get 295.8, the wheel diameter against the shaft is too large, so you may have to file it down; hence I made mine with 1.5mm wire glued
with epoxy around around a plywood wheel of 124mm dia so that I could file it down to be 126.5mm, which is
10 times the dia of the clean bright 1/2 inch dia lathe shaft. I filed the wheel diameter down until I got very accurate
turn counting.

The wheel and counter is mounted on a spring loaded wooden block so it can be slightly swung away from the lathe shaft but is otherwise held against
the shaft with the old spring I found amoung the many old bits of junk in one of my junk boxes.

This allows me to swing out the counter and spin it up or down to a whole number of thousands at the beginning of a new
wind up it make recording the turns easier.

If I reverse wind to undo a mistake, the turn counter winds backwards automatically and accurately.

I think I spent a week building the lathe and it cost less than $200.

I could not find a second hand coil winding lathe and the new ones were way too expensive.
Unless I employed someone to use it all the time to make transformers the cost of a commercial winding lathe wouldn't pay
for itself.
Nobody I know wants to learn to wind transformers accurately for a living unless they gain employment
at one of the dwindling number of commercial winding workshops where mainly mains transformers are
wound repeatedly, and as competition to Chinese imports, which BTW, are usually always
inferior compared to best quality that can be achieved using best materials and neat layer by layer winding methods.

There is no automatic traverse or wire tensioning device on  my lathe. Devising an auto traverse mechanism
where the wire is guided slowly across the width of the bobbin as turns are laid on
was beyond my capability when i made my lathe, but anyone with a better mechanical ability would succeed
at constructing a guide, instead of having to manually lead the wire on by hand and slowly bring one's hand across a hand rest to ensure
no crossed over turns or gaps between turns. So I feed the wire on with the spool of wire
from a bucket on the floor, sometimes with a cloth taped over the wire at the edge of the bucket to prevent
over eager wire loops springing off the spool and tangling or forming a kink in the wire that is then
pulled tight when the wire feeds up through my hand onto the bobbin. I have a wooden rail about 500mm away from the bobbin on
which to rest my hands. This hand rest needs to be about 500mm away from the bobbin for almost all
transformers and chokes, and the leading on of the wire is easiest with this distance.

Speed control for the motor is primitive but it works. There is a foot switch for on and off, and 8 mains lamp sockets
are mounted on a nearby wood block so various numbers of lamp globes or various wattages
can be connected in series with the drive motor, so that I can get from 1/2 a turn per second
to about 5 turns per second.
The extra light from the lamps help me see what is happening while its turning. One has to watch very closely so that the wire layers
are neat, flat, and have the right number of turns, and do not pull down past insulation sheeting at the ends of layers,
and that gaps don't appear..

Apart from the lathe, you will need :-
A Carefully Calculated  design sheet with exactly what is to be wound with the bobbin details set out
on an A4 page beside the lathe so the turns can be recorded at the start and finish of each layer.
If you accidently leave a whole layer of wire out of the design, the work is quite useless  later.
This is likely to happen if you get interrupted in your processes by visitors, phone calls,
debt collectors, or over zealous lady pals.

Side cutters and scissors.
Masking tape and felt marking pen to identify the ends of windings.
20mm x 80mm hard plastic blade sharpened along one end and edge to be an adjust tool when slightly
adjusting wires closer when gaps occur or undoing crossed over turns. Never ever use anything metal to adjust wire positions
on the bobbin.
Pre-cut insulation material.
Tight fitting wire sleeving, preferably fibreglass hi-temp woven auto grade, and not just shrink wrap.
Fibreglass adhesive insulating tape, 25mm wide.
Pre-made bobbins or carefully made DIY bobbins.
Grade 2 winding wire which is double enameled winding wire with high temperature polyester-imide enamel.
Electrical varnish.
Vacuum impregnation tank.
Temperature controlled oven to allow up to 300C.

Alternative to electrical varnish, vacuum tank, vacuum pump and  oven heating:-
Using Estapol 7008 polyurethane epoxy two pack floor coating.

Patience, diligence, willingness, fanatic belief in doing it right or not at all.
Time, for the above, and a good work place.
( Smoke without music is the result of a fool's shortcomings ).

Wind a choke to begin with.

It is impossible for anyone to expect to master the trade of transformer winding in 2 hours.
Once the lathe is fully set up, the first thing to be attempted is a simple one winding smoothing choke.

You will need a bobbin. I try to buy mine already made from high temperature rated moulded plastic
which are available from the small number of suppliers to the transformer winding trades.

But quite often I have made my own bobbins. I start my making a wooden mandrel which is the same dimensions as the assembled core inner tongue.
This has a 15mm hole bored through its centre to allow the lathe shaft to go through. It is vitally important to be accurate
when working with wood or the bobbin with its cheeks will not turn true.
Where the wire bends sharply at each of the 4 corners during the first few layers, the sharpness is reduced by
slightly rounding of the wood at the corners which means the mandrel must have an internal maximum height about 6mm
more than the stack of laminations to be used. To make the base of the bobbin an electrical grade cardboard strip cut to just the right width
is bent tightly around the mandrel and glued with wood glue and held tight with scrap wire turns.
The two layers of 0.8mm thick cardboard ( and 0.8mm is a common size ) will keep the wires at least 1.6mm away from the
core, and when saturated with varnish the cardboard will resist at least 4,000V.
 
When the glue has set, the wire clamp is removed, and the resulting rectangular tube can be slid off the mandrel.
I use 2mm thick fibreglass sheeting for each bobbin cheek plate. It is very carefully cut to size with a hole to take the
end of the cardboard tube. Holes are drilled to allow wires to come out wherever they need to be, but to ensure there is always an available
exit hole for a wire adjacent to a layer end or for a tapping I drill plenty of 3mm holes in the bobbin side to allow wires to exit.
I don't use slots..
In addition to the bobbin cheeks, plates of at least 12mm plywood are cut and drilled to provide support
for the bobbin cheeks, and have 13mm holes for the lathe shaft.
One plate is placed on the shaft, then the mandrel with cardboard tube is placed on the shaft,
then the two cut out bobbin cheeks, with some epoxy glue to hold them to the cardboard tube.

The bobbin cheeks are pressed outwards against the ply plates before the glue sets.
Don't use excessive glue or allow excessive glue to glue the ply plates to the bobbin; you do want to
be able to pull the assembly apart later, but you want the cheeks to remain well glued to the bobbin base.
Applying glue so it **does** get only where you want it is very important.

So the surfaces to be glued both need to be wet lightly, and the easy sliding fit will be filled with glue.
I cannot stress how important it is to work square level and to +/- 0.25mm tolerances so that when you are ready to wind
the assembly will turn true without wobble. Before starting to wind, remove the the glued up bobbin from the
shaft, remove the wood mandrel carefully, and try some laminations, There should be 1mm clearance between cardboard
centre tube and core, and between tops of bobbin cheeks, and E should be able to touch the I with 1/2 a mm clearance.
Obviously, such manufacture is the work of a practiced craftsman if it is to turn out right. To do it well you will need
a good drill, drill set, and jig saw, clamps, and few other tools.

Winding the choke.
This need not be layer wound if the wire is less than 0.5mm dia but can be wound on without making
neat layers or insulation between each layer. The aim is merely to fill a bobbin up with wire to get the hang of lathe working
on an initial project that isn't critical.
The wire is wound on with little guidance so the wires DO cross over each other often.
On a choke the angle of crossing of wires is very shallow, so pressure stresses on insulation remains low.
The wire is gradually  guided by a steady hand so it gradually traverses across the layers to try to keep the surface of the wire built up fairly level and without
sudden wire direction changes. This will give you a useful article at the end and get you used to not using too little tension
and not too much tension and after a couple of chokes you should be used to the lathe, and the way the wire tends to
tangle if you are not careful, and tends to get kinks from loops as it unwinds off the spool on the floor. Arrange the wire and spool so kinking does not occur. If it does with a choke, try not to pull the kink tight, but carefully bend it out straight before proceeding further.
Winding with 0.15mm dia wire is very precarious since it will break so easily. If a wire break occurs, stop to make a join by removing the enamel with a razor blade or fine sand paper and solder the wires, tape around the join
with thin adhesive fibre tape, then continue more carefully.
At the end of the wind up approaches, the level of the wire in the bobbin will have undulations of  about +/- 1.5mm, so make sure
none of the hills in the wire project up past the cheeks of the bobbin or you won't be able to easily
insert the laminations in without having then grind against the wire.
If you have wound too many turns on, unwind them back onto the spool, which means winding the lathe backwards
and winding the wire in sections back onto the spool; this can be a tangling experience, but eventually you will get used to what you have to do
or not do with wire to stop it tangling.

At least you are not in a boat in the dark while out fishing and dealing with fishing line!
You won't catch a feed winding chokes, but you can catch good music if you are persistent.

The layered winding requires more practice and persistence.
The next item one should wind is a layered choke.

For this you will need the insulation material wanted between each layer of wire.
I like to use polyester at 0.05 to 0.1mm thick.
I have bought a roll of umpteen metres from a supplier, and when beginning a new item I cut maybe two metres off the 600mm wide roll and fold it up into maybe ten layers thick and clamp a wooden straight edge down over the layers of insulation.
The straight edge width is a sliding fit between the cheeks of the the bobbin. With a box cutter knife I cut the strips of insulation
about 10 at a time and a metre long. Its a bit wasteful this method, but the polyester  is dirt cheap, and it is heck of a lot better and
easier than trying to mark the insulation and cut strips with scissors!

It is very important to ensure the strip width of the insulation is no more than 0.1mm less than the
distance between the bobbin cheeks, and never more than the bobbin cheek distance.
This way the insulation will lay around the wire layers without crinkling up and occupying too much
winding height which will lead to loose windings, and windings that won't fit under the permissible winding height or into the available
window height of the laminations when inserted. Its precision work.

With OPT, the insulation may be 0.2mm polyester, and stiffer and more difficult to handle but the same approach is used.
The result is that I get  strips of insulation which fit neatly between the bobbing cheeks, and where I want it to be
and in no other place.

The lathe is set ready with a bobbin as described above. An estimate of the turns per layer for the wire size has
been established during the design process allowing for the fact that wire imperfections and very slight gaps between wires
will mean that the calculated number of tight turns per layer will never or rarely actually be achieved, especially if the
wire is less than 0.6mm diva o/a and there are slight variations in bobbin traverse width. So the design should always allow for
two turns less than theoretically possible for wire under 0.6mm.

The in-going and out-going wire entry points should be carefully planned so to allow easy terminations after completion.
Usually this means that one side of the bobbin is devoted to primaries, the other to secondaries. This gives no awkward
1/2 turns added anywhere, although with large power transformers a half turn may have to be
allowed so the correct heater voltage is obtained from the few turns of thick wire involved.
But with a choke winding, there is just one side for the in wire and out wire.
Always start with sleeve insulation on the in wire so it projects 25mm into the bobbin wind area, with 25mm outside.
Allow 200mm of loose wire end outside the bobbin and wind around a screw in the holder plates, and watch that
any wire ends cannot get tangled or gripped in stationery gaps in bearing trunnions etc so thus avoiding
snapping a lead out wire and completely ruining a winding.

Use a choke wire size that will later be useful for an OPT primary say 0.4mm dia.
With neat layer by layer winding, commence winding slowly. Soon the wires will try to cross over each other or you get get gaps.
Stop, unwind crossed turns, and/or adjust the few turns together closely with the plastic blade you have. Its better than a thumbnail.
After some time you will know how to guide the wire to minimize gaps and crossed turns and to be fast about it all.

NEVER use a metal blade or screw driver to adjust layer turns.
Speed does not seem possible at first. I may take 15 minutes to wind 40 turns on properly, especially during
the first few layers where the wire bends sharply 4 times on each turn. When you reach the end of the layer,
apply a small piece of ordinary sticky tape to hold the final turn tight.
The turn count should give about the right number of designed turns. If you seem to have no gaps, and still need to wind two more turns on,
check the wire size. Check that what looks to be gapless is gapless, and try closing up the turns together more tightly with your plastic blade. Often this will give the wanted space for the last two wanted turns. Such adjustments can take the time needed just to get a layer on.
Wind a layer of 0.05mm insulation on and have an overlap of 10mm in an area which is not covered by iron when it is inserted.
cut off the spare insulation with scissors, and tape the insulation overlap tightly.
Consecutive overlaps will increase the height of the wind up
which could make the required wind up too fat to fit into the lamination windows if the over lap and holding tape
is in the wrong place.

The first few layers will be most difficult to wind because the shape of wires under the insulation will tend to cause gaps
between wires, and the wire won't lay properly, so don't use such excessive wire tension that adjustment of wire gaps
and crossed turns is impossible for each 10mm of traverse width.
But you should get the wanted turn count on.

After about 5 layers of fine wire the bend around the bobbin is less sharp and the wire gets easier to manage and by the
15th layer the traveling gets easier but there may be a tendency for layer bulges and bumps at the end of layers to develop.
Also wire may tend to pull down past the end of insulation which doesn't quite reach over to the bobbin flange cheek.
After a few chokes you begin to know just how many turns per layer you should put on and how important it is to
make bobbins with parallel flange cheeks, and cut the insulation correctly, and never to rush a winding project.

With 0.4mm wire on a bobbin meant for 25mm tongue wasteless core, the traverse width will be
about 33mm, allowing about 68 turns. You might get 17 layers on which makes 1,156 turns.
As a layer winding, this may take you at least 5 hrs without the practice that tradespeople develop
who would do this in an hour or two or much less. The problem is that 5 hrs of your time in a western nation could be worth
$150, but a choke could be bought from Hammond Engineering for $25. Spare a thought for the nimble Chinese Person working for $2 per day,
12 hours a day, seven days a week, and driven crazy with turns of wire in lousy
hot noisy conditions with poor lighting and a hard nosed boss. Under such conditions, there is no way
one can be sure there are no crossed over turns or gaps between turns, or a correct turn count, which is why
I NEVER buy any output transformer with a dumbed down design made in asia. Crossed turns mean local pressure
points which lead to short circuited turns and early transformer failure.

For a choke a crossed turn is not a big disaster, although if a shorted turn or turns occur, the
inductance of the complete choke is reduced dramatically, leading to a choke that does not filter. A shorted turn or turns in an
OPT leads to severe bass response problems and probable destruction of the output tubes/devices.
When an OPT fails in an expensive amplifier
it is a real problem because it may be difficult to re-create a spare made to a one off design in 10 years' time.
So hence the OPT must be wound with great care, and used in amplifiers with active protection measures against excessive
tube currents which may overheat thin wire primary windings and power supply choke windings leading to shorted turns
due to heat softened insulations.

The step from winding layered choke windings to an OPT is not a huge step but then you have to learn about having
maybe 30 ends of windings and taps to deal with.
That becomes easy as you learn that 30 connections means that you have 30+ wire ends to cope with so you mark each one with masking tape and label it with a pen so you know where you are as you go.
See the Image 1 below for the way to draw up the winding details to allow the easy winding of an ultralinear
OPT No1 mentioned elsewhere in OPT theory pages.

Image 1.

For the above OPT, when you start winding, set the turn counter at zero and begin with the primary
at anode 2, bottom of the sketch,  and label the wire "1" and proceed left to right and when the layer is finished tape the wire to the side
of the bobbin holder temporarily while a layer of accurately pre cut 0.05 polyester sheet is wound around the
layer and taped into place with a small piece of adhesive tape.
Proceed with the next layer. Before going right across this time from right to left, stop to remove the tape holding the
0.05mm insulation in place and continue to then complete the second layer to connection labeled "2"
which is brought out through a hole in the bobbin cheek and wound around a screw head in the bobbin holder.
It is essential that wire not be allowed to slacken off at any point in the winding process and it must be kept
tight at all times.
Removing the small tape to hold the ends of insulation sheets prevents a bulge developing
after many such tapings during the wind up. Always overlap the insulation ends 10mm and locate the overlap
adjacent to where wire enters and leaves the bobbin lest the bulges from overlapping builds up
the height of the winding too greatly to be able to insert the E laminations.
The process of adding layers of wire and insulation proceeds upwards as shown above.
At the end of each layer, write down the turn number reached and make sure the required number of turns
is achieved in each layer. Having 0.05mm insulation between each P layer which is usually fine wire
between 0.3mm and 0.6mm dia makes it easier to adjust groups of turns together with a plastic thumb tool
so that gaps between turns are avoided, and all turns are pushed up close together all around the bobbin.
No gaps means you should achieve the right number of turns in each layer. Be prepared to find the wire
seems to have a mind of its own and gaps and crossed turns may still occur, especially in the first 1/3 of the wind up
where the wire has to bend sharply around the rectangular bobbin which causes it to easily cross over other turns
or develop a gap, and be prepared to stop, wind backwards a few turns, adjust a gap out, adjust a crossed turn out,
and re-proceed  without tangling or kinking the wire. One needs to be alert.

In the above OPT where any taps in the primary have been designed to occur at the end of a layer,
the layer end is brought out by taping a wire down along a layer, cutting the wire off the spool to allow say 200mm of wire
through the bobbin cheek to a ready screw in the bobbin holder. A label of tape with number is attached.
The 'screen 1' and 'screen 2' connections at "4" and "15" are labeled the same for each in and out wire to the primary.

Where there are taps or ends of windings are brought out from somewhere within a layer of wire, ie,
for secondary connections H, I, J and K,  a layer of fibre sleeving is placed on such a lead out to stop the crossed wires crushing together to form a short. There will be a return wire treated similarly so the winding can continue.
Thin adhesive tape is used to secure such wire lead outs as the subsections of secondary layers are wound on.

When I am done winding, I will have an OPT bobbin with many wires wound around holding screws in the lathe plates.
These wires are all carefully unwound off the screws, and gathered together to allow removal of the plates
and bobbin off the lathe without yanking any wires so tight that a break could happen.
Never force anything.

The Es and Is of laminations should be arranged in piles ready for insertion as soon as the bobbin is removed
and the mandrel carefully tapped out of the centre of the wound bobbin which is fragile, and may try
to bulge and spring apart if not "enclosed" with iron.

There is nothing so boring as stacking in the E&I laminations. It is all too easy to get the correct sequence slightly
muddled with an E leg under another E leg from the opposite direction or one I lam short or one too many.
Check the layers of lams as you proceed each 5mm in height, and redo where mistakes occur.
Have the clamping yokes, taped up insulated bolts, insulated washers, and nuts all ready for assembly.
When assembled, tap up the E&I lams to make the joins gapless and the stack look plumb and square as the bolts are tightened.  When assembled, the bobbin will feel a little loose in the core due to clearances.
Place phenolic scrap plastic pressed in tight to eliminate easy movement and make sure the
end of the windup is well clear of the core.
At this point the circuit boards for terminations should be wired/bolted to the outside of the transformer coils so that
they are well held to the bobbin cheeks with wood blockings or brackets.
The terminals can be turrets but I sometimes use plywood with small brass screws, say No 4 gauge x 1/2" or 12mm long.
These are available from most hardware stores as cupboard hinge screws. Brass plated steel screws are OK.

Where the transformer is potted, a flat phenolic of fibreglass heat resistant board can be placed over the end of the
lams with rows of turrets or screws arranged to face into the chassis area when the item is mounted on the chassis.
Exact details can be chosen by copying well made OPTs buy more serious suppliers.
I never use flying leads of different colours; usually there are far too many terminations on my OPTs to be able to do that and a terminal board is necessary with a removable box screwed down over the transformer.

When the OPT has been tested for termination voltage and phase correctness, it is ready for varnishing or waxing.
Varnish is much more difficult to apply and is best left to someone with a vacuum chamber and temperature  controlled oven.

In my town nobody has either which is easily available, so i made my own vacuum chamber with an old pressure cooker
container and I pull a 95% vacuum by taking a pipe from the chamber to the *intake* of a small compressor I bought.
The oven is a frypan with an extended lid and gives good heat control, although it took awhile to work out what setting of the
temperature dial to use to get an iron transformer up to the temperature wanted to make the varnish cure properly
so that the temperature was uniform within the tranny.
But the second hand vac chamber cost $5, s/h cooker cost $5, and compressor on special since no-one wanted a low power
model from Poland cost $100. I scrounged some auto tubing for vac leads....
Vacuuming the tranny is easy. Place the tranny submerged in a vat of varnish within the chamber and tighten down the cover.
I have a 10mm steel plate 300mm square with rubber seals and 4 x 10mm bolts to clamp over the cook pot.
The vacuum pipe from the cover plate goes into the bottom of a transition glass bottle with sealed top and second pipe
taken to the compressor input. So any fuming or liquid extraction from the vac chamber can be seen in the bottle which will act as a liquid
arrestor to stop varnish being sucked into the compressor which would ruin the compressor.
After a few minutes, the vacuum is as strong as it will ever get and the majority of air is extracted from the transformer
inner cavities, along with nearly all the moisture. The air is allowed to return to the vacuum chamber, and the air pressure
forces the varnish to penetrate inside the transformer to occupy where a near vacuum exists. I repeat the operation 3 times.

During baking, the varnish solvent is expelled by the heat and any tiny unfilled area will be at least wetted with varnish.
The varnish will cure with 4 hours at 125C, which is about 1/2 the temperature rating of the polyester insulation materials used.
PVC insulation will melt and cause shorts so it cannot be used.

Waxing is also OK for OPT and is done  by simply soaking the transformer immersed in a vat of wax kept at
90C for a couple of hours with the transformer placed so air can easily
escape from the many holes drilled in the cheeks of the bobbin. wax is drawn in by capillary action.
Vacuuming isn't needed and would boil the wax. I have used candle wax but it does melt
at 50C, and although a tube amp may have been designed to not have a temperature in the OPTs of more than 10C above ambient, a slight fault could heat a winding and leave a puddle of wax.
And on hot summer days the OPT temperature can climb close to 50C.
The wax does manage to silence the windings from screaming with a sound of their own due to small movements of the wires and iron.
After all, a transformer winding endures magnetically caused forces similar to the forces
in an electric motor where the windings are deliberately allowed to spin. But movement must be restricted in a transformer.
SE transformers usually howl more than PP types due to the gapped core vibration with unbalanced winding currents.
Potting helps stop the noise, but I don't like potting over a waxed OPT.
Many amps I have serviced have had waxed OPTs and have such puddles of wax left by tubes becoming saturated until they eventually became open circuits, sometimes due to the winding fusing open, or short circuits when they caused the mains fuse to blow. Alas, many old OPTs in old amps with thin wire in their primaries would suffer a shorted turn or an open fused winding , regardless of whether they were waxed or varnished.
I have seen this occur in Leak amps, and I have a Luxman amp here with 2 OPTs with shorted turns, and a noisy
mains transformer; its a major amount of work needed for a fix.

I also sometimes use roof pitch for a potting compound which is indescribably messy and smelly
when it becomes liquid enough to pour in around a varnished transformer . The temperature needed is
about 200C, and melting point is about twice that of wax, so it does not soften and flow out of the pot
very easily, and it is a good potting compound which adheres to the potting can which thus is prevented
from vibrating and being noisy.

I find potting with molten roof pitch to be quite good although another smelly process because the pitch
gives off fumes but does need considerable heat to make it melt. I have a Primus camping stove to heat a steel pot
of pitch for potting, and the temperature of the liquid pitch is usually well above 100C and damned dangerous!
But its how they mostly did many transformers many years ago because it was cheap,
and you can SLOWLY heat up a can with a potted tranny and remove it for re-winding if need be.

Quad II amps have a type of pitch which seems to have a type of wax content which makes the compound
have a melt point between pure wax and pitch, but which is sufficiently high if a fault occurs.
This compound remains super sticky, and damps anything loose, but nevertheless I have seen this black goo
leak out all over a Quad II amp from an over heated power tranny after bias failure has occurred.
There MUST be active protection against bias failures in all new tube amps
to prevent damage to precious hard to find power and output trannies because tube saturation from bias failure
may not cause a fuse to blow!

There is an alternative to vacuum varnishing and oven baking !!!

In August 2006 I wound a pair of OPTs for a pair of SET amps using 845 tubes.
I trialled the use of polyurethane two pack floor varnish instead of using electrical varnish.
The product here is known as Wattyl Estapol Polyurethane 7008, and is available in 1/2 litre cans of part A and part B.
This product is a very hard wearing timber floor and bench top coating for general use.
It has good insulation properties and its dielectric constant does not increase the self capacitance of the windings
to any great extent when applied.
When equal volumes of part A and B are mixed together in a clean small container and stirred well the
clear and fluid liquid remains a liquid for about 8 hours at room temperature.
It slowly cures to become a hard plastic which will adhere reasonably well to most insulation materials but not
dissolve them.
I tested polyester and polythene and found no dissolving tendency, and wire insulation is not affected.
After setting up to commence winding up a transformer, a batch of mixed polyurethane
is prepared. Only about 30ml is needed at first. I apply the liquid with a swab made from cloth wound around a
6mm dowel and wired into place; its cheaper than mucking around with a brush which requires cleaning
in a mixture of Estapol Reducer and methylated spirits.
I throw away the application swabs after each fresh small batch of polyurethane is mixed.
The polyurethane is applied generously to consecutive wire and insulation layers both before and after and between all multiple layers of insulation
where say three 0.2mm polyester sheets are piled up to make up the 0.6mm required.
Make sure the workshop is well ventilated and has an extraction fan going to draw away the toxic fumes.

Don't try to thin down the mixed polyurethane; it would ruin the product.
This whole process is indescribably messy, and the polyurethane will try to get everywhere;
it will drip all over the bench, smear onto you hands and gear where it will drive you nuts.
Have a bottle of methylated spirits and lots of clean scrap cloth to clean you hands and gear as required
until things are not sticky.
Be sharp, be alert, be careful and quick, but don't rush and stuff things up.
If you don't complete a wind-up within say 6 hours, STOP, and leave it for 2 days at least before continuing.
BUT, after you stop, and record your turns and where you are, apply the timber blocks and clamp
to the wind-up so that when the curing of the polyurethane occurs over the next two days the
wind-up will have any bulge removed before continuing. If you don't clamp up the wind-up
then the polyurethane will harden and you will never be able to compress the winding later.
When finishing to move again in two days, always finish after an insulation layer and don't paint over it
with polyurethane before the clamping blocks are used. Don't disturb the clamped bobbin during the two days;
the polyurethane fractures easily while 1/2 way along the curing time.
 

The very smelly fumes given off by liquid Estapol 7008 could be toxic to some people or cause an allergic reaction.
Skin damage could also occur if not cleaned off immediately.
A well ventilated work place is essential and an anti chemical fume  face mask recommended.
Gloves will make you very clumsy, so be careful how you apply the material.
Practice makes perfect.
I can say categorically that the process is a complete PAIN IN THE ARSE but the rewards for
taking the extra care to get it right avoids the possibility that the vacuum impregnation of electrical varnish
and baking doesn't quite reach all voids within the wound up bobbin.
Once having applied the polyurethane at a low cost and having taken only 10% more time to wind a bobbin,
the vacuum and bake process does not have to be done at all, and the trauma of heating the completed tranny
does not affect insulations.

When the transformer falls from the pot when stirring it with a poker in the fire, the core with winding can be pushed into the
fire further to heat it till just red, then allow it to cool over night.
Next day the bolts and wire can be sawn off with an angle grinder and the iron is all there for re-use,
and unaffected by the slight additional annealing. The pot may be worse for wear, and may need panel beating
or copying. I roasted a wheel barrow full of old fused or shorted chokes and transformers  in 2003, and now have a stock of hard to get laminations in many sizes for re-use in filter chokes.
There was some GOSS lams in among the lot and the µ of the iron was no different after the firing than before it.

Winding mains transformers is subject to National Safety Regulations laid out in the National Standard Codes of what ever country you
find yourself within and these should all be carefully adhered to before winding anything that is connected to the mains.

Regardless of where you get your information from, if you wind a mains transformer and it causes a  shock to somebody then don't blame anyone but yourself, because you wound it, not me.
The legal systems of most countries will blame you, and nobody else.

The main requirement of a mains transformer from the Authorities point of view is ISOLATION, and SAFETY and to achieve
good isolation a vertically divided bobbin is the surest way. The mains primary is designed to fill half
the bobbin on one side, and the secondaries will fill the other side. Many mains transformers are wound this way but they often use random windings everywhere which is poor quality when one considers that a mains transformer is permitted to have a T rise of 40C above ambient.
Such a T-rise is deadly over time to random windings because of the many crossed over turns
and localized pressure points on wire insulation which tends to crush to cause shorted turn/s especially during a fault
event when the tranny may have a T rise of much more than 40C.
I like to wind all my mains transformers with GOSS and with B = 0.9Tesla max with very well current
rated and nicely layered windings so T-rise is less than 10C above ambient, and the transformer is never highly stressed,
and unlikely to ever fail during the next 50 years. I won't be around to mend one.

Image 2.

A batch of power and output transformers for 300 watt amps on the bench
The clamping yokes are made from aluminium angles. The sizes can be estimated by the 300mm long ruler to the right side.
The two OPTs near the ruler have boards to terminate the ends of the 12 separate secondary windings to get waste-free
load matchings.

Image 3.

Another view of the primary and secondary boards for the 300watt OPT.

Image 4.

In this OPT I have 3mm thick aluminium angle yokes with hardwood blocks with brass screws for the
terminations to P and S windings, P and one end and S at the other.
The sizes can be estimated by the centimetre ruler.  It weighs over 15Kgs.

Image 5.

Here we see the 300 watt amp OPT bobbin being wound on the home built winding lathe.
Behind the G-clamp is a box with the electric drill used for the drive power with lamp sockets on top for
varying the motor speed. The bobbin was hand made and you can see the plywood bobbin clamping plates
with a large plastic handled nut that tightens the assembly on the drive shaft. Just above the roll of masking tape is a hand rest
for resting hands while feeding wire to the bobbin.

Image 6.

Its messy business winding transformers and here we have a picture of a tap being brought out from a 48 turn Secondary
winding on a 300w OPT bobbin. There is fibreglass sleeving and covering tape to keep the last wound turns taught and the tap in position while the rest of the layer is completed over the black coloured insulation material.

Image 7.

Here is a close up of  the 300w amp power transformer with the two hand made bobbins for the OPTs showing the timber mandrel
inserted into one bobbin. The empty bobbin has grey electrical cardboard former and you can see the white fibreglass
cheek plates and all the holes to allow the wires to enter or leave at whatever height is needed.

METRIC WINDING WIRE SIZE CHART
The metric winding wire sizes were kindly given to me by a local Sydney wire and transformer parts supplier,
Blackburn Electric Wires Pty. Ltd. 55 Garema Circuit, Kingsgrove, NSW 2208. Ph. (02) 9750.3133 Fax. (02) 9759.0245
They do not appear to have a website but are VERY good to deal with by mail order.

The original chart contained the same copper sizes as shown for grade 1 with less enamel thickness
and grade 3 with more enamel thickness. I only use grade 2 which is the only grade shown in the chart below.
Grade 2 is the main grade stocked by my supplier because it is the industry norm for 99% of high temperature rated winding wire for electric motors
and stressful industrial applications. Trying to get triple insulated winding wire is almost impossible so making copies of
McIntosh OPT is difficult.

The range of sizes shown are not all obtainable off the shelf, and to get some sizes a wait for an order is involved,
so I sometimes have to design around the wire size available, which adds to the challenge.
Anyone not used to measuring in millimetres better start getting used to metric because here the
diameter measurement matters more than the wire guage, and there are is AWG, SWG, BS, all very confusing,
and I don't have conversion charts so if you work in guages and inches and feet, provide your own solutions.
Before winding anything, make sure you have an accurate micrometer to confirm that the size is correct.

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