Materials for winding transformers.

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TerryG

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Well obtaining supplies to wind transformers isn't easy but with persistence is possible. eBay and Amazon have been wonderful sources as well as Temco Industrial. I would like to deal directly with more suppliers, but you get what you can where and when you can.

A couple of questions I wanted to run up the flag pole to see if anyone here knows anything.

1) Anyone know of any disadvantages of using 200C magnet wire compared to 150C? Is its insulation coating thicker?

2) Is the primary reason for tape between windings to reduce arching in a transformer? And subsequently, in lower voltage smaller transformers would this even be necessary, like in low voltage audio applications?

3) When does multiple interleaving start to become counterproductive by introducing other problems or does it? For example, the most basic interleaving is 1/2 primary, the secondary, and the final 1/2 primary. But I prefer 1/4 primary, 1/2 secondary, 1/2 primary, 1/2 secondary, 1/4 primary interleaving. I am wondering what all the advantages and disadvantages are with side-by-side windings. Where you have a bobbin that is split to separate the windings. To me side by side would be less capacitance but more losses.

In some ways, I am finding experience to be the best and only teacher in this game so far. So I try stuff, measure, test, and then try something else. Sure seems to be the slowest most laboriest teacher, but what choice do I have.
 

Tubetec

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Theres a few different formulations used in magnet wire insulation depending on the temp rating , the stuff used for motors might have more than one coat applied . One of the issues that can arrise with the higher temp stuff is by the time you've burnt or scraped off the insulation the wire is very fragile . Not so much of an issue with thicker gauges but definately a problem with the thinner stuff . One way around this is to double up and twist the wire where the output cable joins . Obviously something like a pickup coil uses very thin wire ,but it doesnt need to withstand any dc voltage , so lower temp stuff is very adequate there. Likewise output transformers destined for use with solid state audio circuits which typically have lower voltages than tube equipment wont require the highest strenght dielectric material either. In tube o/p transformers with high voltages the higher temp stronger insulation is well worth factoring in to your design .

Tape between two different windings with high potential difference definately help prevent the insulation getting damaged and subsequent arcing or flashover . Sometimes a few rounds tape is placed between layers of the same winding , it does help seperate wires with different voltages but also can help to reduce capacitance a bit .

The question about winding geometry/interleave maybe Bill(Mr CMRR) might chime in on . It does seem like the best performing transformers use several sections ,Ive seen as many as 8 sections on the secondary on Williamson type transformers , Partridge also did this in their designs. One of the benefits of multiple secondary sections is it allows either series, parralel or a combination of both to match to different load impedences .
 

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abbey road d enfer

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2) Is the primary reason for tape between windings to reduce arching in a transformer?
Yes, but it also helps maintaining the winding taut when adding the other layers. .
And subsequently, in lower voltage smaller transformers would this even be necessary, like in low voltage audio applications?
Not at all. For example teh bifilar and quadfilar xfmrs have no insulation between windings (other than the coating)
3) When does multiple interleaving start to become counterproductive by introducing other problems or does it? For example, the most basic interleaving is 1/2 primary, the secondary, and the final 1/2 primary. But I prefer 1/4 primary, 1/2 secondary, 1/2 primary, 1/2 secondary, 1/4 primary interleaving.
There is not one single definitive answer. Mc Intosh tube amps xfmrs have an impressive number of sections, for a good reason. Utmost coupling is necessary for the stability of the output stages that are under a heavy dose of NFB. For SS circuits, I had a custom-built quadfilar xfmr. At the other end of the spectrum, you have isolation xfmrs, where the pri and secondary are on separate bobbins.
I am wondering what all the advantages and disadvantages are with side-by-side windings. Where you have a bobbin that is split to separate the windings. To me side by side would be less capacitance but more losses.
Indeed. These types of arrangements also often involve other winding techniques, such as honeycomb. They would be used for inductors, not so much for xfmrs.
 
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CJ

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this is where i buy tape:


McIntosh does wind pri and sec wires side by side on some of their output transformers which run at tube plate voltages.

4-5 is the usual cutoff for sectioning for audio, put your fingers together minus one thumb.
this was used a lot by Peerless. Twin coil stuff requires less sections.

you can get Nomex on ebay

you can use paper grocery bags for Kraft Paper. they come in different widths depending on size and what store you get them from. you need to allow a little extra dryout time at 200 F to get rid of the moisture.

i stay away from the 200 F stuff unless it is heavy wire for power outputs which can be easily scraped with a blade of some sorts.
 

MisterCMRR

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The question about winding geometry/interleave maybe Bill(Mr CMRR) might chime in on . It does seem like the best performing transformers use several sections ,Ive seen as many as 8 sections on the secondary on Williamson type transformers , Partridge also did this in their designs. One of the benefits of multiple secondary sections is it allows either series, parralel or a combination of both to match to different load impedences .
Sectioning is all about leakage inductance (the relative looseness or tightness of magnetic primary-to-secondary coupling). For an output transformer, leakage inductance will kill high-end response if cable capacitance is the load - so tight magnetic coupling is essential and multi-filar winding is the ultimate solution. For an input transformer, the trade-offs are more complex. To have decent CMRR, there must be a Faraday shield (a single turn of copper foil as wide as the inside of the bobbin - with its start and finish insulated so it doesn't form a shorted turn!) between primary and secondary at each layer boundary. Fortunately, the secondary of an input transformer generally has minimal capacitive load - but the insertion of Faraday shields increases leakage inductance, often to the point of unacceptable HF bandwidth. So multiple interleaved layers become necessary to reduce leakage inductance. At very high step-up ratios, self resonance of the secondary can become a factor. And, to drive CMRR even higher, it's important that the primary-to-Faraday shield capacitances be equal at the two ends of the primary winding. Since an input transformer secondary is usually operated unbalanced, it's signal output is often the finish of the outermost layer - to eliminate capacitance to a Faraday shield. To balance capacitances, sometimes extra layers of Mylar tape are used to adjust interwinding capacitances. Making a high-performance input transformer is almost as much art as science, the devil is in the details! Some models in the Jensen line have 7 layers, with Faraday shielding - and they are obviously the most expensive.
 

TerryG

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So what is unique about interstage transformers?

I had one custom made about ten years ago to transformer couple a push pull output with a single driver tube, that could supply 30ma of current to the driver tubes B+. It worked beautifully, of course back then I knew little about what went into those transformers other than they were interleave wound with 150F copper. I do not think I am ready to tackle input transformers, but interstage is very interesting to me, especially when the options come up of possibly using a pentode with a UL tap as a gain stage. The Western Electic 403B interests me, and more options as to circuit possibilities are especially attractive.

I know we have strayed off topic here a bit, but I am sure someone will say if it is best to start another new thread. But this information regarding specific requirements of different types of audio transformers is exciting.
 
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