Question about bobbin materials in Output Transformers

[makinrose]

There seem to be a lot of conjecture about whether modern plastic bobbins are as good sounding as traditional paper bobbins.  I've seen a two different opinion pieces on the subject but  the articles were both published by transformer companies with apposing views on the subject and who each adopt a corresponding different manufacturing  technique. No data or evidence was cited in either article.    I haven't seen anything scientific or logically convincing on why one manner of construction would sound different from another in the same design.   Is there a technical reason to think the two manners of construction should sound different?  Are there other reasons to choose one type over the other?

[K O'Connor]

Hi Guys

Paper bobbins and plastic bobbins are both magnetically transparent, so there is zero technical reason for one to "sound" better than the other. The technique of using paper bobbins is also referred to as "stick winding". A wooden form is used to wrap the paper and then wind the wire, then the whole assembly is slid off the wooden stick.

Proponents of paper bobbins are clinging to traditional methods because those are the methods used to make output transformers in iconic vintage equipment. In general, the "renovationist" end of the MI community does not understand why a given iconic amp sounds the way it does, and they are hesitant to change anything or substitute components with different construction or different materials. I'm going to be blunt and say this view is mired in ignorance - simple lack of knowledge.

Paper preference is then purely aesthetic.

Plastic bobbins are preferred by safety agencies and can make manufacturing easier. Paper must be oil-impregnated and most oils are organic and eventually lose the properties that let them do what they should do for the paper, that is, the paper can dry out over time. paper capacitors are the worst example of such an application and the cap becomes inductive. One saving grace in the transformer application is that the transformer is likely to be vacuum varnished, where varnish is forced into every crevass within the assembly and this greatly extends the life of the paper bobbin.

Plastic is shaped by injection-molding and is inert for life, retaining its electrical insulation properties for life. The transformer can be vacuum varnished or not , although it is usually a good idea. The bobbin also provides a convenient platform to attach solder lugs or pins to for external electrical connections, whether these attachments are externally accessible or wire leads are attached by the manufacturer.

Bobbin material choice should not be something one needs to consider for transformer selection, except for two points: A nontoroidal power transformer should definitely have a plastic bobbin for safety reasons; a nontoroidal audio transformer may be priced higher than it should be if it has a paper bobbin, as many of the promoters of such construction are ethically challenged.

[makinrose]

Wow thanks for in depth reply! That was very helpful. I have to agree with you about the tendency for some builders and some customer to want thing to be "be just like vintage" despite that not always being good thing or even important. It's good to know that the paper bobbin thing is part of that.

[Murrayatuptown]

I used to work in the lighting ballast industry. I wound my own inductors for prototypes & had Heyboer wind & varnish small batches for me once something was finalized. They were 10-15 miles away, and they were kind
enough to give me a tour when I picked up the finished items.

The tour & my inductor design experience got me interested in transformer design, the partial construction of two winding machines, and the hoarding of laminations.

I thought finding bobbins was frustrating...but paper tube was even worse. The insulation companies I reached out to had a US$200 minimum for each type/size of paper tube...so that was a good reason to not complain about bobbins...when I could find the size I wanted.

It is easier to pass hipot/dielectric withstand requirements with polymer bobbins...and (more of a power transformer issue than output), higher temperature ratings that polymers can achieve push paper insulation out of the competition.

I have also disassembled many shorted power transformers...can't prove what failed before what burned, but the books point to insulation system failure (probably localized corona).

I have enough trouble finishing projects, but I know someone who had bobbins 3D printed...I don't know & he didn't care about any 'ratings' for the printed bobbin... he was making an electromagnet for some home project.

Murray