Paper vs. Plastic Bobbins in Tube Amp Transformers

 Among the myriad of “tone” questions about tube amps is question whether paper bobbin sound different from nylon or plastic bobbins in a transformer. Transformer companies take either stance on this issue based on what method they use and publish articles advocating for technique they use. Classic Tone had one about their paper wound transformers and Mercury Magnetic writes about why nylon bobbins are preferred. Yet both companies make well regarded transformers. It is difficult to access the accuracy of these claims. Purist builders claim that paper bobbins are inherently better but can offer no explanation as to why. Let take a look at this issue.

Firstly, we must address the question that if bobbin material makes a difference, in which type of transformers would we expect this effect be heard? The answer would be only transformers in the signal path like output transformers, reverb drivers, or inter stage transformers (only occasionally seen in MI amps). Power transformers and chokes are not in the signal path at all. If wound to same specs they behave exactly the same. Any claim that bobbin material would make a difference in those types of transformers is clearly incorrect.

Second, what role does a bobbin actually play in a transformer? A bobbin is only a form that wire is wound on. It is meant to be inert magnetically. If bobbin material does change tone it would be because the paper is imperfectly inert messing with the operation of the transformer. Fortunately this is not the case—-paper bobbins are inert. Paper bobbins are treated with oils and wax to make them less susceptible to rot but old paper bobbin transformers can and do fail after the paper dries out. This is rare but I have seen it happen. Nylon bobbins are inherently inert and rot proof so they are longer lasting.

If the bobbin is inert and plays no electrical role in a transformer, how can it affect sound? The truth is that is does not. Paper and nylon bobbin are just as magnetically inert. Side by side transformers wound to the same specs with the same materials but different bobbin materials sound the same. I used both and both have had great results. Since different brands use slightly different other materials all brands sound slightly different but this to be expected. It is not due to the bobbin. It is perfectly reasonable for builder to prefer one brand over another but not because of the bobbin material.

So what properties do affect transformer tone? The answer is the specs, style of winding, the accuracy of winding and quality and type of materials. Those are not as easy to understand so purists often go back to the fallback position of “special parts make special amps” and “parts with specific old style manufacturing are magic”. The fact is that the amps they copy were using common and inexpensive parts of the day. Fender and Marshall certainly didn't think of the parts they were using as being special. They were not using high grade steel laminates or any else that was special in the transformers. In fact Marshall and Fender were using surplus radio transformers during their early years. This not to say the components used did not contribute to the sound. Instead it say they contributed to the tone because of their electrical properties. Ultimately it's up to a designer to know how to make an amp sound good. Making good amp really requires that you understand the whys and how of producing tones. It's an engineering problem that requires, knowledge, good ears, and patience. Designing good transformers is similar feat.

So why were transformers made with paper bobbin in the old days? Simple: there were no nylon bobbins and it was an effective and reasonably cheap way to make transformers. So why are so many transformers still made with paper bobbins? Many factories are still tooled to make transformers with paper bobbins using old winding machines. The additional reason is that people will pay extra for them despite the dubious claims of benefit.

Much thanks to Kevin O'Connor at London Power for kindly taking my questions about transformers….