All primary coils are in series and the secondary can be connected in series and parallel thus giving the loudspeaker impedance's of 1.7, 6.8 and 15.2 Ohms. This transformer works! For those who want to try it themselves to make such a transformer the author found out that it is not entirely impossible with a winding machine. The most important feature of such a machine is a reliable mechanical counter.
The transformer, although it is very good is somewhat out of time. The dimensions are not very practical, the copper losses in the primary are with 250 Ohms between the plates somewhat high and a leakage induction with 30mH somewhat high and last but not least, the output impedance's are impractical. And yet, the author has a pair of transformers that are wound exactly to the Williamson specs, it sounds wonderful.
The first thing you must know about output transformers is that the square root of the impedance ratio in the turn ratio. (the quadrate of the turn ratio is
The second thing, leakage inductance can easily be measured with a LCR meter. Short the secondary and measure the inductance that is left over in the primary. Don't be surprised when it is much lower as 30mH. The author reached values as low as 3.8 mH.
The selfinductance of the entire transformer is not so easy to measure since it depends on the excitation of the iron core. Therefore it is more practical to measure the initial selfinductance at 5 volts. If one connects the primary to a 5 V ac supply with a ammeter in series one must measure less then 100microamps for an acceptable transformer.
Since the quality of core material has improved enormously over the years it is not impossible to use a smaller core of grain oriented material with somewhat less turns and still reach a higher primary inductance. Although it is possible to calculate everything of a transformer it is hard to say how a transformer performs once it is wound. The best guaranty that a do it yourself transformer works is sticking to the original Williamson design and diminish the diameter so that you reach a turn ratio of 35 when the entire secondary is connected in two parallel sections that are put in series.
You can also develop a transformer yourself and make use of the modern materials. Be prepared for dozens of transformers that look nice on paper but for reasons that will remain unknown (till you are wiser) don't sound any good. Reject the transformers that don't sound at all and improve the ones that seem better. It is learning the hard way as the author experienced.
The author has after years finally developed a transformer that is better then the original Williamson on a E-I 130B core of grain oriented material. It has sectional windings, has a leakage inductance of 8mH, 120 Ohms dc between the plates, 0.14 Ohm dc secondary and has a secondary impedance of 8 Ohms to a primary of 9000 Ohms. It has connections for ultra linear use with other constructions. Its still weighs 4 Kg (9lbs). The winding details are undisclosed. If you are interested in commercial production of a very good transformer the author is interested in selling his pair of transformers exclusively to you including the winding details. Remember that the development took 2 years and that the negotiation base is two years of wages.
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