Revelation #3 – Transformers


transformer_revelationWhile the topic of transformer design doesn’t always excite me, this article did.

While the topic of transformer design doesn’t really excite me, this article absolutely opened a few metaphoric doors. Since the selection of transformers available today is somewhat limited, and I’ll never have the financial means to allow custom designing an order of 10,000 output transformers, I don’t give too much thought to the theories presented here. With that out of the way, let’s discuss the article above.

The article, which appeared in the April, 1952 issue of Radio-Electronics, is one of many examples of where you must ‘read-between-the-lines’ to discover a revelation or two of your own. Author T. W. Dresser gives a very dry, mathematical analysis of transformer design. This is supposed to help you ‘modify’ a transformer you have to do the job of a transformer you do not have. Surprisingly, I have met many old ham operators who added a 5VAC winding to power transformers that did not have this voltage available! I am usually chided that we ‘kids’ have it good today, where any transformer is readily available. During the war years, many an astute electronics experimenter was forced to ‘roll his own’.

So what has any of this got to do with a ‘revelation’ regarding transformers? I’m glad you asked. Mr. Dresser gives very specific formulae for determining the number of primary turns necessary, the gauge of wire to be used, and calculating the ‘window area’ needed! It makes my head sore just thinking about it all, but the best was yet to come. Mr. Dresser reminds us mid-way through the exercise…

“Due to copper and iron losses, small transformers of this type require about 10% more input to the primary than is taken from the secondary…..”

Now, this is not any great revelation, to be sure. Way back in school, studying transformers, we were always taught to calculate and ‘balance’ the primary and secondary voltages/currents with the axiom ‘power out equals power in less losses’. However, the editor of the magazine make the bold interjection…..

“Britain is said to be a conservative country, and her transformer engineers are held to be conservative by most Britons. This will explain why the core areas given here are much larger than any the average American technician is likely to find on the transformers he tears down as a means of studying transformer design. For practical work, the figures can be cut almost in half, especially for transformers of 100 watts or over.”

After reading this, I immediately began to think about the transformers seen in vintage guitar amplifiers. I obviously knew the practice of using a ‘barely adequate’ transformer, under the misguided notion we would never play an amplifier ‘wide open’. I hadn’t thought about this being an American ‘tradition’, or that ‘British’ engineers usually went ‘overboard’ in their designs. It did make me realize that growing up in Canada, I did not have the ‘experience’ of playing a lot of British amplifiers, much as Keith Richards had little experience playing Traynor or Garnet amplifiers. Does an amplifier have a ‘British’ sound? I don’t buy that sentiment, but I will concede that large beefy transformers (as seen in Hiwatt, as an example) will obviously sound ‘different’ than what is in a tweed ‘Twin’. Which is ‘better’? It isn’t a question of ‘better’, but instead what you prefer; which should be based on your music and guitar playing style. This is why some people like today’s Hammond transformers; they can almost be thought of as ‘British’ influenced.

Next up, I’d like to discuss an article that appeared in the December, 1959 issue of Electronics World. Writer Edward A. Laurent describes ‘improvements’ to a Mullard 520 Hi-Fi amplifier. There is nothing revolutionary in his thoughts, but there are some insights, if we can read between the lines. Here are some excerpts from that article.

  • “….. a few salient facts about the famous ‘British watt’ versus the ‘American watt’ are in order. A watt by any other name is still a watt; in other words, there is no actual difference between them.”
  • “The original amplifier is rated at 20 watts, but measurement of maximum undistorted output will show that there is actually more than 20 watts of mid-band power available. The reason for this ‘extra power’ is quite simple: the unit uses an output transformer rated at 20 watts and the power handling capabilities of an output transformer at the extremes of the audio band become a limiting factor in rating an amplifier. “
  • “In amplifiers that utilize output transformers…….. power rating is sometimes selected at the frequency of maximum power.”
  • “The conclusion to be reached is that many British amplifier manufacturers use the conservative 20-cps figures for power rating; actually there is no mysterious difference in actual audio power between British and American ratings.”

In conclusion, please do not refer to any speaker or transformer, tube, and capacitor, as having a ‘British’ sound. It can simply be an ‘overdesigned’ part, which goes against what I like to hear today, as do many guitar players whose music styles are Blues/classic Rock ‘n’ Roll based. The component may also be rated at a parameter that has no relevance to how it will be used by modern guitar players. If you are transformer shopping, decide what ‘style’ you prefer (underdesigned or overdesigned) based on the vintage amplifiers you like.

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