Reply To: The Mag-Amp A bit of Electronic History

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Ed, I first came across this in the ’60s, where I knew it as a “saturable reactor”. Back then electric kilns were controlled by ON/OFF switching. Sometimes individual elements or zones, sometime the whole lot. At the time I was located on the “Shelly” factory site. Post war, the company had taken two routes. Bone china as for the previous 50 years, but one of the family, Donald Shelley was very technically competent and designed and built his own “Top Hat” electric intermittent kilns. The biscuit firing kilns were designed to heat up to final temperature in 24 hours. A supply of around 80v was required to achieve this, so each kiln had a transformer with  secondary tappings. Depending on conditions, including ageing of the Kanthal elements, the tapping could be changed for faster or slower firing. My parent company took over Shelleys, but the kiln side was separated from the china side, and this was when I arrived, taking responsibility for all firing amongst other things. I was developing an interest in kiln technology, so looked into the possibility of improving the control, discovering this technology. In fact it was quickly decided to replace these electric fired kilns with gas fired ones, so that went no further.

A dozen years later, when I got into colour TV, I specialised in the Grundig CTVs with their thyristor line output stage. The control of the line amplitude was affected by DC control of an intermediate control transformer.

My Pal Martin has a 40+ year old US made “Miller” TIG welder. A foot operated control regulates a DC current through a limb of a big wound component. same technology.