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Re: [TCML] Interesting historical rotary spark gap transmitter transformer.

I read all those QST's as kid in my teens and always cringed when I read of someone destroying a beautiful HV transformer by pulling off the HV secondary to 'convert' it.  20 years later that 'something' was available much cheaper and better. I n my wire scrap bin I still have remains of a Thordarson secondary. Also have a 1/4 kW synchtronous gap. I've run a small TC using an NST and that gap but without proper capactitors. Maybe only 12 inch discharge which makes a quiet 'zhzssssss' with 120 cycle hum. 12" x 4" toroid.


On 6/22/2020 12:07 PM, Bert Hickman wrote:
Hi Chris,

I found an article that might help from a July 1923 QST article about how to modify a similar (Type T) Thordarson HV transformer to convert it into a filament transformer. In those days, spark was rapidly being replaced by vacuum tubes, and old spark equipment was sometimes "repurposed" by amateur radio hobbyists.

Thordarson made both Type R and "Flexible" models T-1 and T-2 for the spark wireless market. In these transformers the magnetic circuit consisted of a main pair of C-cores with the primary winding on one leg, and the HV secondary on the opposite leg. In addition, a magnetic shunt (another core leg) was provided across the primary leg of the main core. This created a magnetic shunt what provided a flux leakage path that preventing some of the primary flux from linking to the secondary. By adjusting the air gap between the primary leg and primary shunt, the proportion of leakage flux, and the primary:secondary coupling coefficient, could be varied.

This is similar, in principle, to magnetic shunts used in modern-day coil-and-core NST's. However, the physical location of the shunts is different between the antique Thordarson transformers and NST's. In NST's, the magnetic shunts bridge across legs of the main core between the primary and secondary windings. In the Thordarson spark transformers, the magnetic shunt forms an external third core leg that was physically external to the main primary-secondary core loop. The adjustable shunt is located below the primary winding in the Type R, and above the primary winding in Type T transformers. Otherwise, the shunts function similarly - reducing the proportion of magnetic flux that linking the primary to the secondary (and vice-versa).

Although the following description in the QST article was written for the type T transformer, it should be similar (in principle) for the model R. See page 33 (page 35 of the PDF file) from https://worldradiohistory.com/Archive-DX/QST/20s/QST-1923-07.pdf

"A filament lighting transformer can easily be made from an old Thordarson spark transformer of the "flexible" type. The one used to make the filament transformer here described was a one kilowatt type "T," which most amateurs of a few years standing will remember. The type "T" had a magnetic shunt in the shape of an extra core-leg above the primary. This shunt was hinged and had a wing-nut and spring arrangement to hold the magnetic leakage tongue (shunt) at any desired adjustment. This arrangement worked very well except at close settings (small air gap) when the rattle was terrific.

"Accordingly in converting the transformer the spring was removed and the shunt set right down on the top of the main core and secured there by U-shaped straps of iron hooked over the shunt. These U-shaped clips are held in place by the bolts that hold the transformer frame together. It is then possible to clamp the shunt down solidly by driving wedges under these straps or by driving the tops of the straps apart so that they slant."

Images of the type T and R transformers can be seen at the Radio Museum site:



Hope this helped and best wishes,


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