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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.
Ed
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).
"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: