Tough Toroid

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "S & J Young" <youngs-at-konnections-dot-net> 

I just built a pair of much larger toroids for my twin 4 x 23 inch TC.  New
toroids are built with six inch flexible black drain pipe (7 inch OD), and
are 34 inches OD.  That is about as small a diameter as one can reasonably
achieve with 6 inch flex pipe. Cost is very low compared to buying a spun
toroid.  They replace 18.5 OD toroids made with 4 inch flex pipe.

Some observations & tips:

1) Joining the ends:  My easy method is to use a short piece (two ridges on
each side of the joint) of the same drain pipe.  Cut out about a one inch
strip along its length so you can compress the joint piece inside the two
ends of the toroid.  The trick is to force the ends of the toroid to be
parallel with each other while you make the joint.  I do it by tying bands
of twine around each end going across the toroid diameter.  The two twine
bands cross each other so there is some force pulling the ends toward each
other as well as toward the opposite side of the toroid.  When the twine is
tightened sufficiently, the toroid is race-track shaped and the ends are
fairly parallel.  Insert your short joint piece and use screwdrivers to get
the inside joint piece to expand and interlock with the toroid ridges.  Then
just drive in a few short grabber screws into the valleys between the ridges
to hold the ends securely to the inside joint piece.  Undo the twine and
reshape the toroid to be circular.  The result is a very tough and not too
heavy toroidal form.

2) Coating:  I covered one with strips of duct tape, then strips of aluminum
tape.  The other one I just applied the aluminum tape strips directly - no
duct tape.  They both came out about the same, so I see no need for anything
between the aluminum tape and the form.

3) Support:  I compared an aluminum foil covered wood disk vs a support of
three 1.5 by 1/8 inch aluminum struts.  The resonant frequency of the
secondary plus toroid was the same for both support methods, so the
capacitance added by a solid center disk is negligible.  Using aluminum
struts is a much lighter support method.

4) Performance:  Fsec dropped from 194 to 138 kHz with the larger toroids.
I had to add some off-axis primary inductance to tune the primary using the
same size primary cap.  Performance was improved and the streamers seem
brighter.  DC power to achieve the same 60 inch streamer length (using
breakout points) compared to the power with the smaller toroids was 5 to 9
percent lower in the 200 - 250 BPS range.  So there was a slight efficiency
improvement with the larger toroids, even with the added off-axis primary
inductance - not sure why.

5) Looks:  a 34 inch OD toroid sitting on a 4 x 23 inch secondary looks a
bit ridiculous!  Small price to pay in exchange for the fun of
experimentation.  If I were to do it again, I would probably use 4 inch flex
pipe and an OD of about 26 inch diameter.  6 inch flex pipe is better suited
for a 6 or 8 inch diameter secondary.

--Steve Young