Winding high power xmit toroids

Steve Harrison ko0u at OS.COM
Fri Jun 25 14:14:28 EDT 1999

At 08:56 AM 1999-06-25 -0700, Lane Zeitler wrote:
>I am beginning the process of winding some toroid coils that are going to be
>used in a 1500 watt hf amp. The cores are T-225 and are supposedly good for
>1500 watts but I figure to be safe I will use two sandwiched together.
>1. Do the two toroid cores need to be insulated from each other or can I
>just place them tightly next to each other and then wrap them as a package?

Treat them as ONE toroid, place them together and wrap them together. If
you wish, epoxy them together with a few spots of epoxy here and there,
just enough to hold them together but to allow you to break them apart
later to save one if needed.

>2. What is the best thing to use to hold them together as one piece? Some
>kind of fiberglass tape? Can I buy the stuff at Home Depot?

Should be able to; and yes, the fiberglass tape has long been touted by
prior toroid winders as being good. Not sure why, though. Where to get it?
I don't really know. Maybe, yes, the stuff at Home Depot.

>3. I am using teflon insulated wire. Do I need to insulate the cores before
>applying the wire?

YES!!! Let me restate that in unequivocal terms: YES!!!!

I've done this before at the 600-800 watt level using T-200 cores in amps
using 813s and 4CX250Bs, and one for a 4-1000A. I've never succeeded in
insulating the cores sufficiently the first time to prevent arcover to the
cores from teflon-insulated wire. Use the fiberglass tape to hold the cores
together securely; then use some sort of teflon tape to create the "first
layer of resistance" over the uninsulated cores. Do NOT assume the
fiberglass tape will provide any high-voltage insulation: it won't.

Then, it will be necessary.. and here, I mean ESSENTIAL... to create some
sort of extremely high voltage insulation. An air gap works well when there
is enough room. What's worked nicely for me is to create a pair of
plexiglass end caps for the end of the stack of cores. The outside
diameter, and the inner hole, of these caps should be sufficient to allow
the wires to be spaced a minimum of 1/8" from the cores when they run over
the outside or through the hole of the core stack; that takes care of the
end insulation of the cores. Then, you can wind some sort of teflon around
the outside of the core stack to provide more insulation; and you can cut
strips and place them inside the hole of the stack.

Be aware, though, that the wires can get hot enough to melt into the
plexiglass. This may or may not cause the wire turns to become loose,
shorting against one another.

The absolute best way to avoid that problem is to use sheets of PC board as
the end caps with holes drilled through which the wires pass. And at the
1500 watt level, you may well have to do that; I worked at about half that

Finally... I've had wires short to one another through the teflon, but
always because I had the band switch on the wrong band, or the antler SWR
was far too high. Large-gauge stranded teflon wire would be best for this
as all the individual strands will provide more surface area than a single
large-diameter buss wire will. But the stranded stuff is EXTREMELY
difficult to get to "form" and lay in place properly. So I've usually used
single-conductor large diameter bus wire with teflon tubing slipped over
the wire. I've never had the tubing arc through but the teflon on the
stranded wire has occasionally arc through where the teflon was crushed
while the wire was wound.

>Any info greatly appreciated. This is my real first attempt at using toroid
>coils for 160, 80 and 40 meters instead of the regular air wound type a la'
>B & W style.

It won't be your last, either!! You will probably have it arc over several
times before you finally figure out how to do it. For 1500 watts, I would
use the largest cores I can for the larger winding-hole in the center, not
because a pair of T-225 cores wouldn't handle the power.

There have been articles published in the years past who did this in 73 and
Ham Radio magazine; not in QST, however: apparently, the "average" ham has
always been presumed by the ARRL of being incapable of building these kinds
of components for their amps. :o|

73, Steve K0XP

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