Re: primary taps

Subject:  Re: primary taps
  Date:   Fri, 13 Jun 1997 20:52:38 -0400
  From:  "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
    To:  "Tesla-2" <tesla-2-at-emachine-dot-com>
    CC:  <tesla-at-pupman-dot-com>

A little cross-pollenation from the Tesla-2 List
> From: Tesla-2 <tesla-2-at-emachine-dot-com>
> To: tom_mcgahee-at-sigmais-dot-com
> Subject: primary taps
> Date: Saturday, June 14, 1997 2:36 PM
> From: Mike Hammer <mhammer-at-misslink-dot-net>
> Date: Sat, 14 Jun 1997 13:36:13 -0500
> Subject: primary taps
> Hello all,
> Since I will be taking some time off
> the next couple weeks I hope to get some
> serious coiling done.
> One thing I need to work on is my primary tap.
> After raising my primary capacitance I noticed
> that my primary tap gets quite warm.
> After several extended runs this piece was too hot
> to touch. So obviously my arrangement of a large
> alligator clip for a primary tap has some amount
> of resistive loss. I had attached 1/2 sections of
> small copper tubing to the jaws to increase the contact area.
> My question is to those of you with operating coils.
> What are you using for a primary tap?
> Just looking for a few ideas to lower the loss in this area.
> Mike Hammer
> mhammer-at-misslink-dot-net

As you have noticed, the resistive losses from the primary TAP
mechanism can be excessive. The losses are from several factors.

1) The currents are just obscenely high. But that's what we WANT, so
we will NOT try to reduce *that* in any way :)

2) With extreme high RF currents you need a large surface area to get
the current transferred. The stupid little teeth on even the best
'gator clips are pathetic for RF use. The points just intensify the

So, get rid of the stinking teeth and increase the surface area.
Grind the jaw structure so that a maximum surface area come in

Add a more robust surface area by soldering on copper pieces shaped
so as to maximize surface contact (not too easy since the shape of
the spiral changes from turn-to-turn). Easier for you solenoid guys,
as the pitch and radius are constant in that form of primary.

Use a MUSHY metallic material to make the contact. Think metallic
Braid. Think of the outside metallic braid on coax cable. Yeah, I
know that everyone tells us not to use braid. But they are talking
about the main power runs, my friend! HERE is a place where the braid
has a quality that overcomes its defect. The great MUSHINESS of the
braid makes it ideal to create a larger surface area contact.

Try *this* with your existing clip lead setup: wrap two or three
turns of braid **tightly** around the desired tap point. Let the clip
lead sink its old 'gator teeth into the braid. Not perfect, but a
definite improvement! (And ridiculously EASY to implement, for all
you lazy bones coilers out there). How many wraps you can tolerate
depends on the size of your 'gator, and the spacing between primary

All right, so now that I have convinced you of the joys of braid,
consider attaching the braid right to the inside of the jaws of the
'gator itself. You can file away the teeth and that will give you
more room for the braid. Solder it only OUTSIDE the mouth of the
'gator clip. We want the stuff between the jaws to remain MUSHY. Be
careful, as the braid LOVES to wick up solder. TIN the outside of the
jaws and lightly tin the end of the braid. Heat up the jaw until the
solder melts and then touch the tinned end of the braid into the
solder on the jaw . Hold the braid in a needle nose pliers jaw,
tightly and near the end. This will restrict the solder wicking by
drawing away the heat. It will also prevent your fingers from having
the permanent imprint of wire braid branded into them.

After soldering one end, wrap the braid along the inside of the jaw
and solder the other end on the other side. What you want is a run of
mushy flat braid across the inside of the jaw. Do both the top and
bottom jaws.

An alternative to going across the width of the jaw is to run it down
from the top of the nose of the jaw, loop it INSIDE the jaw, and
around such that it then comes back out across the bottom of the
bottom jaw. Soldered on the outside, of course.

There are MANY variations on the theme. Pick one. Make a mushy jawed
'gator clip. Amaze yourself. Impress the neighbors.

3) With really huge currents the TYPE of conductor becomes very
important. Heavy duty solid COPPER is best. It is also one of the few
metals that really solders well. So if you are going to modify a
'gator clip, I suggest beginning with a solid copper one if you can
get one. Trying to solder aluminum 'gator clips is an exercise in
frustration. Save your energy and your sanity. Buy copper 'gator

Now, some of you are using those dumb little Radio Shack mini 'gator
clip leads with all the cool little colored booties on them. Doomed
to failure from the word go. Not enough surface area to conduct our
obscene RF currents. These babies get hot enough for the solder to
melt and the plastic booties begin to drip off. Forsake the wimpy
mini gator clip and go for the Macho 'gator clip. Think Surface Area.

4) 'Gator clips are made from two sections. In a high current RF
environment this is bad news, as the second section has only a few
minor points of contact with the main section. This means a high RF
resistance for half of the clip. This in turn means lots of heating
WITHIN the 'gator clip itself. Solder coax braid from the wide top to
the wide bottom. Keep it short and WIDE. Really industrious little
coilers will find ways to do this work from the INSIDE as well as
from the outside. Think Skin Effect.

5) And the final 'gator clip modification: Throw out the stinking
gator clip. Build you own Compression Fitting Primary Tap. This is a
tap that is designed so that it is primarily a PERMANENT TAP, but in
a Temporary sort of way. A 'gator clip is temporary and makes a lousy
permanent tap. Soldering and other such methods are permanent and you
have to live with it. 

A compression fitting can be fabricated in many ingenious ways, but
its main feature is that you can move it around, but once you are at
the right place, you can tighten this sucker down and leave it there
forever if you want. The down side is that it takes a bit longer to
move it around. But it makes an EXCELLENT connection.

I have seen small hose clamps used. Properly trimmed they aren't all
that bad. These work best on larger diameter tubing.

The compression fitting has SOME means to clamp the metal tightly
around the primary coil. The metal that gets clamped should have some
flexibility, but most importantly it should make an extremely tight
connection with a good sized surface area. 

The material should be something that can take a certain amount of
flexing without breaking. I have used copper strapping to good
advantage. You can make your own from sheet copper, or actually buy
pieces that are about 1/2 to 1" wide at building supply stores. You
can also use wide flat braid as the flexible conductor part and use
brass plates to do the compression part.

The compression part consists of some means to pull the strap tightly
around the copper tubing and secure it. I often use brass plates that
I have drilled and tapped so that one plate has holes larger than the
screws and the other has been tapped so that the screw can be secured
by that plate. Also connected to this brass plate is the supply lead.
It may be soldered or also compression clamped. Think surface area.
Sometimes I attach more than one feed lead per clamp. That increases
the surface area of the feed lead while keeping it relatively

I normally have the flexible flat copper/brass or braid section
soldered to the two afore-mentioned plates. Solder them well, as the
solder is going to be providing physical strength to the assembly.
Another method is to use another plate to clamp the flexible metal
band to the master plate.

If using braid, consider first rounding the edges of the plates that
are where the braid passes by. Otherwise the sharp edges will bite

However you decide to do it, get the spacings adjusted so that when
the flexible piece is wrapped around the tubing and you tighten down
the screw(s) the unit is firmly attached with a decent surface area.

Hope the information is of use to someone.
Fr. Tom McGahee