* Original msg to: brad.alheim-at-the-spa-dot-com

Quoting brad.alheim-at-the-spa-dot-com:

> I'm new to SERIOUS coiling but have been an electrical engineer
> for about 20 years, with a few years designing high voltage 
> power supplies. I'm really intrigued with what I've seen you 
> guys doing, and much of it is new to me (I'm used to smaller 
> stuff, smallest being tiny 4kV supplies for night vision 
> goggles, and largest being 35kV at 2A for TV transmitters and 
> RF heat sealing machines)...

Welcome aboard!

> This Tesla coil stuff is really interesting as it is more 
> Physics than Electronics, and most of us don't have the 
> resources to solve a problem with kilobucks.

Indeed. I have found this to be a very rewarding hobby/study. As
for the kilobucks, I am guilty, as are more than a few others
here, of investing a few thousand over a period of years. You can
build a nice small system for as little as $150.00 - $200.00,
assuming you shop surplus and have accumulated a lot of junk over
a period of time (to draw on for the knick-knacks, lots of brass
and copper junk come in handy). As for problem solving, kilobucks
no. We will plop a couple of hundred here and there (typically)
for a good commercial capacitor, rotary gap motor and machine
shop balanced rotor maybe, heavy variacs, and a load of fresh
copper now and then...

> Ingenuity and time has to take the place of cash. 

If you had to go out and purchase custom manufactured systems of
the quality and type I produce you would be spending kilobucks
instead of a couple hundred ever few months... 

> Please excuse me for my ignorance, and my long list of 
> questions.

> I've got a LARGE roll of #22 Formvar coated wire I bought 
> surplus from a company I used to work for. The only drawback is
> that it was from the Vietnam War era when copper was expensive
> and this wire is copper coated ALUMINUM. (it was intended for 
> TV yokes) I haven't measured the insulation thickness yet, but
> I've been wondering if there could possibly be some drawbacks 
> to using this for a secondary coil (it does solder nicely, 
> though)

I see no problem at all with using this wire. The AL core will
have a higher DC resistance, but the RF currents were are dealing
with are severely affected by skin-effect, meaning the RF
resistance of your particular wire is probably not going to be
significantly higher than solid copper of the same gauge. At
Tesla coil frequencies the depth of penetration due to skin
effect is not going to be much over a few mils. Since beginning
projects (lower power-higher frequency) generally run at least a
100 kilohertz or so higher in frequency than intermediate/
advanced coil systems, the skin effect at these higher
frequencies will reduce the depth of penetration a bit more.

The bottom line here is: wind the wire up! 

> I've read that the insulation breakdown voltage of wire can be
> increased by coating the wire with paraffin wax, does anyone 
> have any information on how to do this to a lot of wire, and if
> it is worthwhile? 

Nahh, I have always recommended a few heavy coats of sealer on
the secondary coil AFTER winding for two reasons: it offers some
increased dielectric strength in the construction, and it
protects the winding from physical abuse. Heavy coatings will
subtract Q, but it is a trade-off. I have a few decent coils now
that I would have had to make major repairs on (removing badly
damaged sections) if they had not been protected with coatings.
There is no doubt in my mind that coils are more resistant to
splitting and breakdowns when they are coated. Coating a coil is
not an operational requirement however. Your wire should work
fine either way assuming the resulting coil is respected.

I have never gone out and just plunked down $$$ for specialty
magnet wire. Every coil I have is wound with surplus priced or
donated wire. I do nothing to it other than to make sure it is
clean when it goes on the coil form (some of my cheap wire
resembles D.C.C wire or "Double Cotton Covered" for all of the
lint on the spools).

> How would I initially calculate the distance for a safety gap?
> ( I suppose that I could measure the breakdown with an o'scope
> and vacuum cap divider, but I'd like to get it close to begin 
> with, without frying my transformer!) 

I don't calculate much in this hobby. To determine the safety gap
distance I energize the transformer with straight line current.
No tank circuit! No connection to the coil! Just place the safety
gap with a grounded center post across the HV bushing of the
transformer, and energize the transformer with on/off line
voltage. If the safety gap fires, I power down and open it a tad.
If it does not fire I power down and close it a tad. When the gap
is on the verge of firing: it fires 50% of the time when line
current is thrown straight at it (no variac); it is set properly
for set-up and low power tuning. If required it can be opened up
more once the proper tune is established in the coil system. In a
well tuned system the safety gap may be opened up to the point
that a slight flash (no loud cracks or bangs) will seek the
center post every couple of minutes or so during full power

> Roughly, ( I certainly won't hold anyone accountable if it goes
> up in smoke!) how much could I overcurrent an OLD well made 15
> Amp variac before it becomes a fused lump of copper and iron? I
> don't know the make or model, but it is well made and has a lot
> of iron and a #14 copper winding.

20-25 amps for limited periods (2-3 minutes) will not damage this
unit. When you hear "pinging" due to heat expansion of the
winding you are on the verge of damage and you should shut down.
You could probably pump 30 amps through this unit for periods of
30-60 seconds with a duty cycle of 5-10%. You can increase the
"on" time by mounting the variac in a ducted housing and blowing
a lot of air over it. With lots of moving air you could most
likely run the unit at 20-25 amps for periods up to five minutes
without the slightest worry. The 15 Amp plate rating is
continous; 99% of Tesla coil operation on well designed and
constructed coils is between 3-5 minutes. Many poorly designed 
or poorly built systems can only run 60 seconds or less. 

> Is there any advantage to using a flat disk instead of a toroid
> for capacity, with the intent of making lots of sparks and 
> keeping the secondary resonant frequency down?

The breakdown voltages of flat disks are too low due to the sharp
edges or high radius of curvature on the edges. Homemade toroids
are cheap, fast, and easy to build; everyone is using them with
excellent results.

> Would there be any advantage of using a honeycomb wound copper
> tubing primary over a flat or saucer shape? (other than it 
> would look purdy :)

I am not sure what you are referring to. Please expound a bit.

> Why is a flat or saucer shape primary used (as I've seen in the
> posted photos) over a solenoid winding? 

Better tank circuit caps. Modern plastic film capacitors deliver
higher peak powers. The aspect of the primary coil is reduced
correspondingly to keep from overdriving the secondary coil.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12