[TCML] Fixing up an old coil -- tune-up suggestions

[Jason Goodman]

Hi, all, I'm new to this list.  I'm a physics professor at a small  
liberal arts college.  About 5 years ago, before I was hired, one of  
our physics students built a tesla coil for his senior project.  It's  
been gathering dust in our machine shop ever since.  Some of my  
current Electricity and Magnetism students and I are working on  
putting it back into working order.

We haven't been able to find the original construction plans which  
the student followed to build it; if you recognize the design, a link  
to the plans would be appreciated.

I've got several questions about getting this thing tuned up and  
working well, so let me describe it first.  Here are the vital  
statistics of the coil as we found it.  I'm writing this from home,  
so I'm going by memory here....

125 v->12.5 kv transformer to power primary circuit, toaster-sized  
white box (neon sign transformer?)
6 brine-filled glass bottle capacitors made from root beer bottles
Spark gap: single gap made from two trailer hitch balls mounted on a  
wooden frame
Primary circuit wiring: Heavy gauge wire (4-gauge?  6?) with pretty  
thick insulation, type unknown.  Alligator-clip connectors to primary  
coil.
Primary coil: Inverted conical helix made from 1/4" copper pipe,  
about 7-8 turns, smallest diameter 14 inches, largest diameter 30  
inches, height about 20 inches, mounted on triangular plywood supports.
Secondary coil: 14-16 gauge wire on white PVC pipe, 5"ish diameter,  
36" length
Top load: 30" toroid made from 4" diameter metal ventilation ducting,  
mounted on foil-covered plywood disk.

There are no safety or protection circuits built into the system as  
we found it -- no safety spark gap across the transformer, no filters  
or switches on the mains side of things, no grounding rail above the  
primary, all stuff I've read about.  I've made it very clear to the  
students that the primary circuit is absolutely lethal... I'm not  
sure they took me seriously at first, but they sure did once they saw  
and heard the spark gap firing.

Repairs:
The bottle caps had been sitting filled for five years.  Exciting  
chemistry had taken place between the brine, the oil, and the metal  
bolts used as electrodes, leaving an unholy sludgy mess inside.  We  
just threw these out, replacing them with a six-pack of Corona (heh)  
bottles, built more or less according to http://www.pupman.com/ 
listarchives/1995/december/msg00040.html

We wired the thing with the supply transformer bridging across the  
capacitor (http://en.wikipedia.org/wiki/Image:Tesla_coil_4.svg  ) --  
I now understand that it's better for the transformer to put the  
transformer bridging across the spark gap (http://en.wikipedia.org/ 
wiki/Image:Tesla_coil_3.svg).  The "ground" end of the secondary was  
attached to a copper pipe providing compressed air throughout the  
building.

We placed a grounded pipe (galvanized 1/2" steel electrical conduit)  
near the toroid to draw a spark.

Testing:
We first ran a test of the power supply without the primary coil in  
the circuit, shorting the alligator clips together.  With the spark  
gap too wide, we heard a hum and a quiet snappy crackle.  We could  
see some corona discharge from the aluminum foil surrounding the  
bottles, but they didn't seem to be heating up etc.  With the spark  
gap set at 5-7 mm, we got a nice loud, bright spark, and the students  
were impressed.

When we hooked up the primary coil, a fluorescent tube held in the  
hand lit up from about 6 feet away.  We got corona discharge from the  
end of the grounded pipe to the toroid when the end was about 8-10  
inches away, and occasional dim sparks when the end came within 6-8  
inches.  When the pipe end was raised so the shaft of the pipe rather  
than the end was closest to the toroid, we could draw a steady  
crackle of bright sparks 5-6" long.

Tuning was done by adjusting the position of alligator clips on the  
primary using trial-and-error, just fiddling to try to get the  
longest spark.  Best we could do was 6-8 inches of spark.


So, not too bad, but I feel like this coil is probably capable of  
more.  Questions for you:

1)  What should we expect from this coil?
2)  If we should expect more, is the problem likely one of LC tuning,  
or should we be looking for power loss elsewhere?
3)  Is it worth going to the extra effort of measuring the resonance  
of secondary and primary using an oscillator and oscilloscope and  
tuning that way, or is that just lipstick on a pig at this point?
4)  Is a little corona discharge from the bottle capacitors a  
problem?  If so, how can we correct it?  (Trim sharp edges from the  
aluminum foil?)
5) Do the two circuits mentioned earlier perform equally well, with  
the only difference being protection for the transformer, or does one  
give a better zap?

Thanks for any help you can give.  Please keep in mind that we're not  
interested in redesigning and rebuilding this from the ground up: we  
just want to get it working to original design spec.