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Quoting Ed Sonderman:

 ES> Richard,

 ES> Well, I had an interesting and expensive weekend. 

They usually are...

 ES> I got the new 33" toroid completed and the new ground rod driven in 
 ES> and connected. I test fired the new toroid with  two12kv 30ma trans-
 ES> formers. With one tank cap it tunes at 13 turns on the primary and I 
 ES> get about 29" strikes.  You are right here, I got longer strikes with 
 ES> the smaller discharger at this power level. I tried two tank caps in 
 ES> parallel and the spark gap would not fire until the variac was at 
 ES> full power (and sometimes not at all). The best tune was 9.5 turns on 
 ES> the primary. Why does the spark gap not want to fire with more cap-
 ES> acitance in the circuit? 

The larger capacitance does not fully charge up with too small of a power
supply feeding it. Since the cap is never allowed to fully charge, the
voltage peak in the capacitor is not large enough to jump the gap.

 ES> Next I hooked up my large transformer 12kv 120ma with the large 
 ES> toroid and one tank capacitor. The best tune I got was at 13 turns.  
 ES> I gradually increased the power and got up just over 3/4 on the 
 ES> variac. Things were going well, we had massive discharges and then 
 ES> the secondaries shorted on the transformer and everything came to a 
 ES> quick halt. This is the transformer that I had chisled out of the tar 
 ES> and "saved". I built a clear acrylic box to mount it in so I saw the
 ES> arcing in the secondaries. It looks like they arced to the core.  I
 ES> think it is now a boat anchor. Undeterred, I hooked up all the remain-
 ES> ing transformers I had. Four 12kv 30ma transformers for 1.4kva. At 
 ES> this power level I found the old 14" discharger tuned at 12 turns and 
 ES> the new 33" toroid tuned at 13 turns. Why does the tune seem to change 
 ES> with increased power levels? I did not expect this.  

You don't specifically state if any changes were made to the value of
the tank circuit capacitance as you were changing the power supply 
around. Assuming you did not change the capacitors around, there could 
be at least two causes to the tune changing: 

1) The ion cloud present around the toriod will change the tune, 
and the more power, the larger the ion cloud. 

2) Your capacitors were not fully broken in; the value of these caps 
can drift 10% or more in either direction in a given day of hard use
until the break-in is completed.

 ES> The good news is I got massive discharges - hitting the overhead 
 ES> furnace ducts in the ceiling, the adjacent cement wall, the floor 
 ES> and all the strike shields. I measured one discharge at 50".  I was
 ES> amazed by the show.  With the large toroid, the discharges left 
 ES> mainly in 2 or 3 places.  With the small toroid, there was many more 
 ES> discharges - literally going everywhere. 

The larger the toriod, the fewer the number of streamers that will break
out. The fewer the number of streamers, the longer each streamer gets.
One streamer breaking out at a time will give the highest voltage.

 ES> After running at full power for about 10 minutes (approximately 1.0
 ES> minute at a time with several minutes off in between) all of a sudden,
 ES> everything shut down.  After investigating I found the main tank
 ES> capacitor was shorted. Applying a small amount of power made a kind of
 ES> cliking noise inside the cap.  Watching the capacitor revealed a steady
 ES> stream of air bubbles with a black sooty material floating to the top. 

Hey, there is a bright side of this. You are not coiling until you have 
smoked a few transformers and blown some caps. It will be disappointing,
frustrating, and expensive at times. Most "good" hobbies are. I wish I
could tell you the number of coilers who put in the time and money, and
never saw anything but the smoke... 

 ES> At least I did not have a fire or an explosion.  

It is very highly unlikely that you would encounter a fire or explosion 
from the breakdown of a homemade capacitor. Usually they go "futzz" and 
the show is over. I have blown plenty, I think I know the breakdown 
parameters pretty well by now. 

 ES> I connected the other tank capacitor up and everything still works 
 ES> fine. I am concerned now to even run the system at full power. I am 
 ES> only at 1.4kva, which is still quite a ways short of the 2.0kva that 
 ES> I wanted to get to. I would like to take the destroyed cap apart and 
 ES> do an autopsy (that will be an oily mess). I did a good careful job 
 ES> building these, cutting the poly extra wide. I am guessing it punc-
 ES> tured the poly somewhere. I was sure both capacitors were suffi-
 ES> ciently broken in. I have been running them at 12kv 60ma for over a 
 ES> week and have seen no air bubbles come up. I noticed in reading the 
 ES> KeelyNet capacitor file (CAPACITR.ASC) that he says.0625 poly is only 
 ES> good for 7500 volts AC and for up to 1500 volts AC I need .090 poly.  
 ES> Do I need to build two new capacitors with .090 material? 

First off, you can repair the blown cap in less than an hour. Lay out a
ton of newspapers on the floor. Cut four pieces of 60 mil poly about 
3-1/2 inches on a side to make square "patches". Unroll the cap on the
newspapers. Clean the breakdown site thoroughly with Ronson Lighter fluid 
or similar non-polar solvent. Use some emory cloth to clean the scorched 
and pitted plates where the breakdown occurred. You may need to press out 
a small dent in the plate. With a sharp knife, cut away all of the burned 
plastic where the dielectric failed. Clean the breakdown site thoroughly 
with non-polar solvent again, leaving no traces of carbon soot, or sanding
grit. Place a patch over both sides of the failure point on the dielectric. 
Re-roll the cap. If the cap had blown and you had immediately removed it 
from the oil, the oil could probably have been saved. As it is I would
guess that the oil has been sooting up for couple of days; I would empty
the tank, rinse it with a little solvent, replace the capacitor roll inside 
of the tank, and refill with fresh oil. You will lose some value in the
capacitor, but it will still be quite serviceable.

Capacitors fail. These caps run on the ragged edge of their voltage 
ratings. They will pulse discharge 10 KVAC in a Tesla Tank circuit all
day, even when only a week old; but pushing a brand new cap to very
high peak powers at 12KV is inviting a failure. The true break-in period 
is in fact a matter of months. A loose roll actually breaks in faster 
than a nice tight roll, but a loose roll has a lower capacitance value. 
Break-in requires that not only all of the air be displaced, but that the 
oil soaks all the way to the center of the dielectric. As you unroll your
cap to make the repair, you will see what I mean: despite a week or more
in oil, a majority of the surface area of the capcitor will be dry.

 ES> This project is getting quite expensive. As my wife reminded me 
 ES> yesterday, I have already invested a small fortune over the last 
 ES> two months. If I need to build new capacitors it will be a while.
 ES> I also need to find more transformers.

I saw trouble as soon as you mentioned she had gotten a shock... 
Oh, well.

Figure you got three quarters of the way to your goal, and suffered a
small setback. It is small really. The cap can be repaired, and neons
really suck for Tesla power supplies anyway. I am more sold on my power 
distribution transformers every time I hear a power supply problem like 
this. You can choke, you can filter, you can safety gap, but the Tesla 
coil can just gobble them up; especially at higher powers, or running 
out of tune. The secondary wire on most neons ( the 30 ma neons) are 
about the diameter of a coarse hair. 

To save your capacitors until it is truly broken in, I would drop down 
to 9 kV neons. You will find it very difficult to blow a homemade rolled 
cap of this design with a 9 or 10 kilovolt input. Then place a call to 
our friends at the Transformer Bank, and price a high-voltage power 
supply with a rating around 2 - 5 kVA -at- 10 - 12 kV. You might do better 
in the long run with a heavier step-up transmformer. I sure can't hurt 
to price one. 

It would be a good idea to budget funds to build two more caps also.
With four capacitors you can wire them in series/parallel for about
.02 mFd at 20 KVAC Pulse. I have never blown one wired in series/
parallel, and have put as much as 23 KVAC into capacitor banks wired
in this configuration. In the short run just repair your cap, drop
down to 9 kVAC neon input to prevent further failures, and keep your
eyes down the road.

 ES> It seems that for any combination of discharger and input power,
 ES> maximum output power is obtained with whatever tank capacitors will
 ES> tune at 13 or 14 turns of the primary. i.e. using all of the primary
 ES> turns is much more important than the amount of tank circuit
 ES> capacitance.  Do you agree?  Ed Sonderman

Maximum output is obtained when the entire system is in synergistic 
tune. It is a bit more complicated than what you stated above, but
you are most definitely learning the ropes.

Richard Quick


... If all else fails... Throw another megavolt across it!
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