Re: larger than resonant (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sun, 03 Jun 2007 11:38:05 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: larger than resonant (fwd)

Hi Scott,

As you already know, the coil needs more turns (only 340). But, I looked 
at the coil "as is". I modeled the coil at your tune point of 6 turns 
and inserted a strike ring as a grounded object and 3 other rings to 
represent the 3 unused outer turns (they have a slight effect as an extC 
to the overall frequency). Here is the graphic representation from Javatc.
http://www.classictesla.com/temp/sb340.gif

The reason I took the time to throw the grounded objects in there was to 
back calc the bottle caps. The system frequency is about 710 kHz. For a 
6 turn tap position, the tank cap needs to be 6.28nF, so if your using 3 
caps wired in parallel, then each jar is about 2.1nF, which is not what 
I expected. This indicates a wall thickness abut 0.2" (assuming 
dielectric constant of 7). I expected something nearer to 3.2nF per jar. 
But, it is what it is. "If" your 6 turn tap position is near resonance, 
then 3 caps in parallel are 6.28nF.

BTW, the difference in the 3 extra turns from an extC component is 2.5 
kHz and the strike ring is 10 kHz. So not much affect at all (like I 
said, I just wanted to back calc the tank cap as close as possible).

I tuned the coil for 2 caps (4.2nF) and the primary would tap at about 
7.25 turns. For 1 cap (2.1nF) at 10 turns. So, you could get 2 or 3 jars 
to work, but not 1. I think your running STR anyway, so the 3 jars would 
probably do better. You don't gain much between 6 turns and 7.25 turns.

Well, how to make it better? Wind a new secondary: Example 3.5" x 17.5" 
using #28 for 1200 turns. You would want to rebuild the primary also 4" 
x 16" using 0.25" tubing. This coil would require a 0.0125uF tank cap. 
So, you could probably get away with a single string of 12 caps using 
0.15uF CD type (considering the OBIT's are only 10kVrms). About $40 or 
so for the caps.

See, a whole new coil (but you get to keep the topload).

For the coil at hand, the I see that might be of benefit is to replace 
the needle gap for a nice airblown pipe gap of some type. With the power 
your running, you really should be seeing between 40" and 50" sparks. I 
built a large coil once which was short in the h/d category (about 700 
turns). What really helped that coil out was to add length. Because it 
was sonotube, I literally added 12" to the length (took some work to 
do). This helped in more ways than just h/d and secondary inductance, it 
also allowed me to use more primary turns for the same capacitance which 
helped increase surge impedance and allowed finer tuning. A new rsg also 
helped a great deal.

Take care,
Bart

Tesla list wrote:

>Original poster: List moderator <mod1@xxxxxxxxxx>
>
>---------- Forwarded message ----------
>Date: Sun, 03 Jun 2007 01:10:54 -0400
>From: Scott Bogard <teslas-intern@xxxxxxxxxxx>
>To: tesla@xxxxxxxxxx
>Subject: Re: larger than resonant (fwd)
>
>Ok,
>Toroid, Od 14.5 inches, chord, 3.5 inches, height from floor 46.75 in.
>Secondary 20 AWG, 3.5 in PVC form, wound 12 inches, height from floor 28.75 
>inches.
>Primary, .25 in cu tubing, 4 in ID 16 in OD (turns aren't spaced very 
>evenly, the average space is .3 in, the last turn is spaced wide but not 
>used anyway, it tunes best at 6 turns, and has a total of 9) height from 
>floor 28 inches.
>Capacitors, jar dimensions, 3 in radius filled 12 inches, glass thickness 
>unsure but believed to be .125 in.  I think they probably put out about 3.5 
>nf put together.  (one gallon pickle jars)
>spark gap, single air blown gap "static" (to bolts filed to points) I once 
>tried a asynch, and got good results the faster I turned it, but the SG blew 
>up before I cranked it very fast.
>RF ground, metal floor support (not good enough! but very convenient).
>Power supply, 4 OBITs in parallel, 10 kV at 92 mA current limited total.
>All wiring is 14 AWG high voltage wire.
>
>I don't have any tools to measure capacitance, but I could probably borrow 
>one and find out for sure.  Let me know if you need additional specs.  I get 
>about 12 inch arcs to a grounded object, and the free fliers are about 20ish 
>inches.  It was my first coil, so naturally it was made rather sloppily, 
>hence the unevenness of the primary.
>Scott Bogard.
>  
>