[TCML] Tuning a new coil

[bartb]

Hi Alex,

I'm showing your coil should be nearer to 295 kHz. Your 323 kHz seems a 
little high to me. I know it's a small amount, but in this 
configuration, Javatc is always much closer. Usually in situations like 
this, the inputs to Javatc are not exact. For example, secondary is 
slightly smaller than stated (like 4.25") or things of that nature. 
Turns affect this also, but your 920 turn count seems about right and 
the secondary diameter would affect this frequency quite a bit (20 kHz 
just for a 1/4" difference in diameter).

On the primary side, cap tolerance will affect how much primary is 
needed as well as the primary itself. Good to measure the cap to be sure 
of it's total C. In any case, 1 full turn will affect about 44 kHz on 
the primary side for your coil. Thus, if the tuning is close to begin 
with, the ideal tap will be within inches. I suspect your coil will 
perform well at about turn 6.

When you tune the coil in at low power, you also get small sparks. As 
you increase power, spark lengths increase. Those sparks add a small 
amount of loading changing the frequency of the secondary slightly. So, 
a small amount of increased inductance is often added to match streamer 
loading. Say your coil tunes in at low power at 5.6 turns. When full 
power is applied (and if good streamers are forming), you may want to 
increase the inductance a few inches (but that is something you have to 
play with).

Take care,
Bart

b alex pettit jr wrote:
> Hi All,
>   First post :
>    
>   I am planning on powering up my new coil - designed from many hours of searching on line information and validated by JAVATC.
>    
>   SecCoil 4.5" x 21"  920 turns 24ga PVC  tube
>   3" x 12" toroid top load
>   PriCoil 8.5" ID of 2 TPI .25" copper tubing
>   Cap : 0.0187 uF via two strings of 16 each CDE 942s
>   Gap : ring of 10 pcs 1" dia x 3" long copper spaced 0.02" & fan cooled
>   Power 12KV/60ma NST  ( 133 uF power correction )
>   TerryFilter / NoiseFilter
>    
>   The measured coil/top load rings at 323 KHz. using a Sig Gen / Freq Counter & Scope with a mini antenna .
>    
>   I found that I obtained that value whether grounding the SigGen and 
>   inducing the signal into the base of the coil , or grounding the coil and
>   inducing  the signal through the primary winding via a 50 ohm resistor.
>   ( cap disconnected )
>    
>   Tuning the Primary setting with the Cap installed and Secondary in place was a bit challenging to ambiguous. It seems that the nominal pri inductance may close to what JAVATC predicted, but JAVATC calculated a 10% lower secondary resonance freq. ( is this to be expected ?) But there was no clear resonant point. Half Turn shifts in tap point did not show a great deal (if any) of difference in amplitude shown on the Oscope.
>    
>   QUESTION :
>   How sensitive is the tuning of the Primary Coil :
>    
>   As a benchmark,  lets say I am using 6 turns and that last turn is ~ 15" in dia ( circumference about 45 inches )
>    
>   1) Will the tuning point be anywhere within several inches, an inch, or fractions of an  inch ?
>    
>   2) How greatly does that tuning point shift with power input ? If I tune at a low power and just use a few spark-gap points and then shift to full 720 watts, how much ( in  inches or % of inductance ) will I expect the tuning point to shift ?
>    
>   Best Regards,
>   Alex P
>   Orlando FL
>    
>   FYI, my first Tesla Coil was build in the mid 1950s using a Model-T spark coil I found in the attic and a secondary maybe 2" dia and 12" tall. Thin, 2 - 3 inch sparks  were still impressive to a young boy.    
>    
>   ... times have changed .... as Tim Allen put it : More Power !!
>    
>    
>    
>    
>    
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