Re: Tuning Question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Dennis,

Comments interspersed:

Tesla list wrote:

> Original poster: otmaskin5@xxxxxxx
> I've read plenty about the importance of tuning coils at low power 
> (variac & gap width) to avoid damaging components until resonance is 
> achieved.  I also recall a post discussing, once a coil is tuned at 
> low power, you need to further tune the coil at full power due to 
> additional secondary capacitance created by the streamer itself.  I 
> have some questions about this.
>    * Is additional capacitance of a full power streamer significant 
> enough to warrant the additional tuning excersize?

Sometimes yes, other times no. It depends on the coil geometry and 
spark length. When tuning at half power, you get the coil tuned the 
best your eyes can determine. Then you turn up the juice. Sometimes, 
you'll notice that the coil wants to periodically produce racing 
sparks at those higher power levels. It's real easy to stop and start 
thinking about coupling and raising the secondary. But, don't do that 
yet. Tap outward on the primary first about 5% (the percentage is the 
amount of inductance that would drop the primary resonant frequency 
5%, thus, tapping "high" on inductance). The amount of inductance 
that would cause this change is different for every coil. What you 
will likely find is that the coil will stop frequent racing sparks or 
stop them altogether and sparklength will be a little better. If this 
doesn't help, then go after the coupling. It's actually quite normal 
to run a little high on primary inductance for a smooth running coil.

>    * If so, I would assume you are looking to increase primary 
> inductance to compensate for increased secondary capacitance - so 
> you'd be tuning outward (increasing primary turns) - correct?

Yes.

>    * I tried this & seemed to find increased performance tapping 
> the primary at an additional 3/4 turn - i.e., from 13 7/8 turns to 
> 14 5/8 turns on my 15/60-0.015uF system.  Seems like a lot of 
> additional primary for the small increase in secondary 
> capacitance.  Does this seem to be in the ballpark for full power tuning point?

Yes. I had a coil once that ran best at nearly 13% detuned. I've also 
built a coil that just did not like being detuned more than 3% or 
performance dropped off. So each coil is different as based on it's 
geometry and sparklength (directly related to power). Just experiment 
to find your coils happy spot. The norm is about 5 to 8 percent lower 
primary frequency. Spark loading adds an external capacitance near 
the coil lowering it's frequency during spark episodes.

> My NST died a few days after reaching this higher performance level.
> I'm wondering if I killed it with improper tuning.

Possibly if something bad happened during tuning. I look at it from a 
logical standpoint here: NST was good, but during the tuning process, 
it died. So likely, something occurred, but it's impossible to say 
what. Usually, there's no problem here. You may be running a slightly 
wide gap or had a gap arc incident of some type that allowed the NST 
voltage to rise and overvolt the tranny.

Do you  have a safety gap installed and adjusted? Or a Terry filter 
installed? Without these components, when something bad happens, it 
can be a quick death.

> I want to optimize performance, but would also like to avoid killing 
> another transformer.
>    * Any tips or suggestions on doing this additional full power 
> tuning excersize.  I'm concerned I'm missing something here.

I can't think of anything you are missing. You seem to be doing it 
right, but be sure to install a safety gap and Terry filter to help 
prevent as much "bad stuff" as possible. NST's can't take too much 
abuse, so we have to protect them as much as is humanly possible if 
we want them to live a long life.

Take care,
Bart