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Re: Tuning Question
Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
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?
* 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
* 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.