Wil,
When a VTTC is tuned well (as opposed to just reasonably well
tuned), the sparks might not get much longer, but the vacuum tube will
run cooler (less plate redness). Optimal tune is achieved when
the spark length stays long, but tube redness is at a minimum.
I use a variable HV capacitor in parallel with the tank cap to
find this best tune point which can be quite critical. Then I
replace the variable capacitor with a fixed one of the same
value to reduce bulkiness of the system. I use the formula:
spark length (inches) = 0.5*sqrt input watts (wallplug)
to get an idea what spark length to expect for a given
input power. This assumes no staccato (pulsed) operation
of course. Staccato greatly reduces the input power while
permitting the spark length to stay the same.
Usually the primary is tuned considerably lower than the
secondary for best results. This is done for two reasons;
to run the coil on one of the over-coupled split frequency
peaks, and to account for spark loading capacitance
as the sparks get longer. For this reason a VTTC can
only be tuned at full power. Tuning at full power makes
a huge difference. If it's tuned for best spark
at lower power (using a variac), then it will not be tuned
well for high power. The primary may need to be
tuned 20% to 30% lower than the secondary resonant
frequency in many cases.
Toroids for a VTTC do not have to be very large. For
example a 3" x 12" secondary does well with a 1.5"
x 6" toroid. And of course a long break out spike is
needed to avoid stressing the vacuum tube.
In some VTTC configurations the value of grid leak
resistor, and feedback coil coupling can be rather
critical. I like the make the feedback coil position
adjustable so it can be slid higher or lower above
(or below) the primary coil.
Primary and secondary are not tuned to 1/4
wavelength. The secondary can be made 1/4
wavelength self-resonant, but this gives no benefit
and is best ignored. The 1/4 wavelength idea
is one of the Tesla coil myths that is still "out there".
Regards,
John
-----Original Message-----
From: William Howard <snakeprior@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sat, Nov 1, 2014 11:01 am
Subject: Re: [TCML] Technical Tesla Coil Questions
Dear Paul,
Thank you for your concise explanation.
I forgot that adding a topload lowers the resonant frequency of the coil.
So there is no way to match the topload to the secondary. Tuning is done on
the primary side.
A follow up question: are the primary and secondary oscillating at the same
frequency or at a 1/4 wavelength?
I get confused because I often see the 1/4 wave mentioned. JavaTC seems to
supply numbers for a matched resonance.
Is there a way to get a Tesla Coil to operate at 1/4 wave or do you need to
build a 3 coil system?
I have been battling to get longer than 30cm arcs from my VTTCs. It's
mainly due to my impatience and incompetence but hopefully there is a way
to improve the current system.
I don't have any toroids unfortunately. Will have to get one!
Cheers!
-Wil
On 31/10/2014 7:39 PM, "paul" <tcml88@xxxxxxxxxxx> wrote:
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