Dear Group,
Thank you to all who answered my questions.
My best results were 30cm at around 1500W.
Primary: 54 turns speaker wire 1.5mm
Secondary: 0.5mm 380 turns
Topload: Washer 4cm dia with welding tip for breakout
Tank cap: 125pf 25kv Vacuum Cap
Tubes: Twin GU 81m
Power: Half wave rectified MOT with charging reactor
I noticed a few things but I could be wrong:
1. Secondary and primary work best when they are the same length or a
atleast 80% of the secondary length. Less spot heating issues and better
performance.
2. Under 600 turns is easier to tune and less prone to flash over
3. Higher inductance secondaries with low tank caps run cooler
None of my systems use a grid leak system as I can never place the feedback
coil in the right spot. So I wind 5 to 15 turns extra to the base of the
secondary. The joined wires go to earth and the lowest wire goes to grid.
Adjusting resistance going to the grid lets me know if I have too many
turns. Then all I do is add or remove turns to tune and remove the
resistor. I have not had problems with melting or hot grids (yet).
If the tube oscillates at several mhz I know the primary connection needs
to be reversed. A bit dodgy but I came up with the idea out of frustration
and lack of powerful resistors.
The transformer ratio of my VTTC is 1:7 but I am definitely getting more
than 7 times the voltage out.
I don't understand the technical aspects too well but I was enquiring about
the 3 coil system because if I could just double the voltage of the
secondary with another coil I would achieve my target.
Seems as I am limit to 4kv on the primary side.
But it's probably not that simple...
Cheers!
-Wil
On 02/11/2014 2:26 AM, "Futuret via Tesla" <tesla@xxxxxxxxxx> wrote:
> 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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>
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