Re: [TCML] Small VTTC Progress

[DC Cox <resonance@xxxxxxxxxxxx>]

John:

You might consider metering both your plate current and grid current.  Then
you can make accurate adjustments to the resonance --- very
similar to tuning up an amateur radio xmtr with an antenna.

Dr. Resonance




On Sun, Jan 18, 2009 at 2:19 PM, <futuret@xxxxxxx> wrote:

> Here I describe some more progress on the small VTTC project that I built
> a few months ago.
>
> I had not planned to use a toroid, but I noticed the spark output was
> stronger
> when I put a plate of metal atop the secondary, so I figured maybe it needs
> a
> toroid. I installed a small 1" x 3" toroid, then re-tuned using the
> variable oil cap.
> The spark seemed stronger, but later when I ran the coil again it was
> weaker.
>
> I was using the 4-500 tube which I know is defective. So I installed the
> original
> and correct 4-125A tube. This caused the spark to increase from about 3.5"
> to
> 6" which is not too bad.
>
> Originally I calculated that an 500pF tank cap would be needed for this
> coil,
> but with spark loading and no toroid, it actually needed about 750pF.
> Possibly
> it needed even more tank C, but I didn't add more. So actually I'm not 100%
> sure
> the coil needs a toroid. But it looks a lot better with the toroid anyway.
>
> After installing the toroid, 1500pf total C was needed for the tank cap.
> The more powerful spark output helped to increase the needed amount of
> tank capacitance to the 1500pf value. Using an AM radio and tuning for
> most
> TC noise, the frequency was around 550kHz.
>
> I thought I'd get a little more spark length from this coil, but there are
> many
> factor
> s that may be limiting the output. The MOT is rather small, the tube is
> small, the secondary is small with only about 620 turns, etc. Also, I'm
> just
> tying the two tube grids together and using fewer grid coil turns. This is
> what I did on my previous 4-250A VTTC and it worked OK. I had compared
> the performance back then with the screen grid fed by a dropping resistor
> method, and the results were the same either way.
>
> Probably if I used a larger vacuum tube, I'd get longer sparks, but I want
> the tube to be small because I don't like the appearance of a VTTC when
> the vacuum tube is as large as the secondary coil.
>
> I tried using more grid turns, but this didn't help. I also tried the
> raised grid coil
> position, but this reduced the spark length. I also tried the raised grid
> coil position
> but used more grid turns to compensate. This almost restored the 6" spark
> length, but the coil didn't seem to run as smoothly. I kept the lower grid
> coil position.  There's about 1/2" space between the primary and the grid
> coil.
> The grid coil is positioned above the primary coil on a Lucite form.  The
> secondary is also wound on a Lucite form.  A blue ceramic beehive insulator
> is attached to the top of the secondary coil, and the toroid is place on
> top
> of this insulator.  At full power (140VAC input), the tube plate runs
>
> an orange/red
> color which is normal for this tube.
>
> Coil specs:
>
> Xfrmer, small MOT 2kVAC with level shifter (two 1uF MO caps with MO
> diode).
> Tank cap, single 1.5nF, 6kV rated mica.
> Tube, 4-125A, grids tied together.
> Filament Xfrmer, 6.3V with dropping resistor (50 ohm, 25W) on the input
> line to drop the voltage to 5V under filament loading.
> Secondary coil, 2.5" x 8.875" winding, 620 (?) turns of 28 awg magnet
> wire.
> Primary coil, 4.75" x 2" winding of 16awg pvc insulated close wound wire.
> Grid coil, 20 turns of 24awg magnet wire tapped at 12 turns.
> Grid capacitor, 0.001uF, 2.5kV.
> Bypass capacitor, 0.001uF, 5kV.
> Grid resistor 1.5k ohm, 75W fixed.
> Toroid, 1" x 3" spun aluminum.
> Spark emitter spike, sharpened nail extends 4" above toroid.
> Note 1; no staccato system is installed.
> Note 2; sparks are fuzzy, not swordlike.
>
> Cheers,
> John
>
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