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Re: [TCML] Small VTTC Progress
John -
This sounds like an interesting little coil.
To get a better idea of your primary inductance, how many actual turns are there? You didn't mention if the 2" winding length was close-wound, or spaced.
How long can the 4-125 operate (generating the peak 6" output) before the plate gets red hot?
Regards,
Herr Zapp
--- On Sun, 1/18/09, futuret@xxxxxxx <futuret@xxxxxxx> wrote:
From: futuret@xxxxxxx <futuret@xxxxxxx>
Subject: [TCML] Small VTTC Progress
To: tesla@xxxxxxxxxx
Date: Sunday, January 18, 2009, 1:19 PM
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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