[TCML] Small VTTC Progress

[futuret@xxxxxxx]

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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