An 833A VTTC (using just one tube) can give 23 or 24" sparks without staccato. But to
achieve this it has to pull more than 10amps. It should be pretty easy to get 18" sparks
from the coil as a start. Usually if the sparks are too short despite an orange tube plate,
it means the coil is not fully tuned up. The fact that the tube is red, despite not pulling
all that much power also suggests that more tuning is needed. I'm assuming you're
using a 140V step up type variac, which is usually needed to get full output sparks length.
You should be seeing most of the spark length increase right near full variac voltage.
If you get the 10" spark early in the variac knob rotation, with little or no rise as you
continue to turn the knob higher, that suggests the tuning is not precise.
The interesting thing about tuning a VTTC is that you can get full spark length
even with relatively poor tuning, but the tube will be too red. It won't be able to
handle the power. But by careful tuning, the spark length will remain the same,
but the tube will run cooler. That is the goal. Also tuning must be done at full
power. If for example a VTTC is tuned to give maximum spark at 50% variac
setting, it will never give a real long spark. At 50% variac setting, the spark
should be quite short and weak. It's only as the variac is turned up near the
maximum, that the sparks will suddenly get much longer and stronger. This
is because the capacitance of the spark pulls the coil into tune at the higher
power level.
An easy way to check the tuning of a VTTC is to install a larger or smaller
toroid and see which improves the performance of the system. The larger
or smaller toroid can just be a disc of metal since this is just for testing.
It doesn't need to look pretty. If the performance of the coil gets better
using a smaller toroid, then it means you need to use more L or C in the
primary tank circuit.
It's important to use a strong enough MOT in the circuit, and it can help
to remove the shunts although this makes the MOT run hotter and less
efficiently.
For some reason I've always gotten much better results with my VTTC's by
adjusting the tank C, rather than using a tapped primary. For my
standard setup, I used a 3" x 11" secondary coil (28awg), and a 6" diameter
primary coil using 23.5 turns of 12awg PVC insulated wire. I used
a 1.5" x 6" toroid.
John
-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: Tesla Coil List <tesla@xxxxxxxxxx>
Sent: Sun, Jan 14, 2018 10:03 am
Subject: [TCML] 833a VTTC advice
I just finished my 833A VTTC today and fired it up for the first time. After tuning the primary I am getting 10" sparks. The sparks look very good and are sword-like and brushy. Is this the expected spark length for a single 833A?
I am wondering if I can get longer sparks out of this coil. I am currently not using staccato although that is planned. My coil is based on Steve Ward's single 833A design. The component values are identical to those in his schematic. The coil is pulling 10 amps. The current draw seems to max out at about 10 amps with the variac turned all the way up. I had to raise the height of the feedback coil to about 3 inches above the primary to get maximum spark length (10"). My primary taps are at the bottom of the coil. The bottom of the primary is almost even with the bottom of the secondary. I tried changing the coupling by moving the secondary up and down with no effect. The tube plate glows a bright orange color. The plate is not uniformly orange. There are 2 bright orange spots on the plate. One is at the top middle and the other is towards the bottom middle. The top middle spot is brighter.
Any suggestions or comments?
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