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Re: [TCML] VTTC Poor Performance.
Here I'll discuss a number of issues relating to VTTC
operation.
When only one normal MOT is used, then it helps to
remove the shunts. This increases the spark length
from about 21" to about 24". Using two large MOT's
as you are, I would think you'd have enough power to
get longer sparks even without removing the shunts.
I'm not 100% sure since I never used two MOT's in
series along with doublers.
Your voltage is dropping to 100 volts because the
coil is drawing so much current. Are your power
line wires thick enough to help prevent line voltage
drop? The power wires should be at least 12awg especially
if long. I think the high current
draw is not due to the nature of the MOT's nor
is it due to saturation of the MOT's. Instead it's
due to an aspect of the VTTC ( I discuss this
below). By the way,
I'm surprised you don't get any arcing in the tube
since I've always gotten arcing when I went over
around 5 to 7kVAC to the plate. I realize that
Cameron Prince also used 8kVAC or so and didn't
get any tube arcing. The only explanation is that
the circuit is being heavily loaded thereby holding
down the voltage.
Here's an important key to all this. The power
draw of a VTTC is proportional to the
input voltage. If you double the input voltage, the
coil will try to draw 4 times the power. Steve Ward
used probably around 5kVAC for his coil, and I used
that same voltage for my coils. But if you go to
8kVAC, you have to throttle things back, but not
by ballasting. VTTC's should not be ballasted.
It's the VTTC circuit itself that limits VTTC current draw.
Now I'll mention the key to all this which is
the issue of tank impedance. The higher
the input voltage that is used, the higher the tank
impedance must be. The tank impedance is increased
by using more turns in the primary and a smaller tank
capacitor to stay in tune. For example if you're using
22 turns in the primary, you may need to increase this
to 27 turns or more to limit the current draw of the system.
This I believe is why your coil is drawing so much current.
If you remove the MOT shunts, the system will probably
draw even more current. However removing the
shunts will not do any harm, if you have a suitable
number of turns in the primary. Removing the shunts
will probably make the transformer saturate slightly
easier, but it's generally not a problem. I always run
140 volts into my single MOT VTTC coils with shunts
removed, and all is well.
However, consider Cameron's system. As far as I know
he kept the same number of turns that Steve used for his
primary. However Cameron runs with staccato, and
probably cannot run without it... the system would
draw too much power. So by using staccato, you
can run the tube at higher voltages and higher peak
powers, yet keep the total power within suitable limits.
This is what let Cameron obtain 36" sparks using a
single 833 tube. Cameron mentioned that he had
to raise his grid coil very high above the primary.
I suspect he had to do this to control the system
due to the high input voltage and high peak powers of
the system. At such voltages, the tube is no lower
running within its design parameters.
Cheers,
John
--------------------
-----Original Message-----
From: Phillip Slawinski <pslawinski@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Thu, 11 Sep 2008 1:54 pm
Subject: [TCML] VTTC Poor Performance.
I recently built a 833C VTTC. I've been toying around with the coil and
made little changes here and there which made minor improvements to the
performance. The coil is pretty close to being in tune I believe. Even
with the coil in tune I only get 18" streamers. I'm beginning to think
that
the MOTs I'm using may be to blame. I have a 30A panel meter hooked up
to
measure total system current. At about 70V or so I reach 20A current
draw.
If I push the variac further I hit 100V and never seem to get above
that.
The VTTC causes a volt drop which pulls down the filament voltage.
Lately,
I've been running the filament transformer off a seperate line. The
system
current still soars off the meter when I try to run the coil at full
voltage.
I have two very large MOTs that came from older microwaves. I wired the
MOTs in series for 8kV after the voltage doubler. I have not removed
the
shunts from the transformers.
Would removing the shunts from the transformers help in my situation?
Does
the presence of the shunts cause the transformer to saturate any faster
than
it would otherwise?
My system is almost exactly like the single 833A coil on Steve Ward's
site.
The major differences being that I have a 9x3 topload, my MOTs are
wired in
series, I have an 833C, and I used a 10K resistor in the feedback
circuit.
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