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Re: [TCML] VTTC grig coil help
Hi Jim,
You mentioned the grid coil is 11" dia, but the primary is
8" dia. Is this correct? I would think they'd be about the
same diameter.
Using filtered DC, the staccato circuit won't work using an
SCR. You may have to use grid staccato. When I was first
developing the staccato system, I used grid staccato, but it
was more complex than cathode staccato. According to
Dave Sharpe (I think), grid pulsing (staccato) is the prefered method
however.
In one test, I used a tetrode vacuum tube instead of a triode,
and applied raw AC to the screen grid, and filtered DC to the
plate. This permited cathode staccato (or control grid
staccato) to work. Of course the spark output was as if
I was using raw AC on the plate.
Staccato will cancel the hiss-like discharge, and create the
sword discharges, depending on the pulse rates. However,
to get long sparks, the filtered DC voltage needs to be
equal to the peak raw AC voltage, to get sparks of the
same length as would have been attained using raw AC.
Regarding the coupling of the secondary to the grid coil,
some folks seem to say it matters. I haven't noticed
any particular effect of the secondary. Some have suggested
running the grid coil very high up above the primary.
I tested that idea at low power but saw no advantage.
It has been suggested that the advantage only shows up
at higher powers. In any case, a grid coil that is higher
up (farther above the primary) , will need more turns than
a grid coil that is closer to the primary.
The wire in the grid coil can be somewhat thin. I used
18awg PVC insulated wire for my grid coil for the 36" spark TC.
In that coil I used about 20 turns on the primary, and about
20 turns on the grid coil. I had taps on the grid coil so I
could select various turns. I also was able to raise or lower
the grid coil. If there are not enough turns on the grid coil,
then you'll have to set the grid coil too low.... too near
the primary. This can result in a flashover between the
primary and grid coil. You only need one layer of wire on
the grid coil.
John
-----Original Message-----
From: Jim Mora <wavetuner@xxxxxxxxx>
To: 'Tesla Coil Mailing List' <tesla@xxxxxxxxxx>
Sent: Fri, Jul 3, 2009 7:00 pm
Subject: [TCML] VTTC grig coil help
Hello Esteemed Fellow Coilers,
I am engaged in the long haul of building a 4x 833a coil. I have been
collecting stuff for a few years.one of my prizes are original Johnson
thumb
screw sockets in NOS condition along with Johnson heat sinks - nice
patina.
The secondary is 6" acrylic. The primary is a 8" acrylic form.
Initially it
will be running CW for tuning and power tests ( 4 used tubes cam be
toasted
alright before I turn up the new 833C's), I'll run them near red to
test my
cooling effectiveness. Duty cycle will be solely accomplished by the
staccato circuit. The secondary will be wound 24" with #21awg and will
have
a small aluminum plate on top to keep the frequency up very near 350Khz
(Fo=~348Khz) which some say is a magic number for sword like discharge.
Will
the staccato duty cycle cancel the CW hiss-like high power and cause
the CW
to behave in the aforementioned sword like discharges? I do want to
modulate
this too in CW mode (no antenna ;-^) By the way, what is the best way
to do
that sans a big modulation transformer, since we are controlling
conduction
with a SCR or? which leads to another problem- in CW filtered mode -
how do
I turn off the SCR, power off B+?
I am trying to get my head around the feedback grid coil geometry. It
will
be wound on a small piece of split thin-wall PVC so it can slide up and
down
on the acrylic form. The primary is to be 3/16 soft tubing, (.1875")
spaced
by same a little over 20 turns - math looks pretty good so far. I know I
have to raise the secondary a couple of inches from the base to get K
in the
2X range (pretty tight but recommended in my readings). Cap = .003 mica
in
series parallel. I may be wrong, but I don't think the grid coil
coupling to
the secondary should matter? It seems to me the field from the primary
will
swamp the secondary's influence on the feedback coil. So here's the
question
for our VTTC experienced coilers: How many turns of what size wire
should
this critical grid feedback component have and should it have layers as
my
former is 11"? I am fat on many wire types, magnet and PVC coated in
multiple gauges. Please keep in mind that I have a very stiff PS (big
iron
and filter) that will deliver 2KW real power no sweat at full duty.
Thanks much,
Jim Mora
"The multi-tap big iron came from Tabasco Jack in San Diego, Kudo's
Jack! He
lurks here I think. It will be on its own casters separate from the
coil and
is a heavy two man/women lift! 12 taps: 3KV- 14,400.
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