[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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.



_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla