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Re: [TCML] Proper VTTC Coupling? (coil specs)

John,
My original plan was to use 16 or 14awg wire for that reason of keeping the coil short. The 12awg wire i have is a special type which will give a primary 
winding height of only 2.8".  The 14awg wire would give the same height.
But the 16 gauge wire I have has thinner insulation and would give a much shorter coil. I agree with what you're saying and I'm going to switch to 16awg for the primary. That will increase the primary inductance, so maybe 
I can use fewer turns too.  That will let me put the grid coil on top of
the primary. I've built coils both ways; grid coil on top, grid coil below.
I do have an air variable which I plan to use to find the proper tune point, then I'll replace it with a fixed cap. Actually I have an air variable which 
is in oil to withstand higher voltage and give a higher capacitance.
I've used the 4-125A tubes in previous coils, and also a 4-250A and
a 4-400A.  The last tube was defective however and gave weak sparks.
I have a good selection of mica caps.

Appearance-wise, I prefer a shorter primary coil also, so that's another
reason for me to use the 16awg wire.  It would be interesting to try
magnet wire for the primary coil.  I have some 14gauge magnet wire.
I also have some litz wire, but I forget the gauge.  I seem to remember
hearing that litz wire is particularly helpful above 500kHz if I remember 
correc
tly.

Thanks again,
John


-----Original Message-----
From: Dr. John W. Gudenas <comsciprof@xxxxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Sat, 18 Oct 2008 10:34 pm
Subject: Re: [TCML] Proper VTTC Coupling? (coil specs)


John 
The 4-125A power tetrode should work fine. It is a VHF tube and the plate voltage is 3kv so a doubled MOT won't kill it.  However, consider using perhaps 14 awg or 16 awg for the primary as 22 turns of 12awg pvc is going to make a solenoid  around 5" -> 6" high on a 9" secondary. This is bound to cause some sort of problem, particularly if the grid coil is above it.  With this small coil you might consider putting the grid under the primary.  
 
With a .00005uf tank capacitor the current in the tank circuit should not get very high as long as the secondary is tuned to  the primary frequency and you can use the energy in the secondary and dissipate it with the discharge. That is why I believe  
you can use 16 awg or 14 awg on the primary. 
 
As long as you have a good supply of pico farad micas you should be able to tune it in. It will certainly be sensitive.  If you have an HV air variable from a kilo-watt ham rig it may be useful.  
 
Interesting coil, let's see what happens. 
John W. G. 
 
John W. Gudenas, Ph.D. 
Professor of Computer Science 
 
On
Oct 18, 2008, at 8:13 PM, futuret@xxxxxxx wrote: 
 

Dr. John, 
 
Thanks for your comments. The full specs of the planned 
small, simple coil are: 
 
Tank cap, mica 0.0005uF or as needed to tune 
 
secondary, 2.5 x 9 inch, 28awg (600+ turns) 
 
primary, 4.5" (?) dia, 22T of 12awg pvc insulated wire 
 
grid coil, 4.5" dia, 18T of 26awg on a sliding form 
 
transformer, small MOT with level shift doubler 
 
small 2.5" dia toroid (maybe) 
 
vacuum tube, 4-125A with socket 
 
filament transformer as needed 
 
grid leak resistor, 30W variable up to 4k ohm 
 
misc, bypass caps, tube cap, hook up wire, hardware, etc. 
 
Base, wood at first to test everything 
 
variac, external 
 
I'll try both the grids tied together method, and maybe 
the screen grid fed by a dropping resistor method. I'll 
tap the grid coil at 12 turns or so for use with the tube 
grids-tied-together method. I'm considering using a 
811A tube or some other tube rather than the 
4-125A tube. I may test various tubes in the coil. 
 
I'm trying to decide whether to use a 4.5" dia primary 
or a 4.75" dia primary. The 4.5" form is thick PVC, 
the 4.75" form is plexiglas. I already have the secondaryA

0

coil from previous work. 
 
I may use the coil to research various configurations, etc. 
It should make a nice demo coil if the design works out well. 
 
John 
 
 
-----Original Message----- 
From: Dr. John W. Gudenas <comsciprof@xxxxxxxxxxxxx> 
To: Tesla Coil Mailing List <tesla@pupman 
.com> 
Sent: Sat, 18 Oct 2008 6:59 pm 
Subject: Re: [TCML] Proper VTTC Coupling? 
 
 
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