Re: Post from newbie (VTTC's)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 2/24/03 1:53:22 PM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

>Hi,
>
>My name is Jerry.  I have been a ham radio operator for the past 50
>years but I am new to tesla coils.  I just retired and have more time to
>build "stuff".  I have worked as an electronic tech for most of my life
>and have built a lot of ham gear, including transmitters, receivers and
>antennas from the HF bands up through 432 MHz.  Power levels up to 1000
>watts.


Jerry,

Welcome to the group.  Since you're used to building electronic
equipment, a tube coil will be a nice project for you, and you'll
be able to ramp up quickly and obtain excellent performance.


>I have been reading the mail for about six months but I have not seen
>much info on tetrode tube coils.


I once tested a 4-1000A tube, but it was defective, so that project
never got anywhere.  I use a 4-250A in my smaller staccato TC
which can be seen at my website at:

   <http://hometown.aol-dot-com/futuret/page3.html>http://hometown.aol-dot-com/futuret/page3.html

click on the link for tube coils, etc at the site.   Of course this
is a smaller coil design.  I used the same primary and secondary
for up to a 24" spark length.  A 4-1000A tube could be used with
the same basic coil, since I've used (2) 833A's and got 24" sparks
with that primary and secondary.   Using a larger 6.5" x 24" secondary
wound with 28awg wire, and powered by (4) 833A tubes, I obtained
up to 38" sparks.  I tried using thicker wire on this coil, but the
results were not as good, but I need to do more tests.

I generally use a small toroid on the small tube coils to lower the
frequency, and to add an extra inch of spark length.

In the 4-250A coil, I just tied the grids directly together.  I don't
know if this is OK to do with the 4-1000A.


>I have a 4-1000A tube and would like to try my hand at building a VTTC.
>I plan to use six MOTs wired in series/parallel to give twice the
>voltage and three times the current of a single transformer.


Lately, I like to use level shifted MOT's as a plate supply.  This
uses the MO diode (or other HV diode) along with a number of
MO caps, to create a crude doubler circuit.  This is the same circuit
as is used in the MOT, but is of the opposite polarity.  The level
shifted design is more lightweight and probably more efficient than
using a non-level-shifted supply.  The schematic at my site shows
this design.  Using this method, you most likely can use a single
MOT to power the coil.

    This

>should supply 4 to 5 KV at about 700 mA, close to the specs of the
>tube.  I plan to connect the raw AC directly to the plate tank of the
>tube.  I am going to use the tube in a triode configuration by
>connecting a 10K resistor between the control and screen grid and feed
>the input signal to the screen.
>
>I would appreciate any comments on this design.
>
>Would I need PFC caps on the MOTs?


I never tried using PFC on a tube coil.  I seem to remember that the
power factor was pretty good without it.


>I have some questions concerning the design of the primary and secondary
>coils.  A ballpark design in this area would help me a lot.
>
>Primary diameter?
>Primary wire size?
>Number of turns?


The parameters are not too important it seems.  My small
coils use the 3" x 11 3/4" secondary wound with 28awg magnet
wire, and the primary is 6" dia x 3.25" or so, and is wound with
23 turns of 12awg pvc insulated wire.  A larger primary and secondary
with thicker wire may reduce the losses however and give somewhat
better performance possibly.  In my case I wanted to keep the coil
compact.  I like the appearance when the spark length is about
twice as long as the secondary length also.


>Secondary diameter?
>Secondary wire size?
>Number of turns?
>
>Number of turns on the grid coil?


Often the grid coil should have a similar number of turns as
the primary.  But if the grids are tied together, then fewer grid
coil turns are needed.  For example when I feed the screen
via a dropping resistor from the plate supply (on my 4-250A coil),
I use 20 turns in the grid coil.  But when I tie the grids together,
then I use only 8 turns (if I remember correctly, it's at my website).


>Size of grid leak components?


A 100 watt variable resistor is nice.  The resistance will depend on
whether the grids are tied together.  If tied, a low resistance will
be best, perhaps 1500 ohms.  But if the screen is fed separately,
then a 5k to 10K ohm may be good.  I'm doing some guessing on
the values because I've never used the 4-1000A.  I know that Ross O
built a tube coil with that tube and got 28" sparks I think.  I like to
use an 0.002uF to 0.004uF cap for the grid leak resistor

These large tube coils really roar and are ferocious sounding.
I also like to use the staccato (pulse) feature to reduce the input
power and to add interesting sounds and visual effects.


>I have some large mica transmitting caps to use in the grid leak and
>primary tank.


Excellent.  Sounds like a great project, and I'll be looking forward
to hearing of your results.  Since the 4-1000A tube is rated for 6kV,
it can probably handle an even higher voltage, and it would benefit
efficiency-wise by using a higher voltage.

John


>Thanks,  Jerry  -  W8UJX
>