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       X1 = Neon Sign Transformer, grounded center tap
       SG = Safety Gap. 
       G1 = Main system (tank circuit) Spark Gap.
       C1 = Tank circuit capacitor
       L1 = Primary Coil.
       L2 = Secondary Coil.
       T1 = Toroid discharge terminal                              
      Gnd = RF Ground.

I have deliberately left out bypass capacitors and RF chokes to show 
the correct circuit in the simplest form. Reading from the left to
the right, we start with the line in to the neon transformer. I am 
showing three leads leaving the transformer towards the coil. The
center lead is the internal ground on the transformer and case. 
The transformer case is grounded to the RF ground. A safety gap 
with center post is placed between the two high voltage bushings. 
The safety gap center post is connected to the RF ground and forms 
the ground reference for the high voltage circuit.

G1 is placed directly across the two high voltage bushings. 

C1 is placed in series with the primary coil.

Bypass capacitance is placed across the HV line, and RF chokes are 
placed in series with the HV line, between SG and G1 in this diagram.
(not shown here for simplicity, see additional circuits below) 

The actual oscillator or "tank circuit" is the loop between L1, G1, 
and C1. This circuit, patented by N. Tesla, is the most fundamental 
RF oscillator known. Since the inductance is commonly referred to 
as "L" in circuit diagrams, and capacitance is commonly referred to 
as "C", this is known as an "LC oscillator". The loop between the 
inductance "L", capacitance "C", and the switch (in this case a spark 
gap) forms a "tank" where heavy currents can oscillate with low 
impedance. This circuit is properly called the Tesla tank circuit.
There is no need to "ring" or oscillate the tank circuit off of a 
ground reference. The tank circuit will actually ring better in most 
cases if it is allowed to "float" ungrounded. Any imbalance or kick-
backs will seek the center post at the safety gap for a ground 
reference, so the circuit is as safe as any.

A variation on this circuit (one of many) is shown below, where 
the location of C1 and G1 are reversed:


    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
                                                     ---------
                                                         O
         X1                                              O
                                         G1              O  L2    
           O--------------------------->*  *<----        O
         ||O    |                   |           |   L1   O
         ||O    |                   |           |    O   O
   -----O||O    * SG             -------        |--->O   O
        O||O----*---|Gnd         _______ C1          O   O
   -----O||O    *                   |                O   O
   120   ||O    |                   |                O   O
   Volts ||O    |                   |                O   O
           O-----------------------------------------O   O
                                                         |
                                                         |
                                                        Gnd


Another variation, showing one end of the primary coil grounded 
to the base of the secondary coil is shown below. The safety gap
should be placed across the HV bushings on the capacitor C1.
NOTE: THE CIRCUIT BELOW CANNOT BE USED WITH NEON SIGN TRANSFORMERS!


    TESLA COIL SCHEMATIC                              ---------
                                                      |       | T1
                                                      ---------
                                                          O
         X1                                               O
                                         G1               O  L2    
           O--------------------------->*  *<----         O
         ||O                  |                  |   L1   O
Line in> ||O                  |                  |    O   O
--------O||O               -------               |--->O   O
        O||O               _______ C1                 O   O
--------O||O                  |                       O   O
         ||O                  |                       O   O
         ||O                  |                       O   O
           O------------------|-----------------------O___O
                              |                         |
                              |                         |
                             ---                       Gnd
                             Gnd


These are just a few of the many, many tank circuit configurations 
explored by Tesla. In the circuit below there is a grounded center
tap in the middle of the primary coil. The value of C1 is divided 
into two equal parts (C1 & C2) which are then placed in series with 
the primary coil. The gap is across the line:


    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
                                                     ---------
                                                         O
                                                         O
                                                         O
                                                         O
         X1                              C1              O
                                         | |       L1    O  L2    
           O-----------------------------| |-------->O   O
         ||O                        |    | |         O   O
         ||O                        |                O   O
   -----O||O                        *                O   O
        O||O-------Grnd-------------*--------------->O   O
   -----O||O                     G1 *                O   O
 Line >  ||O                        |                O   O
         ||O                        |    | |         O   O
           O-----------------------------| |---------O   O
                                         | |             |
                                         C2              |
                                                        Gnd


Tesla experimented at length with the circuit diagrammed directly 
above in the latter half of August, 1899. This is a variation of
the "balanced" or "equidrive" Tesla tank circuit. Tesla's diagrams 
and performance notes may be found in the COLORADO SPRINGS NOTES.

Quoting gcerny-at-ix-dot-netcom-dot-com (Glenn Cerny):

> Richard,

> Your follow up to my earlier post has me wondering how to
> connect my bypass caps into my system.  I have the 40000mmfd
> -at-30kv from HVPress.  I was thinking that I would use one on
> each leg to rf ground somthing like below:

I added in my own ACSII artwork to show more detail:

    X1                                           
    ||O--------------------------------nnnnnnnn--TO-TANK-CIRCUIT->
    ||O            |             |        RF1
    ||O         ------- BC1      |                                  
---O||O         -------          |
   O||O            |             *
   O||O--GRND------|-----GRND----* SG                             
   O||O            |             * 
---O||O         -------          |
    ||O         ------- BC2      |                              
    ||O            |             |        RF2                 
    ||O--------------------------------uuuuuuuu--TO-TANK-CIRCUIT->
                                                 
Where:

        X1 = Is the step up xfmr with grounded center tap
      GRND = Dedicated RF Ground for the HV wiring and secondary coil
       BC1 = Bypass Capacitance
       BC2 = Bypass Capacitance
        SG = Safety Gap w/grounded center post
       RF1 = Radio Frequency Choke
       RF2 = Radio Frequency Choke

This circuit should be set up such that the values of BC1 & BC2 are
matched or at least close in value. The same would also go for the 
values of the inductance RF1 & RF2. 

> Using one cap per side with a common rf ground in centerpoint 
> to each leg.  I think you get the picture.  Can you recommend
> if this is good, or perhaps a better design available. I did 
> buy 4 of the caps.  If more are needed, I'll get them.

The diagram above shows how I wire the HV filter/safety gap into 
the HV buss that supplies the Tesla tank circuit. This is assuming 
that I am using a grounded center tap power supply with neons or other 
center tap ground transformers. You will need more than one .004 MFD 
capacitor in series between each HV buss and the ground. You have 
four of these already, I would purchase at least two more, preferably 
four more. 

I have used these exact same caps in several filter boards. I have 
found that you need at least three of these caps in series between 
each HV buss and ground when using 9kv or 12kv neons. You need four 
of these caps in series between the HV buss and ground when using 15 
kv neons. Check the caps and ground conductor for warming as an 
indication that too much bypass capacitance is in the circuit. Too 
much bypass capacitance allows parasitic oscillations between the 
power supply and the ground, a condition which must be avoided to 
prevent fires and power supply failures. Generally I don't like
using much more than .001 MFD of bypass capacitance between any
HV buss and ground, or directly across the HV buss. This is one 
instance where a little less capacitance is really better than a 
little more. 

Word on the Net has it that Nick Andree, 305 Wisconsin Ave., 
Oceanside, CA 92054 has these same capacitors priced under $5.00
each. It does not hurt to have a dozen or so of these on hand.

> If you have a favorite circuit design showing placement of various
> components, please send that along too.  I am really undecided how
> to approach the design for cap, gap, choke, safety gap placement.
> I have seen a lot of designs to choose from and the "classic" is what
> I am leaning to.

    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
                                                     ---------
                                                         O
         X1                              C1              O
                            RF1          | |             O  L2    
         ||O---------------nnnnn---------| |-----        O
         ||O    |BC1   |            |    | |    |   L1   O
Line in> ||O  -----    |            |           |    O   O
--------O||O  -----    * SG         *           |--->O   O
        O||O----|------*--|grnd       G1             O   O
--------O||O  -----    *            *                O   O
         ||O  -----    |            |                O   O
         ||O    |BC2   |            |                O   O
         ||O---------------uuuuu---------------------O   O
                            RF2                          |
                                                         |
                                                        Gnd

       X1 = Neon Sign or other xfmr with grounded center tap
       SG = Safety Gap. 
      BC1 = Bypass Capacitor
      BC2 = Bypass Capacitor
      RF1 = Radio Frequency Choke
      RF2 = Radio Frequency Choke
       G1 = Main system (tank circuit) Spark Gap.
       C1 = Tank circuit capacitor
       L1 = Primary Coil.
       L2 = Secondary Coil.
       T1 = Toroid discharge terminal                              
Grnd, Gnd = RF Ground.

Now for those of you who are using power supplies that do not
have a center tap ground (two bushing pole pigs for instance)
the correct bypass/filter schematic appears below:


    X1                                        RF1
    ||O------------------------------------nnnnnnnnnn--TANK-CIRCUIT->
    ||O            |             |         
    ||O            |             |                                  
---O||O            | BC          |
   O||O         -------          * SG
   O||O         -------          *----|grnd                       
   O||O            |             * 
---O||O            |             |
    ||O            |             |                              
    ||O            |             |                                
    ||O------------------------------------uuuuuuuuuu--TANK-CIRCUIT->
                                              RF2
Where:

        X1 = is the step up transformer WITHOUT grounded center tap
      GRND = Dedicated RF Ground for the HV wiring and secondary coil
        BC = Bypass Capacitance
        SG = Safety Gap w/grounded center post
       RF1 = Radio Frequency Choke
       RF2 = Radio Frequency Choke

When using a transformer without a grounded center tap it is best
to place the bypass caps directly across the HV line without a ground 
reference. I put four of the Sprague .004 MFD 30kvdc barium titanates 
(ceramic) in series and place the series stack directly across the HV
buss for the bypass capacitance. This method has worked extremely well 
for me in conjuction with a moderate RF choke inductance. I always 
place a grounded center post between the two hot electrodes of the 
safety gap to provide a ground reference for the power supply/tank 
circuit in the event of a flashover. 

The coil schematic could look something like this:

    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
                                                     ---------
                                                         O
                                         C1              O
         X1                 RF1          | |             O  L2    
           O---------------nnnnn---------| |-----        O
         ||O     |     |            |    | |    |   L1   O
         ||O     | BC  |            |           |    O   O
--------O||O   -----   *SG          *           |--->O   O
 Line>  O||O   _____   *--|grnd       G1             O   O
--------O||O     |     *            *                O   O
         ||O     |     |            |                O   O
         ||O     |     |            |                O   O
           O---------------uuuuu---------------------O   O
                            RF2                          |
                                                         |
                                                        Grnd


Richard Quick