From rweeks@clark.netFri Jan 19 20:30:03 1996
Date: Wed, 15 Nov 95 10:54:42 0000
From: Robert Weeks <rweeks@clark.net>
To: billbeskimo.com
Subject: Tesla coil instructions

Vacuum Tube Tesla Coil
This Tesla coil uses a vacuum tube oscillator circuit powered by 500 
Volts DC instead of an induction coil and a spark gap.  This updated 
design is based on a Tesla coil described in an early 1950's Science and 
Mechanics article by Tracy Diers.  The silent operation of this coil 
more than makes up for the slightly lower performance as compared with 
the neon sign transformer excited coils with roaring spark gaps that I 
have seen.  Since this is a project that uses parts that may be on hand, 
certain specific details cannot be supplied.  

When operating, a corona about 1/2 inches long will shoot from the 
insulated terminal.  If a hand-held (making good contact with your hand) 
metal rod is brought near the insulated terminal you should be able to 
get a good 1.5 inch arc!  (The tube that I am using is severely 
overloaded when arcing like this is done and such tests should not be 
allowed to continue for more than a few tenths of a second.)

A 36 inch piece of 2 inch (id) pvc pipe makes a good secondary coil form 
and is much easier to find than a 2 1/2 inch paper tube.  The outside 
diameter of this pipe is about 2 3/8 inches.  Clean the pipe thoroughly 
to remove dirt etc. which may have collected on the pipe while it was in 
storage.  The secondary coil is wound using 1/2 lb of #30 enameled wire.  
Drill a 1/32 inch hole in the form 1 inch from one end of the pipe. 
Thread the end of the #30 wire in this hole secure qith masking tape and 
wind the coil until all the wire is used up.  An exact winding of #30 
wire is approximately 90 turns/inch.  Half a pound is enough for about 
28-30 inches of winding.  

Hints for winding the coil:
Use masking tape to secure the coil as it is being wound on the form 
every 1/2 inch or so to prevent the coil from "springing" loose.  For 
such a small coil, a coil winder is not required, but using one will 
certainly make the job easier.  Hand winding takes about 4 hours to 
complete.  When the coil is finished drill another 1/32 inch hole at the 
top of coil and thread the terminal end of the wire into this hole.  
Secure the wire thoroughly again with masking tape.  Carefully remove 
the masking tape that is temporarily holding the coil in place as you 
varnish the coil.  This is tricky because the varnish dissolves the 
adhesive.  I started varnishing the coil at one end and allowed it to 
dry slightly and then removed more of the tape.  After finishing the 
secondary coil, place it aside and allow the varnish to dry.  Don't use 
an oven because of danger of igniting the varnish or softening the coil 
form.  

Wind the primary using #14 solid insulated wiring.  The type that is 
suitable for use in conduits works well.  If you can get enameled wire 
it will wind easier, but recently it seems to be difficult to get 
enameled wire.  You will need about 40 feet of wire.  An oatmeal box 
(small size) makes a good coil form for the primary.  Start 1 inch from 
the bottom and punch a hole in the coil form.  Insert about 6 inches of 
wire into the form.  Wind the plate coil first at the bottom to the 
oatmeal box which consists of 15 turns of the #14 wire.  When you are 
finished leave about 6 inches of wire and insert this into a hole in the 
form.  The liberal use of masking tape will hold things in place until 
you can varnish the primary coils.  Start the grid coil 1 inch from the 
plate coil.  Punch a hole in the form and wind on 20 turns of #14 wire 
in the same direction as the plate coil.  This is important because 
proper phasing of the coils is required for the electronics to 
oscillate.  If the coils are wound in the same direction connection as 
shown in the schematic will be correct.  Varnish the primary and remove 
the masking tape.  

Assembly of the Tesla coil
My coil was assembled on a piece of scrap plywood about 8 X 12 inches.  
If you used pvc pipe for the secondary coil form, a pvc pipe cap makes a 
suitable device to attach the secondary and the primary to the piece of 
plywood.  Drill a hole in the center of the pipe cap.and locate the 
center of the bottom of the oatmeal box.  Secure the pipe cap and the 
oatmeal box to the plywood with a 1 inch screw and a washer.  Do not 
tighten yet because some adjustment will have to be made to get the 
primary leads to reach the appropriate pins on the tube socket.  

Every attempt was made to use on-hand parts.  Vacuum tubes are becoming 
rare and expensive.    The tube that I used has the numbers rubbed off 
it so I do not know what it is.  The basing was like a 6L6 but the third 
grid was not internally connected.  It was an Amperex tube with nothing 
like it in the 1964 RCA Tube manual.  I think that it did service as a 
horizontal oscillator in a color TV.  Use a tube manual and find out 
what you have on hand that will work.  Power transformers are also hard 
to come by.  The best places to get suitable tubes and transformers may 
be trash dumps where one sees an ocassional discarded tube type TV.

The following tubes will work well:  6DQ6B, 807W,6CD6 6GW6.  Consult a 
tube manual for proper connections.  
For lower power coils 6L6GB tubes can be used.  The 6DQ6 has the same 
basing as the 6L6, but the plate is connected to a metal cap on the top 
of the tube.  Most of these tubes work best when connected as a pentode 
with the screen grid biased to around 200-225 V DC.  The use of "triode" 
connection (screen connected to the plate) is not recommended except for 
low power testing.  If you use the voltage doubler circuit obtaining 
screen voltage from the 250 volt point is more efficient than using a 
10K resistor from the 500 volt point.  The 1K resistor may not be 
needed, but the tube that I used seemed to run better with a lower 
voltage at this point.  Just remember that the screen on most of these 
tubes will draw about 20-25 ma at 200 volts and you should not exceed 
the reccommended screen current for the tube.  C4 is used to bypass Rf 
from the screen to ground.  

There was enough space on the plywood board that I used for the 
transformer, caps and diodes.  

I used Jones barrier strips to secure the various components to the 
board and provide proper insulation.  I was able to get a Pomona socket 
designed for bread boarding which gives the finished coil a definite 
retro look.  Other wise attach the tube socket to the board with stand 
off insulators and connnect according to the schematic.  

The coils will be properly phased if the bottom of the plate coil (the 
bottom coil with the 15 turns) is connected to the +500 Volts, the top 
of this coil is connected to the plate of the tube.  The bottom of the 
grid coil is connected to the grid circuit of the tube and the top of 
the grid coil is connected to the ground via the 4.7 k resistor and the 
.0005 cap.  (The 38 ohm 10 watt resistor in the cathode circuit will 
produce about-10 volts of grid bias just in case the circuit doesn,t 
oscillate.)  

Punch a hole in the bottom of the primary form to allow the ground 
terminal of the secondary to be fished through.  I connected a piece of 
solid #22 hook-up wire to the coil ground end since it was more durable 
than the #30 that the coil was made from.  Run the ground wire through 
this hole and insert the secondary into the pipe cap.  It will be a good 
friction fit.  Connect the secondary coil ground to the ground terminal 
on the board.  You can use another pvc pipe cap drilled in the center 
for the HV terminal, but be sure use a smooth brass Phillips head 10/32 
bolt on the inside with a nut and washer on the outside.  (Use the 
Phillips headed bolt which does not have the sharp edges that are on a 
slotted screw.  This will help to prevent a corona discharge on the 
inside of the coil.)  Put sleeving over the #30 wire and connect it to 
the brass bolt using a #10 round solder terminal.  Coat the bolt head 
and the wire with RTV and allow to dry.  I originally assembled the bolt 
with the nut inside the coil which resulted in an internal discharge 
that ignited the top of the inside of the secondary!  Since this was 
inside the tightly closed pipe, I was not aware of what was occuring.  
The resulting fire carbonized the top of the inside of the coil.  This 
ring of carbon acted as a shorted turn in the secondary.  This produced 
a dramatic drop in output from the coil.  The plate of the tube and the 
grids quickly turned red hot.  I discovered the meltdown in the 
secondary coil form while attempting to diagnose the high current-loss 
of resonance condition.  Fortunately I had left about 4 inches of form 
at the top with no windings.  I repaired the secondary by cutting off 
the burned top 2 inches of the pvc pipe.  The burning pvc pipe produced 
toxic gasses such a HCL which had to be cleaned out of the coil form.  


The capacitor (C1) across the plate circuit should be at least 5KV.  I 
am using a pair of 20 KV 500 pf caps in parallel.  When the tube is 
conducting the full B+ appears across the plate coil with a current of 
200 mA..  This energy charges the up the magnetic field in this coil 
(L2).  Feedback from the grid coil to the grid sharply cuts off the 
current in the plate circuit allowing the magnetic field in the plate 
coil to collapse.  This magnetic energy in the coil is converted to 
electric energy.  To conserve energy the voltage across the coil begins 
to rise in an attempt to maintain the current at 175 mA!  This produces 
a 4-5 KV pulse that charges up the capacitor C1.  (Do not use a cap for 
C1 less than this voltage rating!  A "mica" "transmitting" cap is what 
you want to use.  Some of the ceramic caps seem to have a high internal 
resistance at 1 Mhz and tend to get very hot in operation.  If this 
ocurrs find another cap for C1.)  
When the voltage across C1 is a maximum and the current in the plate 
coil is zero, the voltage stored in C1 again flows through the coil and 
brings up the magnetic field.  If the coil had no resistance and the 
capacitor was also perfect, the oscillation would continue, but since 
this is the the real world, the wave quickly damps out.  However the 
power supply continues to make up for the lost energy and the 
oscillations continue as long as the power is on.  

The secondary coil is magnetically coupled to the primary.  The voltage 
is steppped up by a ratio of 90 turns/in X 28 in/15 turns (about 170).  
If the maximum voltage on the primary is 4.5 KV to 5KV then the voltage 
on the secondary is 170 X 4KV to 5KV = 68,000 to 84,000 Volts!  

C3 is used to keep RF out of the power supply where it could cause 
failure of the power transformer.  


Testing the coil:

This coil uses very dangerous voltages and if operated incorrectly can 
cause bodily harm and destruction of property.  Before applying the 
maximum voltage, I connected the filament winding on the transformer to 
the tube and used two 115 volt to 24 volt transformers connected with 
the secondaries series aiding with a bridge rectifier and a 100 uf 
capacitor to test the coil.  This arrangement provided about 67.5 volts 
DC.  The coil oscillated and small arcs could be drawn from the top of 
the coil.  Fluorescent tubes would light up within a few feet of the 
coil with this small amount of B+.  For testing connect the screen 
supply to the B+ terminal on the power supply.  

Schematic and pictorial are in mac Draw.  I can supply in gif format if 
that would be better.

I have the following questions?
The question is this:

In the original instructions upon which I based this design, the 
secondary was wound from #30 DCC (double cotton covered ) wire for a 
distance of 21 inches.  Based upon tables in 1942 edition of the 
Radiotron Designer's Handbook from Australia this is 
54turns/in*21in=1134 turns.  My coil contains 28 inches of #30 enameled 
wire which is 92.5 turns/in*28in=2590 turns.  (the tables contain 
figures for B&S and SWG.  I don,t know whether the wire I have is SWG 
which means Standard wire Gauge or B&S.  Is B&S a British standard or is 
it Briggs and Stratton?  #30 B&S enameled wire is .010 inches  and #30 
SWG is .0134 inches according to the tables in the book.  By using a 
vernier caliper, I have determined that #26=.5mm,#28=.4mm and #30=.3mm 
so converting to inches, I have 
#26=.019 in,#28=.015in and #30=.011 in so the B&S table looks better.       

When I use a neon lamp as a field strength indicator, there is a maximum 
about 12 inches above the bottom of the coil with a subsequent drop of 
voltage as the neon lamp is moved toward the top of the secondary with a 
null occuring at the  19.75 inch point.  The voltage then increases as 
the indicator is moved to the top of the secondary.

Using the B&S table and the original design of 21 inches of DCC wire 
that gives me 54*21/92.5=12.3 inches which is remarkably near where I 
see the first maximum!  (There are 1134 turns of wire at the 12.3 inch 
point.)

Obviously this is a tuned circuit and even with an untuned secondary 
resonance of the entire circuit is important.  However I don't 
understand why there would be maxima and minima along the winding.  
Usually the turns ratio is important.

I wonder if the tuning of the primary needs to be changed to match the 
secondary?  Any clues.  This circuit is remarkably like the horizontal 
output for a TV set so even though there are things about Tesla coils 
that are not well understood, This should not be too hard.  
rww