Kaizer VTTC III article added

I found some old notes and a short video of a quick lash up vacuum tube tesla coil I made in a single day. I …



A VTTC is a Vacuum Tube Tesla Coil and it uses vacuum tubes / valves for the oscillator that is self biasing from a grid leak circuit.


WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm


This is a quick lash up done in a few hours and thus is only working that good. It will properly be out of tune since the primary coil is not adjustable.


Tube 1x PL36 line output pentode
High voltage supply 230 / 2300VAC microwave oven transformer.
Supplied through a variac.
Primary coil 75 mm diameter, 0.8 mm diameter copper wire (0.5 mm2) 18 windings.
Capacitor tank 3x 10 nF / 2kV Panasonic ECWH capacitors in series.
Secondary coil 50 mm diameter, 200 mm long, 1430 windings, 0.127 mm enamelled copper wire
Topload Two bottoms from beer cans / none.
Resonant frequency Around 470 kHz. Higher without topload.
Grid leak circuit 15 windings feedback coil, 2,2 nF capacitor and 68k grid leak resistor.
Input power 100 to 200 Watt, not sure.
Spark length up to 50 mm long sparks.


The high voltage supply is not grounded.


15th June 2009

A quick lash up that uses a single microwave oven transformer as supply for a single PL36 line output pentode from a old CRT monitor.

The microwave oven transformer high voltage is rectified by a diode from a microwave oven.

The Panasonic ECWH capacitors for tank and grid leak comes from the high voltage power supply circuitry found in TV sets using flyback transformers.


Running in CW mode.



A fairly good output of 50 mm sparks for a coil that was put together without any complicated calculations done up front and without being able to be tuned. Primary coil was made according to a quick run through JAVATC to find the resonant frequency.

But despite that effort it still performed better without topload on that with, which indicates it was oscillating at a higher frequency than 470 kHz.


Further reading

http://stevehv.4hv.org/VTTCindex.htm Steve Ward have documented 5 of his VTTCs, the staccato controller and written a good FAQ that is a must for anyone wanting to build a VTTC.

http://drspark.org/ Christopher Hooper have documented 3 of his VTTCs.

http://www.mif.pg.gda.pl/homepages/frank/vs.html Vacuum tube database, almost any tube data sheet can be found here.

Topload design and selection for Tesla coils

Published on: Sep 24, 2015. Updated on Oct 03, 2018.

This is chapter 10 of the DRSSTC design guide: Topload


The topload of a Tesla coil serves more than one purpose. The most important and why it is there in the first place is that it acts as a capacitor that can store the high voltage charge and it will also lower the resonant frequency of the secondary circuit.

The magnetic field around a topload will prevent sparks from forming on the top winding of the secondary coil and also in some degree prevent sparks from going inwards towards the secondary coil, primary coil and strike rail.

It also adds more distance out from the center and height to the entire assembly to avoid spark length being limited if it would always strike to the ground because of a short distance.



Round and smooth surfaces are preferred to get the longest possible sparks when corona losses is avoided. A uneven surface will result in many small sparks, barely visible to the naked eye where energy is dissipated and this means there is less energy to be put into the spark you are aiming to get.

Round and smooth surfaces almost only leaves us with two shapes, spheres and toroids.

Spheres can result in problems with sparks going downwards to strike the primary coil or result in racing sparks along the secondary coil. This is caused by the electromagnetic field shape from the sphere. If a Sphere is used it is best to place the break out point on the top of it so sparks would be forced as far away from the secondary coil as possible.

Toroids gives the best electromagnetic field shaping where sparks are much less likely to travel inwards towards the secondary coil. If the diameter of the toroid is very large a secondary smaller toroid can be used underneath to ensure that there is no breakout from the top winding of the secondary coil.


Solid vs skeleton

From an efficiency point of view it doesn’t really seem to matter whether you use a solid metal toroid, or a piece of flexible ducting bent round into a ring, or a skeleton frame made out of tubing, they all work much the same. However, if you are wanting to run the coil without a breakout point, you’ll find that a smooth surface gives a single large streamer that wanders around the topload, whereas a rough one can generate several shorter streamers.


Do use these materials

Copper: One of the best conductors in terms of price/resistance ratio.

Aluminium: About 2 times the electrical resistance of copper.

Corrugated aluminium air duct is made entirely from aluminium that is twisted to form a continues lock seam.

Two frying pans mounted opposite against each other can form a small, uniform and cheap topload. The goes for two savarin cake molds, mounted opposite against each other, they come in a wide variety and some even have a filled middle which makes them easy to mount on a secondary coil.

Styrofoam torus/donut/toroid shapes from a hobby store covered in aluminium tape makes for a light weight and cheap toroid, it can however be hard to get a very smooth surface.

Two bottom parts from beer or soda cans can form a tiny topload.

Brass: About 4 times the electrical resistance of copper.

For connections through a threaded rod or bolts to mount the topload on, brass is the prefered material as its resistivity is still good compared to how rigid and strong it is.


If possible, avoid these materials, they are not optimal

Metallized polyester flexible air ducting: The polyester film is very thin and only rated for a maximum operating temperature of 71 C. A break out from the surface of the film could easily damage or melt the film. The spiral is made from spring steel.

Stainless steel: About 40 times the electrical resistance of copper.

The stainless steel salad bowls from Ikea are cheap, almost round and easy to build a sphere from.


Current and losses in toploads

Udo Lenz at the 4hv.org forums made some very specific measurements on topload and arc current on a DRSSTC on three different power levels, where the highest was producing 80 cm arcs. His measurements for the DRSSTC was inspired by the measurements by Greg Leyh from the worlds largest SGTC Electrum.

UPDATE: these links are now dead, try archive.org to find a copy, pictures of the coil can be seen here: https://www.lod.org/electrum.html

Hydron at highvoltageforum.net has made some other very specific measurements with a battery driven data recorder: https://highvoltageforum.net/index.php?topic=117.0

The current through the topload is

\mbox{Power dissipation}=\mbox{Current}^{2}\cdot\mbox{Resistance}

The current through the topload is given (at a given operating point of your coil). If you increase the resistivity, the dissipation in the topload goes up (see equation above) and so your Q factor goes down. If the topload was superconductive, it would not dissipate anything and won’t do anything to your Q factor.


Choosing minor and major diameter

A rule of thumb for choosing the minor diameter for a DRSSTC topload toroid is to take the secondary coil diameter and use the same value.

Kind of the same rule can be used for the major diameter, take the length of the secondary winding and that is your toroid major diameter.

These rules apply if you follow the general design guidelines for sizing of the secondary coil.

The minor diameter will affect the breakout voltage, so if the minor diameter is too large then the coil will not break out without a break-out point. Too small a minor diameter will cause it to break out at a lower voltage depriving the coil of some of the voltage build up on the toroid, usually resulting in multiple smaller streamers.

The major diameter affects the field control under the toroid, so too small and you will get strikes to the primary coil / ground rail or if way too small strikes to the secondary coil which might destroy it!


Choosing distance to top winding of the secondary coil

The underside of the toroid should align with the top winding of the secondary coil or be raised a distance at up to secondary coil diameter.


One or two toroids

If you want to raise the topload higher above the secondary than the above figure, it will be a good idea to add a second smaller toroid for field shaping. It can also be necessary to add a second smaller toroid if the main toroid is very wide so field shaping around the secondary coil top is weak.


Calculating the capacitance of a sphere

K is the dialetric constant 1.01, R is the radius of the sphere. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.

C=\left ( \frac{K\cdot R}{9\cdot 10^{9}} \right )


Calculating the capacitance of a toroid

D1 is the major diameter of the toroid and D2 is the minor diameter of the ring of the toroid. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.

C=2.8\cdot \left ( 1.2781-\frac{D2}{D1} \right )\cdot \sqrt{\frac{2\cdot \pi ^{2}\cdot \left ( D1-D2 \right )\cdot \left ( \frac{D2}{2} \right )}{4\cdot \pi }}


Breakout point

A breakout point should be a good conductor with a even surface to avoid a large corona spray all around the wire, rod or what is used for it.

The tip of the breakout point can be tungsten if heating or even melting damage to the tip is a problem.

The breakout point has to stand out far enough from the very close and heavy magnetic field that is close to the topload surface. At least 10 centimeter would be a good start, even longer and upwards pointing can be used to get arcs further away from striking down in the primary coil, earth rail or just get longer arcs to ground from the bigger distance.

Running a DRSSTC without a breakout point can be very stressful on the IGBTs as the Tesla coil have to build up a large potential on the surface on the topload for it to break down from the very even surface. The IGBT inverter is much happier when it can just keep feeding energy into a arc.

Tales of running a DRSSTC without a breakout point tells that nothing happens until it almost hits full input voltage from f.ex. a variac. Suddenly large arcs will start lashing out in unpredictable and random directions. Half of the time the sparks would either be from topload, racing sparks on secondary or even inside of the secondary.



The art of making a metal toroid is called metal spinning. A sheet of metal is formed into shape with a metal rod against several different wooden master models mounted in a lathe.

Aluminium ducting is easy to mount around two circular pieces of wood with spacers between them.


Previous topic: Secondary coil Next topic: Tuning

Good MMC capacitors

Here is a list of capacitors tested by the high voltage community to be known to withhold the use as primary capacitor in Tesla coils.

Capacitor specifications are taken from data sheets at 100kHz and some values for peak current, rms current, ESR and dv/dt are estimates(* marked) from similar capacitors and graph read outs.

Product Ipeak Irms ESR dV/dT Rth
data sheet V μF A A V/μS °C/W
Aerovox RBPS20591KR6G 1000 2 854 22 7 427 15*
2000 0.15 432 13.5 5 2879 11
2000 0.1 171 8.3 7 1712 11
1000 3.75 2500* 152 1 810* 10*
900 0.047 28 4* 135 600 51
2000 0.15 345 10 26 2300 23
1600 0.033 198 3.5 350* 6000 40
1600 0.047 282 4.3 250* 6000 40
1600 0.056 333 5 150* 6000 40
3000 4 5000 80* 0.75 1250 6.9
1000 0.1 1100 6 10 11000 33
1600 0.1 1100 6 10 11000 33
1250 0.22 2420 6 9 11000 33

Capacitive reactance Xc = 1 / ( 2 * π * f * C)

ESR can be calculated from the tangent of loss angle given as TANδ in the data sheets. ESR is frequency dependant. Capacitance is given in Farad, frequency in Hertz. ESR = (1 / (2 * π * f * C)) * TANδ = TANδ * Xc.

Thermal resistance (Rth) when given in data sheets are either Watt needed to raise the temperature by one Kelvin or degree Celsius the temperature raises by one Watt dissipation. Conversion from W/K to °C/W is to divide one by W/K dissipation factor.  °C/W = 1 / (W/K).

Ipeak is calculated from the dV/dT rating times the capacitance of the capacitor. Capacitance given in micro Farad times pulse rise time given in micro seconds will give a result in Ampere. Ipeak or Ipulse = C * dV/dT.

As a rule of thumb ESL is about 1.6 nH per millimetre of lead distance between the capacitor itself and the rest of the circuit. This also includes the leads of the capacitor itself. This only applies to well designed capacitors.


MMC calculator

MMC tank design calculator for SGTC, VTTC, DRSSTC and QCWDRSSTC Tesla coils. Results are guidelines to designing a MMC and should always be double checked in your final design! Most importantly is that voltage rating is the DC voltage rating, from experience this can used for good quality capacitors, AC voltage rating with frequency derating would be much lower.

Capacitor specifications are taken from data sheets at 100 kHz and some values for peak current, rms current, ESR and dv/dt are estimates from similar capacitors and graph read outs.

Inputs are in green. Outputs are in red. Formulas used can be seen below the calculator.

Basic MMC configuration – List of good MMC capacitors
Capacitance uF
Voltage rating VDC  
Capacitors in series
Strings in parallel
Price per capacitor  
MMC voltage rating VDC  
MMC capacitance uF  
Total capacitors  
Total MMC price  
Advanced options
MMC capacitor parameters
Peak current rating A  
RMS current rating A  
dV/dt rating V/uS  
ESR rating Find correct ESR rating
for your resonant frequency
dissipation factor
Tesla coil parameters – Examples are small, medium and large
Frequency kHz
Primary inductance uH  
Primary peak current A
On time uS
Advanced results
Primary impedance
(single cap)
Power dissipation
(single cap)
Temperature rise
(single cap)
ºC 0-5 very good, 5-10 good
10-15 not good, 15+ bad
  Actual values MMC rating
Peak voltage MMC VDC VDC
RMS current MMC A A
dV/dt for MMC V/uS V/uS
Peak current for MMC A A

Theory used

MMC voltage rating: MMC voltage rating = DC voltage rating * capacitors in series.

MMC capacitance: MMC capacitance = (single capacitor capacitance * amount of capacitors in parallel) / amount of capacitors in a string.

Primary impedance: Zprimary = SQRT(Lp / Cp).

MMC Xc, reactance: Xc = 1 / (2 * PI * F * C). F is frequency in Hertz. C is capacitance in Farad.

MMC Zc, impedance: Zc = SQRT(ESR^2 + Xc^2). ESR is the combined ESR for the MMC. Xc is the MMC reactanse from above.

Peal voltage MMC: DC peak voltage over MMC = Zc * primary peak current

RMS current MMC: Irms = 0.5 * primary peak current * SQRT(on time * bangs per second). Steve McConner.

dV/dt MMC sees: Actual dV/dt in V/uS the MMC sees = (2 * Pi * V) / F. V is peak DC voltage over MMC and F is frequency in Hertz.

dV/dt rating MMC: dV/dt rating in V/uS = Primary peak current / MMC capacitance.

Peak current for MMC: Peak rating = capacitor peak rating * amount of capacitor strings in parallel.

Kaizer VTTC I


A VTTC is a Vacuum Tube Tesla Coil and it uses vacuum tubes / valves for the oscillator that is self biasing from a grid leak circuit.

I chose to build a VTTC because the operation and components were simple and few. It did turn out to be hard to find all the required components at a reasonable price in Denmark.



WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety! http://www.pupman.com/safety.htm



After spending many evenings reading about vacuum tube Tesla coils and comparing tubes, I chose to build a Tesla coil utilizing two 811A transmitter tubes. These tubes have been used by others with great success, they are fairly cheap and have a moderate output power for a tabletop size Tesla coil.

Having studied and read about all the VTTCs I could find on the internet, it became clear to me that there are no pre written text book example on how to design your own VTTC from the bottom, as there do for other types of Tesla coils. Its about finding the old radio amateur books and manuals and combine some of the theory from those about oscillators and the knowledge gained by those who have build VTTCs.

The schematics for this dual 811A VTTC are by most people recognized as Steve Wards, but they do in fact go way back to a article from the magazine “Science Experimenter” fall 1963 issue, which are the drawings most dual 811A VTTCs are based on. I have not been able to find earlier material about this Tesla coil design.

I will build a Tesla coil that resembles the one Steve Ward build, in order to be able to compare the two coils. He succeeded in producing 180mm long sparks, due to it being his first VTTC he wrote there was room for improvement. I have tried to correct some of the issues he described and my goal was to achieve at least 200 mm long sparks. A major limitation to this project is money, so its being designed around components I have already available.

Here are some of the things I want to improve.

The high voltage supply needs to be able to exceed the maximum ratings of the tubes. For very short run times the tubes can be driven at a higher voltage / current than they are rated at, in order to achieve longer sparks.

The secondary coil will be bigger, from the unconfirmed theory saying that larger secondary coils gives longer sparks at the same power input in a VTTC.

Adding a Staccato controller to be able to adjust the break rate of the sparks, this will make me able to apply a higher voltage without excessive heating of the tuber, longer sparks can be achieved this way.



Tubes 2x RCA 811A transmitter tubes
High voltage supply 230 / 2300VAC MOT with voltage doubler.
Supplied with 160VAC through a variac for 3200V.
Primary coil 180 mm diameter, 1.78 mm diameter copper wire, 26 windings, tapped at 22 windings.
Capacitor tank 4 nF Fribourg capacitor 20kV pulse rated.
Secondary coil 75 mm diameter, 440 mm long, 1600 windings, 0.25 mm enamelled copper wire.
Topload 45 mm small diameter, 152 mm large diameter, John Freud toroid.
Resonant frequency Around 300kHz.
Grid leak circuit 19 windings feedback coil, 1 nF capacitor and 500R to 2K5 grid leak resistor.
Input power 300W at 150V.
900W at 180V.
50W for the tubes heating.
Spark length up to 330mm long sparks.



Here is the schematic with all the component values. Here is a few notes on choosing some of the components.

Tesla Coil circuit schematic

All ground connections are made at the same ground rail. R2 is a wire wound resistor with L4 wound around it, one for each tube. C4 is a decoupling capacitor mounted directly under the socket, one for each tube. R1 needs a high wattage rating, bigger is better, or supply sufficient cooling.

Staccato controller schematic. There are slight changes from the original schematics. I do not use a center-tapped transformer and have removed the polarity switch.

Here is the layout for a veroboard, green traces are the traces of the veroboard, remember to cut them where they do not join. Red traces are copper wire jumpers on the component side of the board. Notice that the red jumper next to D2 is connected to 14VAC in N along its way, this is the only jumper that does.

The ExpressPCB files for the PCB layout, however made for veroboard, can be downloaded here:





Nearly all the following equations is originated from John Freau and is to be found in Steve Wards VTTC FAQ, there is a link at the end of this article. These calculations are not exact and will only work as a guide to building a system that is easier to tweak into good performance.

My goal with these calculations are to determine a Q value and a value for my tank capacitor.

First I will calculate the load impedance for my two 811A tubes. This equation is for class C amps supplied with AC or pulsed DC. If you want to calculate the tube impedance for a filtered DC supply use R = V / (2 * I) instead.

The maximum ICAS plate current rating for my RCA 811A tubes are 175 mA each.

R = V / (4 * I)

R = 3200 / (4 * 0.350) = 2285 Ohm

Next I want to find the primary reactance, f is my resonant frequency in Hertz and L is the primary inductance in Henry.

X = 2 * π * f * L

X = 2 * π * 300000 * (102 * 10^-6) = 192 Ohm

Now the Q value can be determined

Q = R / X

Q = 2285 / 192 = 11.72

Generally a Q of 10 to 20 is ideal. Lower Q gives less tube efficiency, more difficult coupling, but less tank losses. Higher Q gives higher tube efficiency, easier coupling, but more tank losses.

Now I can calculate the capacitance needed for my tank capacitor to work at the desired Q for the tubes I use. Result will be in nF.

C = (Q / (2 * π * f * R)) * 10^9

C = (11.72 / (2 * π * 300000 * 2285)) * 10^9 = 2.72 nF

The closest value capacitor suited for this high frequency and current was three 10 nF capacitors in series for 3.33 nF. The resulting Q for this tank capacitor value is as follows.

Q = 2 * π * f * R * C

Q = 2 * π * 300000 * 2285 * (3.33 * 10^-9) = 14.34

For my final tank capacitor I ended up using a 4 nF capacitor to only have one capacitor instead of a series connection of other types, this capacitor is intended for use in radio oscillators so it can handle the frequency and current, the losses are despite the higher Q value not enough to heat the tank capacitor. The resulting Q value for the 4 nF tank capacitor is as follows.

Q = 2 * π * 300000 * 2285 * (4 * 10^-9) = 17.23



14th July 2008 to 6th  August 2008

It takes its time to find all the special and at times rare components and materials at a price where I feel its worth it. It would take 1½ month to have found all the things that I needed.

The two 811A vacuum tubes are imported from USA through Ebay. Sockets and terminals are imported from England through Ebay. Heater transformer is bought from a private person in Denmark. The high voltage supply are from old microwave ovens with their capacitors and rectifiers. The rest of the components and materials are mostly thrown out stuff from various industry and private persons.

9th August 2008

The staccato controller is assembled on a breadboard and tested.

12th August 2008

The  staccato controller is build on a vero board.

30 – 31st August 2008

The secondary coil is wound and enamelled with 3 layers of varnish.

75 mm drainpipe with 1600 windings of 0.25 mm enamelled copper wire. End terminations are made from brass bolt and brass nut.

5th September 2008

A test setup have been build to experiment with the optimal number of windings on the primary and feedback coils, adjustable from 18 to 26 windings. With these options I can adjust the systems efficiency, frequency and coupling. In the first nights of testing 80 mm long sparks are achieved.

6th September 2008

The Tesla coil is tuned and adjusted

2300V high voltage supply, 1K/1nF grid leak circuit, 24 primary coil windings, capacitor tank 3.33nF.

160mm long sparks are achieved.

This is where I discover that my vacuum tubes are not conducting evenly, which they should as they were bought as a matched pair. I solved this problem by driving the weaker tube with 1 winding less on the grid leak coil, that was enough to adjust the screen voltage to a level where the tubes would conduct very close to matched conditions.

20th September 2008

A voltage doubler is added to the high voltage supply.

3200V high voltage supply, 3K/1 nF grid leak circuit, 24 primary coil windings, capacitor tank 3.33 nF.

260 mm long sparks are achieved.

3rd October 2008

3200V high voltage supply, 0.5K/1 nF grid leak circuit, 22 primary coil windings, capacitor tank 4 nF.

280 mm long sparks are achieved.

15th October 2008

The staccato controller is connected to the Tesla coil and it is now a completely different world, to modulate the sparks. It can vary the number of sparks per second from 1 to 50 outbreaks, the sound and shape of the sparks changes all the way through that scale.

Up to 330 mm long sparks can now be achieved through over driving the tubes further with a lower break rate.

27th October 2008

New clear PVC have been bought along with nylon supports for the primary, screws, shoes and fittings.

1st November 2008

Removing the isolation from 15 meters of 1.78 mm hard copper wire took a real long time as I need it to stay in its circle form. It did cost me a deep and violently bleeding cut to my thumb because I did not pay enough attention when cutting off the isolation. But I got the job done in all my bandages.

A platform was build from thrown out wood and clear PVC, that does not mean its not in good condition.

High voltage, filament and 12VDC transformers are placed on the platform, the capacitor is for the high voltage doubler.

All wiring is done, staccato controller added and the grid leak circuit is in place, everything is in its place.



The complete coil in all its beauty, the blue and purple sparks go very well with the warm soft glow of the tubes.

This series of close up pictures really show the thin sword like sparks that are characteristic for a VTTC.



Running in CW mode.

Running in interrupted mode


The most important goal was to achieve at least 200 mm sparks and I went through the roof with sparks leaping out to 330mm.

I had assumed that the grid leak voltage would be so low that the coil could be wound on top of its own layers, but eventually it burned through and I had to find a place for the new grid leak coil wound in one layer. The 3 positions I had made for the grid leak coil in the nylon supports were no longer of any use unless I took the whole primary support apart and remade them, which I decided not to do.

The new grid leak coil is still made from 0.5 mm diameter enamelled copper wire, this is not optimal as its isolation is not thick enough, ideally normal wire with PVC isolation would be to prefer.

A higher Q value seems to be preferred if you have a tank capacitor that is able to withstand the heavier load.

Too much of the construction was not drawn in hand before building, that is a sure way to run into problems, but I built most of it from my head and took the battles along its construction, something that I do not advice others to do and I will try to avoid this in the future. 2D/3D CAD drawings would have been a great help, so for the future this will be standard in bigger projects.


Further reading

http://stevehv.4hv.org/VTTCindex.htm Steve Ward have documented 5 of his VTTCs, the staccato controller and written a good FAQ that is a must for anyone wanting to build a VTTC.

http://drspark.org/ Christopher Hooper have documented 3 of his VTTCs.

http://www.mif.pg.gda.pl/homepages/frank/vs.html Vacuum tube database, almost any tube data sheet can be found here.