Introduction: How to Build a Vacuum Tube Tesla Coil (VTTC)

Picture of How to Build a Vacuum Tube Tesla Coil (VTTC)
Can't find a neon sign transformer? Want to build your first Tesla Coil without facing the complexities of going solid state? Here's some good news: Vacuum tube Tesla Coils, which have been making a comeback in recent years, can be just as rewarding as any other type of coil without breaking the bank. This is one such coil that I built during the spring of my 8th grade year.

You can even use it to wirelessly transmit electricity to a lightbulb! (12/3/12)


While this project does work in its current form, I have detected some problems and and working to fix them. You would best be advised to postpone your building until then - it seems that theses tubes could operate more efficiently at higher frequencies and my primary RLC tank circuit's natural frequency is much higher than my secondary side RLC circuit's natural frequency; a new secondary coil with a frequency of approximately 1.5MHz is being designed and the primary circuit will be retuned. I expect a great leap in performance, with sparks possibly as long as 7" to 9".

3/10/10: I decided to try to estimate the resonant frequencies of my primary and secondary circuits using deepfriedneon's formulas, and I found that my coil is oscillating about 100kHz above my primary circuit. I don't have any parts to fix this now, but will add a capacitor or two to the primary circuit to lower its frequency soon. IMPORTANT: I found a 6.3V at 12A Hammond power transformer and replaced my 5V computer power supply - the results were truly impressive; I am getting better performance with one tube than I ever got with two, filament voltage really matters! Here is a quick video:

4/16/10: The MOT (plate transformer) burnt out because the secondary windings were damaged by previous experiments (SGTC,s, Jacob's Ladders, etc.). It was replaced with a larger one and the sparks are now almost 7" long - this coil performs as well as Steve's did, but with only one tube and with a poorly tuned primary oscillator!


Step 1: Vacuum Tube? What's a Vacuum Tube?

Picture of Vacuum Tube? What's a Vacuum Tube?

In the early 21st century, many of us have never even heard of vacuum tubes, and the few who have only know that they were used in old electronics. Therefore, before I begin this project, I feel the need to explain how they work.

Lets take a look at picture 1. This is the standard symbol for a vacuum tube diode. A diode only conducts electricity in one direction. They can be used to turn an alternating current into a direct current. In the diagram, the bottom half hexagon is the filament. It is just like the filament inside of an incandescent lightbulb. The line above it is called the plate. The circle around the filament and plate represents the (usually glass, sometimes metal) envelope of the tube. Almost all of the air inside of this envelope has been evacuated, there is a vacuum. This will become important later.

Now, lets take a look at picture 2. Here we have applied a voltage between the filament and the plate. The filament is negatively charged, and the plate is positively charged. While the electrons in the filament are attracted to the plate, there is not enough voltage for them to do so on their own. So how can we get them to jump? Take a look at picture 3.

In picture 3, a few new things have appeared.First, we have a 10 volt power supply connected to each side of the filament. Just as in an incandescent lightbulb, this heats the filament up. The negative side of the power supply is still connected to the filament, but the positive side is not. Notice that now, the negatively charged electrons are stil flowing into the filament from the 100 volt power supply, but something is different. Why are they floating around the filament? As the filament heats up, thermionic emissions occur. Essentially, the electrons are shaken off of the filament by its thermal energy. This can happen because there is a vacuum. So now, the question is: What happens when we connect the positive side of the 100 volt power supply to the plate? Take a look at picture 4 to find out.

In picture 4, the positive side of the 100 volt power supply is connected to the plate. We have zoomed back towards the tube. In the picture, the electrons floating around the filament are moving towards the plate! There are no air particles to hinder their passage, so after the thermionic emission occurs, the positively charged plate attracts them, and they accelerate towards it, hit it, and move along the wire back into the power supply. That's how a vacuum tube diode works.

The principle of operation is relatively simple, but a Tesla Coil such as the one that we are building is an oscillator. That means that there is a feedback system that turns the diode on and off, to accomplish this, we use a triode. Read on the find out how it works.

Step 2: The Triode

Picture of The Triode

The first true electronic amplifier was the vacuum tube triode. It works because like charges (in this case, electrons) repel each other. Take a look at picture 1. It looks similar to the symbol for a vacuum tube diode, but it has an extra part that looks like a grid between the plate and filament.

This grid normally allows electrons to pass through itself for diode operation, but as it grows more and more negatively charged, it allows less and less electrons to travel from the filament to the plate due to electrostatic repulsion. In this way, you can regulate the flow of a relatively large current by using a relatively small one.

Step 3: The Vacuum Tube Oscillator

Picture of The Vacuum Tube Oscillator

A Tesla Coil is essentially a very large oscillator. When the primary side of the coil oscillates at the natural frequency of the secondary side, resonance is achieved. This is a fundamental concept that is used in all Tesla Coils and other resonant transformers (such as the ones found in many switch-mode power supplies, and CRT television sets). The Vacuum Tube Tesla Coil that I detail here uses a configuration known as an Armstrong Oscillator.

In the standard model of a transformer, there are two coils, a primary and a secondary coil. Currents are usually induced from the primary coil to the secondary coil (although the opposite sometimes happens, usually with destructive results), this is a concept that we will not go over now, if you are unfamiliar with it, then this is a good place to become acquainted: However, an Armstrong Oscillator works by introducing a third coil, called the feedback, or sometimes "tickler" coil.

Currents are not only induced from the primary coil into the secondary coil, but also into the feedback coil. This feedback is then used to turn off the oscillator by blocking current from flowing into the primary coil. However, when the primary coil is turned off, current is no longer induced into the feedback coil, and it no longer blocks current from flowing through the primary coil. In this way, the cycle repeats indefinitely, until it is interrupted, or the power is switched off.

The basic schematic for an Armstrong Oscillator using a vacuum tube is given in the first picture. (This picture is from Steve Ward's site:, you can read more about VTTC operation there)

Step 4: Our Tesla Coil Schematic

Picture of Our Tesla Coil Schematic

Here is the particular schematic for the Tesla Coil that we will be building. I do not take credit for its creation - it was made by Steve Ward and you can find the full - sized image on his site here: A few things that you should note are that I have found that you should make the primary coil (L1) slightly larger but allow for it to be tapped every other turn. Also, I've noticed that a slightly larger (~2nF) tank capacitor (C1) works better for my coil, but this could vary. Also, if you are adventurous enough, you might consider using a level shifter to double the voltage to the 811A tubes to 4000VAC RMS and then use a staccato circuit (something you should consider even without the level shifter) to keep the tubes running cool. However, since this is a slightly more advanced project I will not cover it here (yet!).

Step 5: Parts!

Picture of Parts!

Here are the parts that I used, and the approximately how much each one cost:

~$30 (2) 811A Vacuum Tube Triodes
~$0   (1) Microwave Oven Transformer
~$10 (1) 30kV 1.0nF Polystyrene Capacitor (2 or 3nF will also work here, I found that larger capacitance increased the performance slightly)
~$15 (1) Bundle of 1000 ft. of 28 AWG Magnet Wire
~$10 (1) Bundle of 100ft. of 16 AWG Insulated Audio Wire (Can be purchased from Radio Shack)
~$3   (2) Small Circuit Boards
~$10 (1) 1' x 1' x 1" Wooden Board (This price is for about 10 of these boards)
~$5   (1) Box of Nails
~$0 (2) Ferrite Toroid Cores (Should be at least about 1/2" in inner diameter, these are not critical parts)
~$2   (4) 3kV 1.8nF Ceramic Disk Capacitors
~$10 (1) 50W 5k� Resistor (�  = Ohm Symbol)
~$7   (2) 20W 30� Resistors
~$0   (1) 6.3 VAC 10A Filament Transformer (You can use 5 volts from a modified computer power supply instead if you want to save some money, but the performance will decrease)
~$5   (1) Small Container of Epoxy Glue (This is for gluing the secondary coil down, if you want to experiment (like me) then you shouldn't glue the coil down)
~$5  (1) 1' of 4" Diameter PVC Pipe (Primary Former)
~$3  (1) 1' of 2" Diameter PVC Pipe (Secondary Former)

Total: ~$100

This is actually a very crude estimate, and shipping costs will differ depending upon where you buy from and where you live. The parts marked $0 were either salvaged (like the Microwave Oven Transformer) or they were free samples (like the Ferrite Toroid Cores) or were already owned (I used a computer power supply instead of a costly filament transformer).

Step 6: Assembling the Base

Picture of Assembling the Base

While there are no general guidelines on how to do this step properly, you should strive to fit everything on one board and keep connections as short and simple as possible with as few overlapping and/or twisted wires as possible in order to minimize stray capacitance and inductance. Here, you can see my main board with all of the major parts.

One thing you may have noticed is that some of my photos appear to show a coil with a different base - this was the original variant of the coil which did not work as well because of the long hookup wires.

NOTE (4.5.11): This circuit layout is now obsolete - arcing between the top leg of the primary capacitor and the microwave oven transformer's iron case has become problematic to the point where I completely reassembled the circuit and put the transformer on its own board. Hopefully, I'll get some pictures of this soon.

Step 7: Winding the Secondary Coil

Picture of Winding the Secondary Coil

Unfortunately, I did not take pictures of this part of my construction - both of my hands were too busy winding and holding!

The ideas involved in winding an effective secondary coil are very simple, but some things are easier said than done. All that you are really doing is winding wire around your 2" PVC pipe former. Here are a few general guidelines for winding your own coil:


1) Break the wire - if the wire snaps half way through, it is better to buy a new roll (or buy are bigger one in the first place) than to solder the broken wires back together. This is not a good idea because you will risk serious damage to your coil - there will be problems with unwanted discharges (the secondary coil could potentially arc to the primary coil and ruin the entire primary circuit; very bad) and the coil can also destroy itself by burning through the plastic form or by melting the solder you used to hold the rip together, thus unwinding the secondary coil.

2) Drill Holes in the Secondary Coil Former - this is the most common mistake, and you will pay the price for it with this coil. If you drill holes in the secondary former, there will be a huge risk of the coil discharging through the inside of the pipe or discharging upwards though the secondary former (at the top) and damaging itself (not to mention that the impressive sparks won't be flying into the air, but rather melting through the secondary former).

3) Wind the Secondary Coil Haphazardly - If you cross windings or wind them on top of each other, the performance of your coil will suffer greatly and the secondary coil will be at risk of damaging itself. While its okay to make one or two small mistakes (with emphasis on small!) near the bottom of the secondary coil, you will regret it if you do not wind well.


1) Use relatively thick wire for winding the secondary coil - increasing the thickness of the wire you are using will make it easier for you to wind the secondary coil and will decrease the chances of the wire snapping.

2) Wear gloves while working with your coil and/or wash your hands very well - some of the various molecules in your sweat and on your hands, if caught on the secondary coil, can decrease performance. While you can wind the coil with sweaty hands, you will notice that the sparks will be shorter than if you had used dry, washed hands or worn gloves.

3) Work slowly and deliberately - its not a race, you will make fewer mistakes if you are willing to commit a few hours to winding the coil. Sometimes, if you make a mistake earlier on, you might want to unwind and then rewind the coil entirely. For this particular coil, I wound the secondary half way through before I noticed some crossed windings and rewound the whole thing.

4) Use shellac (I used the spray-on type) or polyurethane to cover your secondary coil - this will help to prevent the coil from unwinding and will hold everything together well, it also looks and feels very nice. Give it a good day to dry off after the shellac (even if the can only says 15 minutes) as the secondary coil might erupt in flames if the coating has not dried thoroughly.

Step 8: Testing

Picture of Testing

When you're finally ready to turn your coil on, be sure that you have a large open area to work in where there is no danger of sparks from the coil setting anything on fire. I would recommend testing the coil at a lower voltage first (using a Variac - I would start by testing it around 30VAC input first and then working up to full power) instead of plugging it in to see what happens. Also, a 10 ampere FAST safety fuse is REQUIRED in series with the mains electricity you are using in order to prevent electrical fires and other nasty scenarios in the event that your coil does not work properly.

Before you plug anything in, however, you should use a multimeter to make sure that your connections are all correct and that your vacuum tubes are not damaged (ie. burnt our filament, shorted grid and filament, etc. - all of these have happened to me when working with vacuum tubes)

On the first attempt, do not expect to immediately be rewarded with roaring sparks - be glad if your coil works at all. Once you have established that it does, then attach a small topload to the top of the secondary coil (I like to use a filment lightbulb wrapped tightly in aluminum foil) and use the taps on the primary coil to tune your coil for maximal spark length. Note: you will need some sort of breakout point like a sharp nail if you use a topload of any sort).

Here are some pictures of my Tesla Coil's first light:

Step 9: Sparks!

Picture of Sparks!

If you're satisfied with the way your tests are looking, then you can plug everything in and enjoy the plasma! Interesting experiments you can try include observing discharges inside of an argon filled lightbulb, inside of a vacuum tube (be careful some tubes might produce small amounts of X-Ray radiation if you do this by way of Bremsstrahlung), and you can light up fluorescent tubes at a distance. Also, if you remove the breakout point and tune a nearby radio (and sometimes a faraway radio too) to the resonant frequency of your coil (Usually somewhere on the AM band) you will be able to hear the 60Hz buzz of the coil.

Step 10: Conclusion

Picture of Conclusion

This is a page where I will answer general (and sometimes specific) questions that you have and will try to help to explain some of the deeper operating principles principles of the Tesla Coil.

On a different note, the 811A also makes a great audio output tube!



; D

NeenadJ (author)2017-06-13

Can this tesla coil run at 10 Mhz or higher ?

ELECTRONICS3 (author)2017-02-13

hi I just bought my 811a from ebay and can this circuit work with 1 tube ??

my coil secondary is around 65cm 1260turns !

pls answer ! or I will cancel my order and buy GU81M

I dont have MOT so is there some other options for power supply ?

can I use voltage multiplyer on 220v ?

SamS378 (author)2016-12-14

im just curious ... how long could you leave this running?

rah187 (author)2016-09-09

Is there any other transformer I can use for this project other than a MOT? I'm only asking because there is only one microwave in my house and I'm sure not going to take it apart so I can build experement with high voltage.

JacquelineX (author)2016-08-05

Thanks X, I'm so intrigued!

MichaelH66 (author)2016-08-02

hat analogy to a merry go round is the most ignorant thing i have seen today.

craftyv (author)2016-08-02

This may be very clever but all I could see was the danger involved, not only the "machine" but look how close it was to the light flimsy curtains blowing in from the open window. Maybe it's a mother thing.

EdwardC75 (author)2016-08-02

Look - the way this is built is horribly dangerous. At a minimum put a plexiglass case around the circuit with only the secondary coil getting to the outside. Anyone touching the plate cap of the 811 or any part of the plate circuit is going to get seriously hurt or killed.

I'm also sure that the FCC isn't going to be happy with a randomly tuned, high power oscillator running in the AM broadcast band. If you happen to drift up in frequency a short way then you'll be in an amateur radio band - and they will hunt you down and report you to the feds.

crazy-blender (author)2012-06-16

will this thing kill you

Xellers (author)crazy-blender2012-06-16

If you stick your hand in the wrong part of the circuit, yes. Tesla coils, especially SGTCs and VTTCs, are frighteningly dangerous - I'm kind of scared by the fact that I did this all the way back in eighth grade and got away with it...

crazy-blender (author)Xellers2012-07-27

Witch part of the circuit will kill you also will the discharges from the top load kill you

cvaughn3 (author)crazy-blender2014-03-05

the sparks can give you REALLY nasty burns, but it cant kill you unless you try to arc to your eyes

mddeming (author)cvaughn32016-08-02

cvaughn3crazy-blender ANY arc back to the primary circuit will kill you if it touches you. The RF from the secondary may not kill you outright, unless you are the shortest path to ground, but all high frequency discharges do cause nerve damage which may not show up for years. (I built my first coil in 1954 and I still have all my parts.)

crazy-blender (author)Xellers2012-07-27

Witch part of the circuit will kill you also will the discharges from the top load kill you

tesla man (author)crazy-blender2012-09-18

the part of this circut that could potentially kill you would be the primary coil (everything from the outlet, to the first coil of wire), since that has an amperage that the heart cannot withstand. the secondary(everything after that) could also kill you, but that is less likely.

cvaughn3 (author)crazy-blender2014-03-05

if you stick your hand on the side of the hv transformer, aka parts of the tube and the primary depending on what type of primary you have, I have been struck by a transformer exactly like that and am being lucky to survive as it was about 100 amps and the amount that can kill you is .5, not saying dont build this project but excersize extreme caution, hoped this cleared things up

mattcintosh (author)2016-08-02

Are any of the tubes getting overdriven? I heard running tubes the wrong way can produce X-rays

rah187 (author)2016-05-26

Nice 'Ible! I understand most of it, but I don't have any triode tubes around my house. Is there a model number I should look for? Most of the vacuum tubes I tracked down are for audio and amplifier applications. Will any of those work?

asrivastava19 (author)2014-05-27

Is there a possibility of using three or four 10kV Capacitors instead of the 30kV one in any sort of arrangement?


crazy-blender (author)2013-04-11

if i was to run this guy on 4 KV what would I have to modify and what should be replaced. could you give me a circuit diagram that showed how to wire the MOT's in series


Xellers (author)crazy-blender2013-04-11

Hi there!

You'll need to pick a different tube; peak voltage from a single MOT is already pushing it, but when I run with a doubler I get arcs inside the tube. Is there any reason you're planning to wire two MOTs in series rather than use a doubler? (The diode and the capacitor you get from the microwave are all you need!)

crazy-blender (author)Xellers2013-04-12

Thanks. How would I make a voltage doubler with the transformer could you give me a schematic? Also the reason I wanted to run it on 4 KV is because I made a dual MOT stack. Do you know of any tubes that can handle the power?

Tvmender (author)2013-01-03


Do the 811a need to be a matched pair?

Xellers (author)Tvmender2013-01-03

Ideally, yes. You can tap the feedback coil differently for each one if they aren't. Kaizer Electronics has a good writeup of this.

Good luck!

Jimmy Proton (author)2012-01-02

I have a 6.8V 5A filament transformer from an xray head, will that work?

Xellers (author)Jimmy Proton2012-01-03

If you're using only one 811A, yes.

At the moment, I would strongly recommend upgrading to a 572B tube - it's a drop-in replacement the 811A and solves the plate reddening problem.

Good luck!

Jimmy Proton (author)Xellers2012-01-03

These tubes are almost twice as expensive as the 811A on ebay and yeah...16 years old..not much money lol

Xellers (author)Jimmy Proton2012-01-03

I'm 16 too! =)

Try local hamfests/electronics events - I got a few American-made 811A and 572B tubes at Swapfest in Cambridge MA near MIT.

Jimmy Proton (author)Xellers2012-01-03

lol nice! but i live in NC :/ I don't know of any places like that down here.

science rox (author)2011-11-02

how do you assemble the base is there any special way to do this?

jam2197560 (author)2011-10-25

could you use a metal halide ballast instant on a microwave transformer?

Xellers (author)jam21975602011-10-25

You certainly could use your MOT with such a ballast, but I don't see any reason to do so in this coil.

ZachFejes (author)2011-07-19

True, it won't burn you, but you still shouldn't touch it. You may not feel it, but it is can burn out (permanently) your nerves, which you do NOT want.

Teslaling (author)2011-04-14

I just got my coil working, and I'm wondering if I would be able to add a voltage doubler circuit to the plate voltage. It's a 2k mot, and 2 811a's. Do you think the tubes would arc over or would they survive???

Xellers (author)Teslaling2011-06-23

I tried adding a voltage doubler to this coil, but my tube started arcing after a short period of time, so I took it out. I was using a staccato circuit, so if you try this, I would recommend the same. My tube is the cheapest Chinese variant available, so if you're using NOS American-made 811As, you might encounter more success than I did.

You don't need a particularly complex staccato circuit to pull this off - just get an SCR rated for several amps at several hundred volts (dirt cheap on eBay), put it in between the filaments and ground, and add a simple 555 timer circuit - I used my 555 timer-based DRSSTC interrupter and it worked fine.

Good luck!

Teslaling (author)Xellers2011-06-23

Thanks for the advice! Looking for an SCR now!!!

Xellers (author)Teslaling2011-06-23

Just in case you were wondering, the SCR I used was a BTA16-600B (600V, 16A) -

Note: 16A at 600V is extreme overkill for this application, so don't worry about using a weaker SCR.

Teslaling (author)Xellers2011-06-23

Ok, thanks!

Teslaling (author)2011-04-04

Stupid 12ax7a's!!! They cut out for the most part at 50khz!!! I need 1.5 Mhz for my super-mini-plasma globe-ish vttc!!!

Xellers (author)Teslaling2011-04-04

12AX7s are designed as low power preamplifier tubes - they only have 1 watt of plate dissipation, so I don't think they'd stand up to much VTTC use. A better tube for your purposes would probably be the Russian-made GU50. It can run right off of a MOT and I've seen some people get pretty impressive results for a tube of its size:
(check out the other VTTCs on this website too)

Teslaling (author)Xellers2011-04-05

I know its not meant for vttc use...I was bored last night and i had a few 12ax7a's laying around, so i got out the signal gen, scope, and powersupply. It's looking horrible for 12ax7as but i dont really care :D

electricfan (author)2010-04-20

in the plan are all grounds commen ? all the tubes and transformers are grounded so doesn't that te primary and seccondary share ground correct me if im wrong.


Teslaling (author)electricfan2011-04-04

They would share the same ground. For (small)Solid State coils and most Vacuum Tube coils, this is quite common.

Teslaling (author)2011-04-04

If you vary from the design, you may want to run some calculations(if you are up for it)!!! It will seriously help, trust me!!! If you change the top load size, or the secondary size, or the capcitor value, etc. It will throw the tuning off. I found that if you calculate the resonance of the secondary coil, then tune the primary coil and feedback coil accordingly.

This is just a suggestion for the more advanced coilers, for beginners, you will want to just stick to the design as the calculations can get pretty crazy really fast!!!

jp333 (author)2011-03-11

i have built a vttc i have found the plans on your web site and
nothing is happening wen i test it there was a loud hum sound and
that was it . the tubes are not heating up and the grid circuit
does not seem to be working but the continuity is ok. also
continuity is ok thou the rest of the Tesla coil please help

1 k mot
1.85uf microwave oven cap
two 811a's
veritable resistor in gird with 0.0022uf cap

jp333 (author)jp3332011-03-14

hello i am still having problems with my coil. i have done some readjusting to my coil and also remove 1.85uf microwave oven cap as the tank cap and replace it with an a new one but still no success . it seems like to me feedback coil isn't picking any thing up at all and i don't know y . i hope these photos can help.

Xellers (author)jp3332011-03-14

What are the dimensions of your feedback coil? From what I see, it looks like only 1 turn of wire! One things you should consider is that the LC circuit formed by L1 and C1 is a tuned circuit that should resonate with the secondary coil and topload! You MUST make sure these are tuned properly, or else you will get little or no output - I would suggest making a completely new primary circuit (new primary coil and new capacitor) with the specifications I give in the instructable, that way, you know that everything is already approximately tuned. You CANNOT use a microwave oven capacitor in your primary circuit because the voltage rating is a bit low and its capacitance is way too high!

About This Instructable




Bio: My name is Daniel Kramnik - I like building Tesla coils, quadrotors, and robots!
More by Xellers:Build a Robotic Arm for the Science OlympiadDIY Electron Accelerator: A Cathode Ray Tube in a Wine BottleHow To Build A Spark Gap Tesla Coil (SGTC)
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