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!


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asrivastava192 months ago

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


will this thing kill you
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
Xellers (author)  crazy-blender2 years ago
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...
Witch part of the circuit will kill you also will the discharges from the top load kill you
the sparks can give you REALLY nasty burns, but it cant kill you unless you try to arc to your eyes
Witch part of the circuit will kill you also will the discharges from the top load kill you
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.
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-blender1 year ago
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!)
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?

Tvmender1 year ago

Do the 811a need to be a matched pair?
Xellers (author)  Tvmender1 year ago
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!
I have a 6.8V 5A filament transformer from an xray head, will that work?
Xellers (author)  Jimmy Proton2 years ago
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!
These tubes are almost twice as expensive as the 811A on ebay and yeah...16 years old..not much money lol
Xellers (author)  Jimmy Proton2 years ago
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.
lol nice! but i live in NC :/ I don't know of any places like that down here.
science rox2 years ago
how do you assemble the base is there any special way to do this?
jam21975602 years ago
could you use a metal halide ballast instant on a microwave transformer?
Xellers (author)  jam21975602 years ago
You certainly could use your MOT with such a ballast, but I don't see any reason to do so in this coil.
Thanks! :)
ZachFejes3 years ago
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.
Teslaling3 years ago
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)  Teslaling3 years ago
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!
Thanks for the advice! Looking for an SCR now!!!
Xellers (author)  Teslaling3 years ago
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.
Ok, thanks!
Teslaling3 years ago
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)  Teslaling3 years ago
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)
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
electricfan4 years ago
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.

They would share the same ground. For (small)Solid State coils and most Vacuum Tube coils, this is quite common.
Teslaling3 years ago
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!!!
jp3333 years ago
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 jp3333 years ago
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)  jp3333 years ago
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!
Xellers (author)  jp3333 years ago
Hello, I'm quite glad that someone actually went and tried this instructable out! As for your coil not oscillating, one possible culprit is the feedback coil. In any circuit involving an Armstrong oscillator, I always try reversing the feedback coil connections if it doesn't start up. As for the parts you listed, what what do you mean when you say a "1 k mot"? What parameter is " 1 k" describing? Also, where did you use the microwave oven capacitor? There is no place in this circuit for one and if you inserted it somewhere, it might be causing problems. What sort of grid resistor are you using? If it's not a high enough wattage resistor, it will quickly die and if it's too big or too small, the coil might not oscillate properly. When you say that the tubes are not heating up, do you mean that the filaments are not lighting? If so, there's something wrong with the filament transformer circuit (for example, if you inserted the microwave capacitor across the tubes' filaments, then that would cause the filament power supply to short circuit). Finally, what do you mean when you say the "continuity is ok"? What exactly were you testing?

The easiest way for me to try to diagnose the problem is to look at what you built. If possible, can you upload and post some pictures of your coil from different angles so that I can see what you did (if you do, please make sure the photographs are detailed enough for me to see what's going on)? Also, if you have access to a video camera, can you post a video of yourself quickly demonstrating what happens when you turn the coil on?

Good luck!

jp3333 years ago
(removed by author or community request)
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