Introduction: GU-81 VTTC

In this Instructable I am going to show you how to build a Vacuum Tube Tesla Coil (VTTC)!

I try to focus more on math and give you the possibility to build your tube without just looking up values on the internet. You should be able to REALLY BUILD YOUR OWN! And of course you can use bigger tubes. More power, more fun!


With this, you are an evil professor trying to achive world domination, but:

This project contains very high voltages which can kill you instantly! Be carefull! The capacitors can store energy which is also abel to kill you even if everything is unplugged! I am not responsible for anything you do with this device!

Let's start :D

Step 1: Scematic Overview

This is the scematic I designd for my GU-81 VTTC. Nevertheless you can use any kind of big transmitter tubes instead of the GU-81. In this case, or if you want another frequency, you need to change some values as shown below in the text.

I try to make an instructable helping you to make a vttc with nearly every tube, primary capacitor or frequency! Sorry, there will be math :D

Step 2: The Tubes

But wait. Vacuum tube? what is that? If You want to learn more about tubes and a 811A coil, please check out this amazing Instructable by Xellers!

Well, in this Project we are using a GU-81 pentode but you can use nearly every triode, tetrode or pentode you can find ! There are plenty big tubes out there on ebay and if you are motivated to do some math and some research, you will be abel to finish this project with other tubes too! I bought my tube for 28€ on ebay (Germany).

Step 3: Find the Datasheet

You have bought a cheap tube on ebay? Great. The first thing you need to do ist the HV power supply for the plate and the high current power supply for the tube heating. Find the datasheet from the tube you have bought and find out the required filament voltage and the maximum anode voltage! In my case the filament voltage was rated at 12.6 Volts at 11 Amps and the maximum anode voltage was ratet at 3 kV.

Datasheets from tubes often found on ebay





Step 4: The Filament Transformer

Technically it doesn't matter how you achieve the ratet voltages for your tube given in the datasheet but there are two main aspects to keep in mind:

1. The filament Voltage should be around 0.1 Volts - 0.2 Volts higher than shown in the datasheet!

Most tubes are made to run many hours a day but I don't think you want your VTTC to stay on several hours :D . To prevent corrosion, you should drive your tube at higher voltages. This way it will last longer.

2. The powersupply has to be able to draw the required current.

Some really big transmission tubes need up to 20 Amps heating current. Dont burn your powersupply!

3. the effective voltage counts! 12.7 V DC = 12.7 V AC

You can use both kinds of Voltage. But please dont think you need 2^(1/2) times less if you use AC. This kind of math does not come into play yet.

Powersupplys you can use:

MOT: I use a rewound microwave transformer with an output of 12,7 Volts AC. I used very thick wire so it can draw over 30 Amps. Moreover you can adjust the output Voltage easily and it ist for free ;)

Toroidal transformer: They are available on nearly every voltage and they can draw a lot of current. The only negative aspect: They are really expensive.

Computer PSU: If your tube needs 11.8 - 12V, this is the way to go! In addition you can use the DC Voltage to power the relay for the preheating circuit!

Step 5: Pre-heating

Be nice to your Vacuum tube and use a pre-heating circuit. This way the tube will survive much longer.

Simply power the tube with a 2 Ohm resistor in series for 1 minute. After that bypass the resistor using a microwave relay (16A). I cut out a part from a microwave PCB containing a transformer, a rectifier and the relay for the magnetron.

Step 6: The HV Power Supply

Depending on the tube you are using, you will need a power supply with a Voltage between 2 kV and 7 kV. Your tube can handel about twice as much voltage for short amounts of time. This was what i went for.

The easyest and cheapest way to achive this are mikrowave transformers (2.1 kV). You can use a doubler cirquit to achieve 4.2 kV Halfwave AC . If you need even more power you can use more MOTS in series.

This time the maximum voltage counts! Remember to multiply with 2^(1/2)

In my case: 3KV rating --> 6KV Max > (MOT plus doubler)2* 2.1KV * 2^(1/2)=5.94KV

If you have decided to use a doubler (like I did) just use the microwace capacitors to do the job.


Step 7: The HV Diode

Well why shouldn't I use these microwave diodes?

The HV diodes inside microwaves can handel about 12KV at 350 mA. If you use more than one microwave capacitor for the doubler (I use 3 right now) 350 mA is not enough. Furthermore 12 kV is not much as well. I simply used 24 1N4007 diodes in series to get to 24 kV at 1A and glued all into a PVC pipe. Cost for 30 diodes: 0.85€ !

Don't put 2 MO- diodes in parallel. This doesn't work!

Step 8: The Primary Capacitor

The primary capacitor is very important and needs to match perfectly to your systhem! You can find the main calculations you will need in step 10.

Nevertheless I am going to throw in some general stuff about the primary capacitor right now.

Which capacitor to use?

There are many types of capacitors you can use for this project. They just need to have the correct capacity and need to be HV proof. The cheapes way to achive this are ceramic disk capacitors or film capacitors connected in series. Remember 1/c1 + 1/c2 = 1/c3

The best way are doorknob capacitors but they are pretty expensive so I decided to use 20 film capacitors. In step 10 I calculated 750pF for my GU-81 VTTC so I used 1,5nF capacitors (1250V) in series. This way I only paid 5€.

Step 9: Limit Grid Current

You need something that limits the amaount of current floating through the tube grid and the feedback coil. It doesn't really matter if you use a high wattage resistor or a light bulb. The most important thing is that the calculations are correct !

All calculations for the GU-81 pentode

R= V(Anode voltage)/(4*I(Anode current))


=2458 Ohm

--> Use at least a 2,5kOhm resistor! (at least 130W)

W= (V(Grid Voltage)/R)*V(Grid Voltage)

= (600V/2458Ohm)*600V

=162 W

--> Use definitly not more than a 160W light blub (I use 100W)

Step 10: More Difficult Math

This is the hardest part of creating your own VTTC. But if you want more than just copying values, you need to focus on this :D

This calculations will help you to find out which number of coil-turns and which primary capacity you are going to need to get the frequency you want!

1.Primary Inductance:


A = wire cross section u0 = 4Pi*10^-7 N = secondary windings F = Frequency l = coil height

2.Primary reactance


f(pri.)= (the frequency you want) I choose 350 kHertz

Calculate C for the primary capacitor

C=(R/X)/(2*Pi* f(pri.) *R)

Now that you have calculated the needed capacity and number of primary turns, you need to build a secondary which has abou the same frequency (inclusive topload).

1. Secondary capacity:

C = 0.75* l + R + 2((R^3)/l)^2

l = secondary height R = secondary radius

2. Secondary frequency:

F(sec.) = 1/(2*Pi*(((N^2)*(u0*A)/l)*C)^-1

A = wire cross section u0 = 4Pi*10^-7 N = secondary windings F = frequency

The closer F(sec.) gets to f(pri.) the better!

(Toploads add additional capacity to your secondary, but there are a lot of calculators or java scripts on the internet, so I dont mention that here)

In my case:

primary capacitor: 20kV 750pF

Primary windings: 36

FB : 8

Secondary windings: 600 (0.5mm)

Secondary hight 30cm

Secondary diameter 8cm

Step 11: Make the Coils

This is not fun at all. It took me around 5 hours to make all three coils. The primary and the feedback coil have taps for easy tuning. Remember that the number of secondary turns does not effect the output voltage directly. I used 0.5mm magnet wire for the secondary (0.25mm is also okay) and 2mm wire for the primary and the FB-coil. After I have finished the secondary I put some clear sprayshellac on the coil to hold everything in place.

Step 12: Test It!

After some testing, the discharges reach over 23cm! (27cm against ground) Furthermore it lights up neon tubes from 2 meters away. If you have the equipment (variac) to test on lower voltages first, you should definitly do this! use a neon tube to check if there is any output. You will not see any big discharges at 30V AC.

Step 13: Make It Glow !

You can use chemicals to colour the discharges ! This is really simple. I just got some chemicals from the kitchen (NaCL ;D ) and from my old chemical-experiments-kit and put them into ceramic tubing which I put on top of the topload. The results are pretty impressive (they look better in real!) even though this can't be healthy at all can it? Please don't throw random chemicals into the flames! Use Google first :)

Chemicals I used:

NaCl --> Yellow

CuSO4 --> Green

KMnO4 --> orange/red

In addition to that you are now able to light up neon tubes and other gas discharge lights like a sodium discharge lamp or a mercury arc lamp! But the most amazing thing you can do with a vttc (at around 350kH!) is turning on sensor lamps (well I mean these lamps you need to touch to trigger them) from over 50 meters away! I am able to turn on my neighbors bedside light which is definetly really spooky because it triggers these lamps pretty fast giving you a haunted-house feeling :) I am really happy with these results!

Step 14: Finally...

If you have any questins or need some help, just write a message or a comment under this Instructable. I am going to help if I can. And please post pictures if you made one too!

  • Costs: ca. 65€but I bought more parts than I use right now! Maybe enough vor other HV experiments!

If you've enjoyed this, please vote for me :D

Step 15: Edit:


I redesigned my coil and made everything more compact. This way this project doesn't take to much space in my garage which is pretty good :)


Moreover I found out that a bigger feedback coil (13 turns) works even better.

In Addition I made a controller out of a transformer, a capacitor, two relays and a status LED for each relay powering the preheating circuit and the HV-supply.


Secondary math added.

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    8 years ago on Introduction

    Add modulation to it to make it a Plasmaspeaker ..


    Reply 8 years ago on Introduction

    Great idea! This can't be that hard can it? I just need to get some missing parts...



    Reply 7 months ago

    although audio modulating a VTTC is possible(they did it at the boston museum of science) it is really hard to do so i suggest building an SSTC using the circuit from this video(link in the description to download files for custom PCBs for the circuit)


    Question 1 year ago on Introduction

    can i use a audio tube 12AX7B its has rated at 330V, and can i just use it as a transistor in brovin kacher circuit and can i supply directly to the main voltage with out the MOT?


    Answer 7 months ago

    NOOOOO. If you think that u can use a tube in the brovin kacher circuit u should not be building tesla coils. vacuum tubes work very different than transistors and you can buy an IRFP460 MOSFET for 3 bucks on eBay and make the brovin Kacher circuit just fine. if u do make a brovin Kacher SSTC i highly recommend watching this video
    also you should never run 12ax7 tubes in VTTCs either as their current ratings are pathetic(several milliamps).


    Question 4 years ago on Step 10

    Hello, not clear to me how you have calculated C for the primary capacitor. What is R in the related formula? Thanks in advance for your answer...Alex


    5 years ago

    Hello! This is an amazing project and I want to build. There are some questions though:

    1. What bulb shall I use? Normal 220v 100W bulb or something special?

    2.What is the purpose of the 700pF 12.5KV capacitor? Can I use a 1000pF 15KV cap instead ?

    3. What is the purpose of the 4.7nF 6KV cap in the filament transformer output?

    4. Should I use a separate grounding rod for the Coil and if yes, to which part of the circuit should i connect it? (I have RCD breakers son any ground current will trip them)

    5. What is the purpose of the "UKW Trap" ?

    6. Can I run both transformers from the same outlet?

    I would also like to appreciate the amount of work put into making this instructable! I really love this project but I don't want to do anything before fully understanding the schematics. Sorry If my questions are stupid/noob but i'm new to VTTC stuff.



    6 years ago

    I'm thinking about building a VTTC following your schematic. A few years back I bought a GU-81M, so I will finally put it to use, but I have a few questions. First of all, when I input your values to javatc I get a resonant frequency of around 885khz, far from your 350khz. I am in the US so no metric lol. I am going to design a coil that is based loosly off your dimensions, just as a baseline.


    8 years ago

    really cool!!! I'm remodeling my home w a steam punk theme I'm thinking of actually building several of these and using them as accent lighting lamps


    Reply 8 years ago on Introduction

    Great idea! Keep in mind that the tubes become really hot if you overdrive them ( like I did). My tube glows red after 5 minutes :D


    8 years ago on Step 10

    What are the dimensions of your coils? Turns on the secondary don't matter but the length does make a difference.


    Reply 8 years ago on Introduction

    Sorry, I totally forget about the secondary-math! (going to add that soon!)

    My secondary has a height of 0.35m and has a diameter of 0.08m. The primary and the feedback coils have a diameter of 0.12m.


    Reply 8 years ago on Introduction

    You need to main things to produce X-RAYs: Very fast electrons(25kV < U < 600kV) and a anode material which slows the electrons down. (most common wolfram and copper). My input voltage is only 6kV and the anode material used in transmitter tubes is graphite (which is one of the best materials for absorbing the energy, that is why the anode glows red after 2 minutes). I don't think that this tube is able to produce noticable amounts of X-RAYs (hopefully :D )


    8 years ago on Introduction

    I like this and the long sparks are very impressive. If I had more space in my house that would be a really good project to build!



    Every Mad Scientist needs one of these right next to a jacobs ladder. I love the glow of tubes.

    Tom Hargrave
    Tom Hargrave

    8 years ago on Introduction

    I like your project but you don't need to use a vacuum tube to drive a Tesla coil.

    Years ago I built a Tesla coil using a neon sign transformer, a aluminum foil & glass plate capacitor and a spark gap to drive the primary. The primary coil and glass plate capacitor made a tank circuit and the spark gap generated the HF needed to make the whole thing work. The primary was an open coil of #14 wire wrapped around a oatmeal box and the secondary coil was a single wrapping of magnet wire wrapped around a paper towel tube with the bottom end tied to the ground side of the primary.


    Reply 8 years ago on Introduction

    You are right! Actually there are are many different ways to drive tesla coils! The most commons are Solid State Tesla Coils and Spark Gap Tesla Coils!