Step 3: 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: http://en.wikipedia.org/wiki/Transformer. 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: http://www.stevehv.4hv.org/VTTCfaq.htm, you can read more about VTTC operation there)
<p>Is there a possibility of using three or four 10kV Capacitors instead of the 30kV one in any sort of arrangement?</p><p>Thanks. </p>
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
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 <br>
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 <br>
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 <br> <br>thanks
Hi there!<br><br>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?<br><br><br>
Hi! <br> <br>Do the 811a need to be a matched pair?
Ideally, yes. You can tap the feedback coil differently for each one if they aren't. Kaizer Electronics has a good writeup of this.<br><br>Good luck!
I have a 6.8V 5A filament transformer from an xray head, will that work?
If you're using only one 811A, yes.<br><br>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.<br><br>Good luck!
These tubes are almost twice as expensive as the 811A on ebay and yeah...16 years old..not much money lol
I'm 16 too! =)<br><br>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.
how do you assemble the base is there any special way to do this?
could you use a metal halide ballast instant on a microwave transformer?
You certainly <em>could</em> use your MOT with such a ballast, but I don't see any reason to do so<span style="font-style: italic;"> in this coil.</span>
amazing!! <br>
Thanks! :)
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. <br>
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???
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.<br><br>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.<br><br>Good luck!
Thanks for the advice! Looking for an SCR now!!!
Just in case you were wondering, the SCR I used was a BTA16-600B (600V, 16A) - http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXywxqt.pdf<br><br>Note: 16A at 600V is extreme overkill for this application, so don't worry about using a weaker SCR.
Ok, thanks!
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!!!
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:<br><br>http://www.youtube.com/watch?v=wpSBRJETQDg<br><br>http://teslacoil.ru/devices/fakelnik-na-gu-50/<br>(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
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. <br /> thanks<br /> <br /> paul
They would share the same ground. For (small)Solid State coils and most Vacuum Tube coils, this is quite common.
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. <br> <br> <br>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!!!
i have built a vttc i have found the plans on your web site and<br>nothing is happening wen i test it there was a loud hum sound and<br>that was it . the tubes are not heating up and the grid circuit<br>does not seem to be working but the continuity is ok. also<br>continuity is ok thou the rest of the Tesla coil please help<br><br>specs<br>1 k mot<br>1.85uf microwave oven cap <br>two 811a's <br>veritable resistor in gird with 0.0022uf cap
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.
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!
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 &quot;1 k mot&quot;? What parameter is &quot; 1 k&quot; 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 &quot;continuity is ok&quot;? What exactly were you testing?<br><br>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?<br><br>Good luck!<br><br>Xellers
can you tell me the values of all the parts? I'm planing to make an table top version of this... also, i will tweak it so that is an AM transmitter!
The values of the parts are all specified within the instructions, and you can visit Steve's site (there's a link) if you need some more information. Also, this *is* a tabletop coil! Regarding the AM transmitter, this is probably not the best way to go about building one, the 811A is better suited for building a linear amplifier or a modulator than it is for making an oscillator. I would use something like a 6146A (or B) tube as an oscillator and a pair of 811A tubes in a 500W RF amplifier for an AM transmitter. If you do build a transmitter, then please post some pictures and tell us how you did it!
it will be a while before i build it as a gather info on it, but i think this schematic is useful... i took your schematic, (the second pic) and took out all the static, moved some components around, and then added a audio transformer and ac-to-dc converter. the third schematic is similar to the first schematic without modification.
I have 2 nice new EI-niš PL-519 tubes. It is a penthode rated at 7Kv. I also have new mot. I don't want to use schematic with transformer T3 for audio-modulation, but because I have a penthode I would apply audio-input to some of the grids. What grid should I use? Should I apply audio-input to both tubes, or just one? <br>
You can follow the link in step 4 to the complete schematic. The parts are listed in the schematic, as well as in step 5. A cost estimate is also given in step 5. In order to modulate the coil, you need steady DC power supply; in this particular schematic, the oscillator is only active during the positive half cycles of the current provided by the MOT. This means that you are already modulating a 60Hz signal into the coil, and this is reflected by the loud buzzing noise it makes when it is turned on. To modulate audio, you would need a full wave rectifier and a filter capacitor in the power supply (I can already see a half wave rectifier in your second schematic, but this will still cause some hum, use a full wave rectifier). However, if you continued to use the MOT in this configuration, it would destroy the tube almost immediately - you would need to significantly reduce the power supply voltage, which would in turn reduce the spark size. Also, this would add about $50 to the cost of the project because MOTs are relatively easy to come by and you can find one for next to nothing (if you can't find one for free inside of a broken microwave oven), while other high voltage plate transformers are rare nowadays and would probably have to come out of vintage radio gear. (I have also posted this comment an answer to your question)
can you give me some info on the tubes why they will break? and do i have to use a MOT, cant i use another transformer like a BIG wall-wart (wall adapter) or a bunch of batteries?
1) Vacuum tubes will arc over, resulting in catastrophic failure and destruction. 2) I'm sorry, but if you do not know why you need to be using a microwave oven transformer, then you probably shouldn't be using one in the first place. I do not want to give you information that could be potentially harmful; the &quot;hobbyist approach&quot; (learning by doing, rather than by understanding) works well in some cases, but I do not believe that it is wise to be building Tesla Coils without some proper foundations in electrical engineering. Find a book or website that explains vacuum tube operation, and then read the article on Steve Ward's site about designing VTTCs.
i just want a safer transformer. don't vacuum tubes work at low voltages? not a lethal 2.5KV @ 100ma+ ?
You are correct, many vacuum tubes work with relatively low voltages on their plates. In fact, some types of later &quot;space charge&quot; tubes (like the 12DZ6, for example) were even <em>designed</em> to work with only 12 volts on their plates. Some smaller space charge tubes have been run with only 3 - 6 volts on their plates! However, these types of tubes are usually not capable of any significant power output (most of them can't handle more than a watt or so of output power in class A), usually they were designed for use as RF amplifiers in the first stages of a radio circuit. Some other types of &quot;subminiature&quot; tubes and small 7 pin tubes can work with relatively low voltages of several dozen volts on their plates, but these too suffer from weak power handling abilities. The smallest type of tube that can be used in a VTTC is a strong RF amplifier from a radio receiver, or a weak RF oscillator (or amplifier) from a radio transmitter (when I talk about &quot;strong&quot; and &quot;weak&quot; here, I am talking about how the particular tube compares in terms of plate dissipation power with other similar tubes). The smallest VTTCs can run with only about 200 or so volts on the plates of their tubes, but they only produce sparks a few millimeters long. I'm sure you've already seen this, but this is more or less what you should expect if you plan on building a small VTTC: <a href="http://www.youtube.com/watch?v=dYJ2U0IHJ00" rel="nofollow">http://www.youtube.com/watch?v=dYJ2U0IHJ00</a>. Another popular tubes for small VTTCs is the PL504. However, the voltages on the plates of even the smallest VTTCs are still lethal. The minimum voltage required to operate a tube like the 811A is only a few hundred volts, but don't even expect it to be able to oscillate.<br>
will a NST work for it?
This question has already been answered, and the answer is no, unfortunately, it will not.

About This Instructable




Bio: My name is Daniel Kramnik - I like building Tesla coils, quadrotors, and robots!
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