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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 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. </p>
Thanks X, I'm so intrigued!
<p>hat analogy to a merry go round is the most ignorant thing i have seen today.</p>
<p>This may be very clever but all I could see was the danger involved, not only the &quot;machine&quot; but look how close it was to the light flimsy curtains blowing in from the open window. Maybe it's a mother thing.</p>
<p>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.</p><p>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 <strong>will</strong> hunt you down and report you to the feds.</p>
will this thing kill you
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
<p><a href="http://www.instructables.com/member/cvaughn3" rel="nofollow">cvaughn3</a> <a href="http://www.instructables.com/member/crazy-blender" rel="nofollow">crazy-blender</a> 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.)</p>
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 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
<p>Are any of the tubes getting overdriven? I heard running tubes the wrong way can produce X-rays https://www.youtube.com/watch?v=yLSu_UjrcUA</p>
<p>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?</p>
<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>
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!

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