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High power plasma tweeters were all the rage on the 4hv forum a few years ago, so I decided to join in on the fun and bought myself a GK-71 power pentode. Cue several years, later I finally slapped together the simple circuit this Tesla coil consists of, and had a working high-frequency-vacuum-tube-Tesla-coil, or HFVTTC. This project is quite simple and easy to get working, but absolutely lethal, so take care if you attempt something similar. The GK-71 can operate at anode voltages up to 1500V, which coincidentally is the voltage a microwave oven transformer will output once full-wave rectified. For the longest time I fussed over how to connect the transformer, as MOTs have one secondary wire connected to the core which should be grounded. For full-wave rectification to work neither end of the secondary winding can be grounded, but to simply disconnect the grounded end of the secondary might cause flash-overs from the secondary to the core. It stands to reason that the winding insulation is only designed to cope with low voltages near the core, so the threat of flash-overs is very real. Despite this I disconnected the grounded wire, rectified the output, and grounded only the core, all without problems. Which honestly, was just luck. The transformer could have just as likely gone up in flames, but in my case the insulation was sufficient.

Red hot bolt Warmly glowing bolt Partially melted steel carriage bolt
Some bolts at various stages of heating. The same bolt after getting the unit working correctly, melted like wax.

To filter the microwave oven transformers output, two microwave oven capacitors were used in parallel for 2uF. When powering vacuum tube amplifiers from a supply with filter capacitors, it is recommended to use a "glitch resistor" to limit the surge current in the event of an internal arc in the tube. This will protect the tube from damage, often at the price of the glitch resistor. I used a 30W, 22R sandbar resistor. To power the filament I used an old laptop charger, which provides a shade over 19V DC at several amps. The high frequency noise from the coil doesn't seem to bother it.


The estimated power dissipation of the 11k7 resistor is 85W, so be sure to use something that can handle a lot of power. I used some 5W and 10W resistors wired together for 20W, but they melted through the plastic bubble-wrap in just a few seconds. The feedback capacitor seems critical to the design. I initially used cut brass sheet which had sharp edges, and breakouts would occur along these edges unless the two plates were spaced a few centimeters apart. With the plates so far apart the power from the coil was severely limited, and the breakout was only a few cm tall, similar to a regular plasma tweeter. I then used two metal jar lids with smooth edges and enamel, which seems to have forced the breakout to occur only at the breakout point. This has allowed close proximity of the jar lids, and higher feedback capacitance. Which in turn has increased the plasma streamer to roughly 16cm! It should be possible to use purchased capacitors instead of making your own with jar lids. I haven't measured the capacitance, but I assume it is in the range of 5-15pF. I haven't run this HFVTTC for more than 30s at a time, but so far the tube has shown no signs of turning red from losses.


4hv Forum Thread 1
4hv Forum Thread 2

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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

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