Vacuum Tube Tesla Coil

Vacuum Tube Tesla CoilIn around 2000 I built a Tesla coil for producing high voltage electric discharges. Rather than use the conventional spark-gap circuit with all its noise and low-tech behavior, I decided to build one using vacuum tubes. I have been obsessed about high voltage phenomena and Tesla coils since my teens when I was experimenting with taking TVs apart to see how they generated their high voltages.

I went through around three iterations of this circuit to improve the performance. It is still not as good as it could be for a variety of reasons. Maybe one day I will re-visit the design with the benefit of some computer simulations to really nail the working parameters.

Tesla Coil Circuit

I refined the circuit a number of times. Various classic vacuum tube tesla coil circuits are shown on the web. However if you build them its very easy to get lots of bad effects. I got advice from kind folk on various discussion boards and improved the circuit by adding components to prevent parasitic oscillation, prevent leakage back into the mains supply, and to ensure a good solid radio frequency circuit. The first version I built crashed my central heating system so that it was continuously on and the room became very hot. Also when I scoped it with a high voltage probe there were multiple frequencies present which is not a good thing. The new version has a nice pure sine wave oscillation.

Vacuum Tube Tesla CoilI’ll give you a description of the function of this circuit now. The power supply consists of a microwave oven transformer that gives 2kV at around 1kW power and this is doubled to 4kV by a voltage doubler circuit. Actually this is too much voltage. I did not run it typically at 100% on the variac. The 572B tubes are not specified up to 4kV, but they will just about take it. They did arc over once or twice, but it was a rare occurrence and they were not damaged. The arcing was probably because of too much negative grid voltage in actual fact (the grid swings extremely negative during cutoff). If I rebuild the circuit I will use a lower high voltage supply and try to get the efficiency higher.

Vacuum Tube Tesla CoilThe circuit consists of two Svetlana 572B tubes in parallel. These drive an air core transformer and there is a feedback wiring that goes to the grids to create oscillation. The tank capacitor is a huge 15kV 1nF doorknob capacitor, which seems indestructible. The grids get feedback using a self biasing scheme where the positive feedback voltage causes grid current to flow which via the 4nF capacitor biases the working point below ground. The RF circuit consists of the tank circuit connected to the anodes, then the 20nF 10kV HV bypass capacitor to ground, then from ground through the heater bypass capacitors at the cathode and then via the tubes back to the anode. It is important that good quality RF capacitors are used which can take a high voltage and RF current. The parallel arrangement of HV bypass capacitors that I used was not very good, and some of them have blown from too much RF. I would replace them with a single high value high current high voltage low loss capacitor.

The metering allows you to see what cathode current you are drawing through the tubes. The parallel inductor/resistors at the anodes are to prevent parasitic oscillations. The heater common mode transformer and mains RFI chokes are to prevent RF getting back to the supply.

Vacuum Tube Tesla CoilThe main difficulty with this circuit is getting the right turns ration, oscillation frequency, coupling and number of secondary turns, as well as the size of the top capacitance. This all has to match at the operational frequency and also it has to be optimal for the tube output impedance so that it maximizes the power that is delivered to the spark load. I would not say that this circuit was well optimized, even if it did produce quite nice 10 inch sparks as a result.

I was able to let the secondary sparks jump to a metal object held in my hand without being electrocuted. Also fluorescent tubes would light quite brightly held free in the hand anywhere in the room pretty much.

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Tesla Coil Spark

I did an experiment where I added a piece of piano wire to the break out point at the top of the secondary. Very quickly it was consumed in a shower of sparks like a firework sparkler.

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