Ok, so Adrian Marsh shows on his YouTube channel a VTTC with fractal fern discharges: https://www.youtube.com/watch?v=Oo_mINYLqOI

It turns out to be a class C Armstrong oscillator, and Adrian has a nice explanation of its operation on his website. To quote:

Circuit operation is as follows. When first turned on the impulse current from the tank circuit B+ conducting through the vacuum tube causes a ringing oscillation (ping) in the primary circuit LpCp. This oscillation couples to the secondary circuit LsCs which is further coupled by the pickup coil to the grid bias leakage circuit. When the phasing of the pickup coil is the correct way round for positive feedback, the grid bias leakage capacitor C1 becomes negatively charged during the positive half cycle in the primary circuit, pushing the grid voltage down and progressively restricting conduction in the vacuum tubes towards the off state with much reduced plate current. The negatively charged grid leakage capacitor C1 then discharges through the grid leakage resistor R4. As this happens the grid voltage on the vacuum tube starts to rise progressively towards 0 volts turning on the vacuum tubes with an increasing plate current. The plate current through the primary circuit LpCp again is coupled to the secondary LsCs and the cycle repeats. With the grid bias leakage circuit correctly adjusted the VTG will oscillate with a linear sinusoidal output optimised for maximum plate voltage and current swing, (maximum power transfer at the resonant frequency of the secondary), whilst keeping the grid bias currents within the maximum ratings for the vacuum tubes used.

So now it has become quite clear, but it may still be of interest to learn what kind of signal is at the grid of the tube in this setup?

If we could connect an oscilloscope that would withstand the conditions that exist there, what would it show?

The schematic is: schematic of the fractal fern discharge GU-5B VTTC

Thanks in advance.

  • 2
    \$\begingroup\$ Show a schematic of what you're talking about. \$\endgroup\$
    – Hearth
    Sep 4, 2023 at 17:16
  • \$\begingroup\$ A sine wave, give or take. Do you understand what an electronic oscillator is, how it works? Explaining a VTTC won't do much good without a basic understanding of operation, plus device characteristics; easier to start with the oscillator itself. \$\endgroup\$ Sep 4, 2023 at 17:52
  • 1
    \$\begingroup\$ Videos of these kind of coils in operation can be found on YouTube. ... people will not go searching youtube \$\endgroup\$
    – jsotola
    Sep 4, 2023 at 20:12
  • \$\begingroup\$ It seems I'm beginning to understand the oscillator a little. \$\endgroup\$
    – Hermolaou
    Sep 8, 2023 at 15:42
  • \$\begingroup\$ FYI, datasheet for the tube: frank.pocnet.net/sheets/018/g/GU5B.pdf \$\endgroup\$ Sep 8, 2023 at 15:48

1 Answer 1


The grid receives feed back from the coil system (a resonant transformer). This feed-back signal (basically AC pulses) then switches the triode (on-off-on...) to create a high current AC signal at the plate, this high current drives a primary coil that induces a much higher voltage on the larger secondary coil. A smaller coil samples the AC voltage signal and that becomes the feed-back signal to the triode grid.

Here is a much more complete explanation of a "Vacuum Tube Tesla Coil (VTTC)". (Note that in the schematic diagrams the group of 3 coils are actually arranged to create a transformer system, see the assembly pictures.)

  • \$\begingroup\$ Are you sure that there are AC pulses and the triode ever gets completely switched off, like in radar systems? The other possibility is that there is just an uninterrupted oscillation going on, with the tube swinging between more or less conducting states. \$\endgroup\$
    – Hermolaou
    Sep 8, 2023 at 15:49
  • 1
    \$\begingroup\$ @Hermolaou - With enough feedback the triode can easily reach an off state, however following that the feedback would stop, the triode would conduct once again, that creates another feedback signal, the cycle repeats continuously. A full on/off cycle would result in a basically a square wave, (repeating square pulses). A square wave would then be smoothed by the LC values of the primary coil section that favor one resonant frequency. If the feedback were to be carefully adjusted the triode could be made to output more or less of a continuous sine wave, but that isn't required in this case. \$\endgroup\$
    – Nedd
    Sep 9, 2023 at 0:13

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