I'm building a vacuum tube Tesla coil (VTTC,) and although I have experience in SGTC (spark gap), there are some questions left unanswered.

When picking a tube for your coil, does it matter which tube you pick, except for the voltage and current ratings? Because as far as I understand, the feedback coil's job is to make the tube turn on and off at the right time. If the tube is going to resonate anyway due to the feedback coil, the only reason why there are different coils is because of different voltage and current ratings.

When looking at more complicated VTTC circuits, there are a lot of capacitors. What is the meaning of all the extra components, compared to an SGTC, there are only 4?

enter image description here

This is not my Tesla coil schematic, as I havent started building it yet. As you can see, there are three capacitors as compared to a SGTC, which has only one. Do the capacitors, resistors and diode change the primary frequency or are they just there to smooth things out?

  • 3
    \$\begingroup\$ Can you show some schematics of what you're referencing? \$\endgroup\$ Jul 4, 2022 at 20:03
  • \$\begingroup\$ We can't help you without some schematics. Tesla coils are not the kind of thing most people here work with on a daily basis--show us what you're talking about and we can help. \$\endgroup\$
    – Hearth
    Jul 4, 2022 at 20:05
  • \$\begingroup\$ instructables.com/How-To-Build-A-Vacuum-Tube-Tesla-Coil-VTTC \$\endgroup\$
    – Antonio51
    Jul 5, 2022 at 8:40
  • \$\begingroup\$ Emil Adser - Hi, Your "answer" (which wasn't an answer to your original question, and therefore should not have been posted in the "Your Answer" box) has been added to the question as an edit i.e. an update. Unless you are writing the full answer to your own question (i.e. unless you have solved the problem yourself, with no further help needed) please don't use the "Your Answer" box below. Instead, you should edit the question to add new information. This is part of how Stack Exchange differs from typical forums. See the tour and help center for more rules. Thanks. \$\endgroup\$
    – SamGibson
    Jul 5, 2022 at 20:58
  • 1
    \$\begingroup\$ stevehv.4hv.org/VTTCfaq.htm \$\endgroup\$ Jul 5, 2022 at 21:42

1 Answer 1


I very recently built my first VTTC using a modified version of the schematic you included in your original post. Bruce Abbot commented a link to Steve Ward's VTTC FAQ page which I cannot recommend enough. That was my biggest source of information and guidance throughout my build.


It describes the circuitry in detail, the functions of each component, and in many cases, how he determined the values and specifically, how he made his.

The answer to your question is simple, and yet not simple at all. In its simplest form, no - you cannot use just any vacuum tube. A more complex answer would discuss the many characteristics that come into play, for example rated voltage and current (as you already suggested), power dissipation, operating frequency, mu (gain), cutoff voltage, transconductance, and so on. Not to mention the type of tube itself, be it a triode, tetrode, or pentode. All of these factors have an effect on the behavior of the tube and its ability to operate in a VTTC. However, there are quite a few tubes out there which are tried and true. My VTTC used the relatively-cheap GU-81M. Other popular choices (though perhaps more expensive) are the 811, 833, GU-5B, etc. I have seen very small (2 inch tall) VTTCs running off of miniature radio tubes, but were only able to produce a small corona spray. If you'll forgive the generalization, builders of VTTCs usually aim for longer sword-shaped arcs. This is really only achievable by larger, more powerful vacuum tubes.

  • \$\begingroup\$ Thank you so much for your ansete it helped alot. I will definetly be looking into that site. But What about all the Capacitors do they chance the frequency, or do i just calculate the the frequency, by the primary coil, primary cap and transformer. And does the vacuum tube change the primary frequency \$\endgroup\$
    – Emil Adser
    Jul 6, 2022 at 15:45
  • \$\begingroup\$ The website explains the function of all the capacitors. C1 is the main resonant capacitor and that is primarily the one which changes frequency. The others have a variety of functions, but they do not really change the resonant frequency of the coil. The tube also does not change the frequency, but its characteristics may affect the shape and length of the arcs. \$\endgroup\$
    – DerStrom8
    Jul 9, 2022 at 2:47

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