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Old 12th Nov 2018, 7:26 pm   #1
Sean Williams
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Default Vacuum Tube Tesla Coil (VTTC)

Spurred on by Albert's musical musings, I quite fancy building something sparky.

Whilst I am aware of several "designs" that are available on the WWW, and numerous conflicting pages of advice, I thought I might pop a few questions down here.

I envisage using an 813 to drive the coil - I have a couple in stock, I also have bases and top caps.

I have a custom wound 2250V at 1100ma transformer that is sitting here not doing very much at all.

I have enough rectifiers, and caps to make a very robust DC power supply.

From research so far, the MOT designs are favoured due to the rough pulsed DC output, making the duty cycle of the whole system much lower, allowing some quite serious liberties to be taken with the ratings of components.

If I am to build such a beast, I would prefer reliability over spark length, of course, the wow factor has some importance, but I really don't want to be tearing it apart every month to replace the latest overstressed component.

My idea so far is to build a coil that will work sub 500khz - this will of course mean lots of turns - I am fairly happy with building the TPTG oscillator side of things, after all, this is just an amplifier with positive feedback, what has me slightly baffled is the design of the secondary coil.

Obviously there needs to be a massive increase in turns to encourage a huge step up in voltage, what I cannot find spelled out anywhere, is the question of resonant frequency - in my mind, the secondary needs to be able to form a tuned circuit at the same frequency of the primary, thus being able to gain maximum power transfer, yet, nearly all of the design data I can find so far indicates the secondary coil resonating at around 1/10-1/20 of the primary frequency - am I missing something fundamental here?

Very happy to learn here - and have all my pre-conceptions blown out of the water, but it seems to me to be a simple class c oscillator, using the secondary coil as a loosely coupled antenna.

Steps back and awaits a flurry of expertise to prove all my theory wrong
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Old 12th Nov 2018, 9:30 pm   #2
astral highway
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Default Re: Vacuum Tube Tesla Coil (VTTC)

Quote:
Originally Posted by Sean Williams View Post
... it seems to me to be a simple class c oscillator, using the secondary coil as a loosely coupled antenna.

Hi Sean, welcome to the club!

You're absolutely right. It's a classic class C oscillator.

The main difficulties arise with the grid bias and feedback circuits (and coupling generally) both of which are experimentally determined. And also, not getting flashovers, but as you have power RF design experience, this will be trivial for you.

I don't think that's the main reason why MOTs were popular. It was rather that they were easily available from junked ovens. They are designed, obviously, with a magnetron as a load and they get very hot very quickly in Tesla coil operation. I think they're bad candidates.

Quote:
Originally Posted by Sean Williams View Post
...what has me slightly baffled is the design of the secondary coil....nearly all of the design data I can find so far indicates the secondary coil resonating at around 1/10-1/20 of the primary frequency - am I missing something fundamental here?

I don't know what designs you've seen but yes, the tank circuit comprises rather few turns and hence the L component on its own has a relatively low inductance. But the parallel LC circuit together is resonant at approximately 75-80% of the secondary coil resonant frequency.

All except low-powered valve tesla coil designs I've seen over the years have a parallel LC circuit. The capacitor is typically strontium titanate, (one of those common RF doorknob types) and usually between 3-5nF. Or you can build what's called an MMC from tens or more pulse caps.

The streamers at the antinode have some (series, obviously) capacitance with the secondary coil. It might only be a few tens of pifs but it does detune the coil variably variably.

Usually the self-resonant frequency of valve tesla coil secondaries is lower by means of a large topload, typically toroidal, but spherical will do. A break-out point is essential.

It's recommended to build an interrupter to lower the duty cycle of the valves if you don't want to red-plate the anodes in seconds. The 'on'-time of the square pulses only needs to be 20mS for persistence of vision to kick-in. Typically you modulate the cathode via a big MOSFET thesedays.

The RF circuit needs to be good, but you're expert on this - very modest post, considering your background!

This design was John Freau's and is one of the best out there. I got 22 cm of streamers from a single Svetlana 572b after my first build in 2001, when I built the first of a total of five (to date) coils.

(I abandoned the last EHT one because of my orthopaedic injuries and being unable to safely shift a circuit that already weighed 35kg and wasn't finished! I think Albert's rig is twice that or something.)

Happy to help esp with typical glitches and I'm familiar with good resources and bad!

P.s - I am still working on my solid-state build, very slowly. I thought I'd give Albert plenty of elbow-room to exhibit his amibtious project! So there are now three known Tesla-coil activists on the forum!

Spark length in inches for a valve Tesla coil is estimated by 0.5 *Sqrt (power in)
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Last edited by astral highway; 12th Nov 2018 at 9:44 pm.
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Old 12th Nov 2018, 10:05 pm   #3
Sean Williams
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Default Re: Vacuum Tube Tesla Coil (VTTC)

Thanks Al,

Yes, I like the idea of an interrupter circuit, the 813 isn't a great fan of excessive plate colour.

Plumbing some rough figures into a calculator gives me a tank frequency of around 400Khz on that design, not a million miles from where I want to be

I will look at the secondary coil in a little more detail when my brain is more switched on - Parallel LC circuits are easy to calculate, need to refresh my memory on series tuned stuff, especially when the streamers add a variable to the calculation.

I'm planning to use a 68mm former for my secondary, and was hoping for 110mm for the primary, but am quickly realising that I may need to go somewhat larger to maintain a decent loose coupling, and reduce the risks of flashover.

Lots of work to do - will start digging parts out over the weekend - think I will be shopping for doorknob caps - most of mine are more suited to VHF stuff
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Old 13th Nov 2018, 3:28 pm   #4
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Default Re: Vacuum Tube Tesla Coil (VTTC)

Hey Sean,

It sounds like you could comfortably use readily- available 110mm dia pipe for the secondary, with a big enough topload. It helps to have at least 20 coats of varnish to protect the secondary winding. Otherwise , it’s so easy to bash a turn by accident and destroy the whole winding .

I sometimes use Kapton tape over the part that is surrounded by the primary.

I have some titanium strontanate doorknob types - do PM me if you are looking for a source .

I have found some excellent resources to help work out the best sized tank cap based on the reactive power in the circuit. Anyways , happy to share my experiences of they’re any help along the way !
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Old 20th Nov 2018, 5:14 pm   #5
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Default Re: Vacuum Tube Tesla Coil (VTTC)

Hope your project goes well, Sean, especially with Al's sparky experience to help you.

I doubt if I can help except to say that a variable HV capacitor across the tank will quickly find the sweet tuning spot. Then it can be replaced by doorknobs etc to suit.

I used 110mm white drain pipe for my secondary and an 150mm acrylic plant pot holder (ebay) for the tapped primary coil. I bought 3 core flex cable to wind the primary. Cores were split to make one long length.
(brown,blue,green)

I did have a big flash over but put thin flexible PVC sheet to 'separate' the coils,
which cured it.


If you ever wish to audio modulate it (in a mad moment ! ), your power supply will have to be DC of course, as all mine are.

Look forward to hearing more from you in due course.
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Last edited by Viewmaster; 20th Nov 2018 at 5:19 pm.
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