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Old 12th Nov 2018, 7:26 pm   #1
Sean Williams
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Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
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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