Part 1: The Large Ladder

Before starting my DRSSTC I had to build a large Jacob's Ladder for a friend's business. It obviously HAD to be safe. This meant no NST's, OBiT's, etc. that run on 60Hz. Needed to be enclosed thoroughly and high frequency. I built the Information Unlimited JACK-50, a 5' design that runs 16-18 KVAC at 15-50KHz. Guessing about 1KVA or so, didn't think to use a clip-on meter on the AC line - oh well. Here is a link.

https://www.amazing1.com/products/jacobs-ladder-5-foot-plasma-arc.html

The kit is run by a small full bridge unit (chopper) that outputs a 300V p-p square wave with the usual gate driver circuits, gate transformers, etc. What distinguishes this kit is that it has you scratch build a high freq ferrite core grounded center tap transformer. I found this very instructive, very difficult  by hand, and pretty cool. Some of the really tough parts were locating structural pieces for the transformer that fit together (bobbins, etc.) I used polycarbonate tubing in 1/16" x 1 1/2" for the bobbins, and larger for the enclosure. You do have to vacuum pot it to prevent arcs and corona.

I used Ferroxcube 3C91 or 3C94 Ferrite in the U100/57/25-3C94 size. Either material will work due to the frequency range.
https://www.digikey.com/en/products/detail/ferroxcube/U100-57-25-3C94/7041696

The arcs are huge, white, and fuzzy - it will bridge electrodes at up to 4" or more with a 1" thick arc. Downloadable plans do have some small issues (obsolete parts, etc.) but do work and are, IMO worth the 10.00 for just the transformer design. One of the really nice features is the cooling fan for the full bridge is dual-purpose. Someone very carefully designed ports in the base to provide air to prevent arc hangup and give a smooth arc climb.

Here are some pics of the test setup - the first set of electrodes were made from steel coat hangers and without the fan, suffered quite a bit of damage from hangups. I also applied salt and baking soda dissolved in water for a little entertainment    There are other fun chemicals - don't use any Barium compounds, vapor is toxic.

Cheers / Robert

If you want to do it, there's a reprint of a 1902 book called "Induction Coils" by Norman Hugh Schneider, available from Franklin Classics (UK). You can find it on Amazon. There are chapters on homemade everything in it, including an adjustable primary cap that looks pretty dodgy (but cool)!
 If you wanted to do a period coil (aka 1890's) that's 100% real it's your reference.
The ads for the book company's other offerings in the back are very entertaining too.
It also includes a chapter on "experiments" or making cool glowing things. I am very happy with the purchase.

This is the proper subforum! 

I spent some time going through the DC schematic on Richie Burnett's website and realized the charging inductor is basically a boost-mode SMPS setup. 2xDC is Awesome! Here are all the thoughts on this Coil idea. My dry-erase board in the workshop is full - attached sketch.

http://www.richieburnett.co.uk/dcreschg.html
Below is from Richie's Website:
"It should be realised that the function of the de-Q-ing diode can be performed by the DC bridge rectifier in 3-phase designs where no HVDC smoothing capacitor is used. The lack of a separate de-Q-ing diode and potentially lethal DC smoothing capacitor make such 3-phase supplies ideally suited to variable speed DC resonant charging."

This is a good idea! So here is Project "10-Horse," a DC Primary "Boost Mode" Asynchronous Rotary Spark Gap Tesla Coil.

You can buy a cheap 7.5KW (7500VA) or 10HP VFD ($300.00 US) that runs on 240VAC mains at 35A and outputs 208/240VAC 3 Phase from 1-400Hz, 1-600Hz or in a few cases, 1-999Hz. "You get the three-phase source without paying the power company for it."  It will require adequate impedance in the transformer primaries or the drive will fault as it will be "expecting" to see 10 horsepower motor windings or the equivalent.

I have a box of 30 12KV 1A NTE rectifiers (block mount on heatsink)
https://www.digikey.com/en/products/detail/nte-electronics-inc/NTE541/11655505

RECTIFIERS
1.) Actual Rectifer output will be 1.414 x Transformer Output; diode voltage ratings must take this into account and have leeway above it.
6000VAC transformer output (25.0 : 1 Step Up) = 8.5KVpp rectifier output. DC doubling = at least 12KV
6500VAC transformer output (27.1 : 1 Step Up) = 9.1KVpp rectifier output. DC doubling = at least 13KV
7000VAC transformer output (29.2 : 1 Step Up) = 9.9KVpp rectifier output. DC doubling = at least 14KV - near rectifier diode limit
7500VAC transformer output (31.2 : 1 Step Up) = 10.6KVpp rectifier output. DC doubling = at least 15KV - rectifier limit

NTE Rectifiers chosen for the project because I have them and they need a home. If a pair of NTE diodes are used in parallel for 2A, then 7-7.5KV are tops.
If a pair are in series, then higher primary voltage may be utilized with lower current.

- 2 NTE 12KV Diodes in series = 1A at app. 13-18KV, possibly 20KV - assumes voltage drops are balanced between the 2 series diodes. All diodes series or parallel will be tested, I don't desire to use balancing resistors. Defeats the DC de-Q-ing diode!

- 2 NTE 12KV Diodes in parallel = 2A at app. 9-10KV max - more current but less voltage

First question on primary circuit - better higher current or voltage?
   
2.) Step-Up Transformers (3)
Drive output is 240VAC at 0-400H, possibly 600Hz. Using higher output frequency allows a smaller step-up transformer to be utilized. Possibly cheaper?
For 6000VAC, a 25 to 1 step-up
For 6500 VAC, a 27 to 1 step-up
For 7000VAC, a 29.2 to 1 step-up
For 7500VAC, a 31.25 to 1 step-up


Design difficulties are the step-up transformers and the choke. I'd like to think I could wind the choke myself given the correct core, but transformers in that power range will probably have to be ordered.

Difficulties:

1.)  3 strong step-up transformers

3.) Choke for resonant charging: high value, high current, high voltage

Has anyone built this design?

In my build queue I have the current Jacob's Ladder with a friend and a McCauley 8.0 DRSSTC to learn that aspect of the hobby. I prefer to study lots and then build a kit or proven design before I try stuff on my own.

For a home-designed project, the above sounds pretty awesome. Choke protects the transformers, diodes and DC keeps the resonant rise in the primary down.

And an awesome rotary gap : )

If you see anything wrong, please post!

Hydron: I don't think so. I'm hand-building a ferrite core grounded CT transformer for HF high voltage and it doesn't look like running it low is an option. I could do a small signal test but that will only measure coupling, not saturation or heating effects from being run at low freqs. Too much work and winding to risk  smacking it with 60 or 400Hz! Big learning experience that I'd prefer not to repeat.
Running a Variac / full bridge / step-up transformer / rotary gap does seem sideways reasoning : )  especially given the DRSSTC design polish and losing the expensive transformers.

A VFD would drive the Richie Burnett 3 phase version but wouldn't be voltage-adjustable other than soft start. You always seem to end up back at the Variac : ) or a big switch!

Thank you / Good Conversation!

Suppose you just happened to have a pair of 5KVA 40:1 step up matched transformers rated to 5000V each. 20GA wire on the secondaries so they are good to app. 27KHz. Well matched enough to connect primaries in parallel and run the output grounded center tap. Think a 1A 10KV "NST." Yes, they would make a scary full wave multiplier.

Then suppose you had access to higher frequency than mains, say a Variable Freq Drive or a full bridge converter.

Could you drive an SGTC at higher than mains frequency and get any sort of gain from it?

I would assume an SGTC would really benefit from 400Hz or so with lots more breaks and a more annoying buzz, but that going up to the KHz would just create a constant arc unless there was a mechanical break device.

Since I do have the two supposed transformers on the way, suggestions?

Cheers / Robert