Herr Zapp wrote ...

Eniyuki -

Your link to the data on the 100W resistors did not work for me, but I think I found the Tyco 100W power resistors in the TME on-line catalog. Are they the parts with the gold anodized housings? These resistors are not rated for high voltage, but may work, depending on how good the internal insulation is. Be careful to mount them so the housings are isolated from anything at ground potential, as the flashover distance from the resistor terminals to the housing probably will not withstand 15KV.

Don't worry about too many air holes reducing the cooling or quenching performance of your spark gap. You definitely do not want airflow restriction on the "inlet" side of the housing. If you have a powerful vacuum motor, especially is it has a two-stage impeller, it can generate a significant negative pressure. If your gap is surrounded by a negative pressure, the gap breakdown voltage will be reduced, which will reduce performance. In some cases you'll get better performance if you turn the vacuum motor around so that it blows air through the gap, rather than sucking air through the gap. Given that your gap assembly is essentially completed, I'd just add 2X or 3X as many holes in all three plates of your gap enclosure, and test the system.

For your MMC, be careful of mounting the capacitors directly to the surface of a plastic sheet, with holes drilled for the leads. I've seen several MMC failures due to surface tracking across the plastic, probably caused by residue from the soldering flux, sweaty fingerprints, dust, etc. I prefer to mount the capacitors slightly elevated above the board surface using "turret standoffs". These may be hard to find, so you could use any type of insulating spacer (ceramic standoff insulators, short segments of plastic tuning, nylon spacers, etc). In any case, leave planty of spacing between leads that have a large potential difference across them, and after assembly carefully clean everything using isopropyl alcohol.

A few questions for you:
1. What is the gap width that your static gap assembly is currently set to, as shown in your photos?
2. Is the gap spacing adjustable?
3. What are the two objects shown on the LH side of the "black spray paint" photos? The NSTs are obvious, but I can't determine what the other items are.

Here are a few photos of small MMcs using the standoff construction method:


Regards,
Herr Zapp

Herr Zapp wrote ...

Eniyuki -

I'm not sure what you mean when you say "what if I add a pipe from the exhaust to the electrodes". Do you mean make it into a closed-loop system, with the air constantly recirculating from the exhaust back in to the inlet? This would be a bad idea, the intent is to flush the hot ionized air out of the spark gap and replace it with cool, un-ionized air. I'd just add some more ventilation holes, and run your spark gap "as-is" for your initial testing.

Your answers:
1. OK, .25" should be fine for initial tests.
2. Good, you may decide to make some adjustments later to see how total gap width affects performance.
3. OK, I didn't notice that the top covers were removed from the NST housings.

Have you calculated the primary-secondary coupling coefficient ("K factor") that your primary will provide? That is another variable that can have a large influence on coil performance. From your photos, it looks like the plane of the primary coil may be quite a bit lower than the bottom turn of your secondary, which would yield a low coupling factor. Coupling factors of between .12 and .15 are typical values for spark-gap coils. If you design-in some way that you can raise and lower either the primary or the secondary, it will be easier to find the "sweet spot" during tuning. Adjustment increments of .25 inch are fine. Use MandK, JAVATC, or another Tesla coil design tool to calculate your coil's K.

Regards,
Herr Zapp

Herr Zapp wrote ...

Eniyuki -

Your initial coupling factor of .13 should be fine. epecially if you have some means to adjust it.

I have never used MOVs in my Terry filters because I frequently used the filters with various NSTs of different voltages, and I didn't want to "rework" the filter each time. With a properly adjusted 3-ball spark gap at the output side of the filter you should be OK without the MOVs.

On my Terry filters, I actually have TWO 3-ball spark gaps, one at the input side, and one at the output side of the filter. Curiously, during some operating conditions I have seen the spark gaps at the INPUT side of the filter sparking while the gaps at the OUTPUT side were not. (All spark gaps had the same ball diameter, and were carefully set at the same gap distance.)

You may find it harder than you think to get the primary conducter routed through the holes in your primary supports, especially if you secure the supports to the baseplate first. I made a flat spiral primary once using the "feed it through the holes" technique, and I'll NEVER attempt it again. I was using relatively thick primary supports (3/8") and PVC insulated wire, which created more friction in the holes than bare copper tubing will. The first few turns were easy to slip through, but as the wire moved in to the smaller radius holes, the friction became extreme, even with everything "lubricated" with soapy water.

A more common (and MUCH easier to assemble) technique to just slot the primary supports so the conductor can be "snapped in" from above.

Remember NOT to try to straighten out your copper tubing in any way. Annealed copper will work-harden to a shocking degree with even a little straightening or bending, and it will become impossible to make a clean, uniform spiral. That's why the "open slot" primary design is so easy to assemble: just have someone hold the coil of copper tubing slightly above the primary supports, and guide the free end down into the slots.

Also, it looks like the hole locations in your primary supports are identical for all the supports. This would be OK if your primary was a series of concentric circles, but it is in fact a spiral. Each hole in each primary support should be offset slightly to allow for the "pitch" of the spiral (each support's holes offset by primary pitch/number of supports). Each support needs to have a unique identification mark so you can position them in the correct sequence before you start installing the primary conductor.


Regards,
Herr Zapp