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Herr Zapp

Thu Sept 09 2010, 07:25AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Mr. duck -

Get rid of those 1M ohm bleeders resistors, and replace them with 10M ohm components.

".0146 ohm loading": what is this?? The target value for your tank capacitor with a 9kV, 30MA, static gap system is around .014 microfarads (.014uF). This is strictly the capacitance value, and has nothing to do with "ohm loading", chicken gizzards, or Vespa scooters.

Certainly paralleling 2 NSTs (with careful attention to proper phasing of the secondary outputs) will substantially increase your coil's performance. However, to extract maximum performance you'll need to roughly double the value of your tank capacitor.

The main issue with excessive gap spacing vs the voltage rating of your cap strings is capacitor failure due to overvoltage, not charging time. Your MMC has a DC voltage rating of over 25kV, moderately conservative for a peak supply voltage of ~13kV. I wouldn't adjust the spark gap to exceed JAVATC's 99% of max voltage value.

Herr Zapp

modularduck

Fri Sept 10 2010, 12:17AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

Herr, Ohm loading is a term we use in AV circle for distributed audio and amplifier loads, i totally misused it, i sometimes say what i think as opposed to what i mean in the excitement of getting an idea out. I did however order enough caps to increase the size of my MMC to eliminate the resonance issue pointed out by you and quicksilver. too, the 10M resistors to change those out.

I have figured out a good way to redesign the spark gap with just the tools i have available so this weekend i will get that all fixed up and should be much more adjustable than my current one, with more electrodes and better cooling.

I was wondering after looking at different variacs, about solid state versions, much like a simple motor speed control, much like i have for my router, is this possible given its rated 15 amps and current draw from my NST is only 4.5 amps? I would think not as i would see more people using this less bulky alternative to lower power coil but can someone explain why?

Herr Zapp

Fri Sept 10 2010, 02:15AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Mr duck -

Although useful for controlling resistive loads or universal AC-DC motors, most of these inexpensive solid-state speed controllers provide a chopped DC or severely distorted sine wave output, not suitable for conventional iron-core transformers like NSTs.

A good 10A variable autotransformer (Variac, Powerstat, etc.) is well worth its cost ($20-40 at hamfests, electronic swapmeets, etc.) for ramping up input power to a spark-gap Tesla coil. Many variacs are "tapped" so they can wired to provide a 10-15% increase in output voltage, allowing you to increase the input voltage to your NST, increase your coil's streamer length, and increase the risk to your MMC and NST.

Herr Zapp

HV Enthusiast

Fri Sept 10 2010, 01:03PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

You can also get a good inexpensive variac on Ebay. Look for a 10A-20A variac. Variacs can generally be used for twice their rated current for short run times (duty cycle)

modularduck

Thu Sept 16 2010, 02:36PM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

All parts for upgrade to my MMC arrived yesterday and got the machining part of it done, now just need to solder it all together tonight.

In looking around town for variacs I stumbled into a surplus ham shop run by a great old guy who just happened to have a matched pair of 811A's he had just gotten in, I couldn't resist...So perhaps a VTTC my be in the not so distant future also. I did however find a variac on craigslist that i am going to pick up tonight!

modularduck

Fri Sept 17 2010, 04:26AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

I got the MMC finished up this evening and picked up my variac! much better perforance! its very cool to be able to control input voltage, still needs a bit of tuning, and I think the Toroid needs to be modified a bit but here are some pics. no worries my foster dog only hangs out for the rewiring phase. I can get near 15" fully cranked. I suspect once i get the new spark gap constructed, i should be able to hit 20"!

oddly enough my cell phone actually takes better pics in the dark.

[MOD EDIT] - PLEASE WATCH the double posting.

modularduck

Sun Sept 19 2010, 02:13AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

Finished the new SG today. got it installed, but will have make some changes to the layout inside the structure to make it pretty. Also as you can see in the pics i used metal angles as clamps to hold the fan on, it quickly became apparent that this was a bad plan. A quick fix latter and problem solved. I will admit that the changes i have done in the past week have made a huge improvement to the performance of this coil! I need to adjust the segments in the gap while i measured them out before installation i think they may have moved a bit during installation.

I do however now need to do something about my toroid, now even with a breakout point i get random streamers all around the toroid anywhere there is an irregularity in the surface. this is what got me thinking about my gap being set a bit to open, and over charging the MMC. I am hoping that once i get the toroid properly smoothed out, of can find a reasonably priced spun one, I should be able to get good distance from a single point.

modularduck

Mon Sept 20 2010, 12:31AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

I find it very interesting that one can adjust the BPS with the size of the electrodes used for the SG. I suppose that is why people use rotary gaps, i.e. for total control of variables. which brings me to my question.

is the a reason that one would want to use an A-synchro gap as opposed to a synch gap?

ScotchTapeLord

Mon Sept 20 2010, 12:50AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

In short, an asynch is preferable in DC-supplied coils and is acceptable in AC coils if it's fast, whereas synch is most reliable for AC coils.

modularduck

Mon Sept 20 2010, 01:07AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

Thanks Scotch! I am then right in my thinking that in a SSGTC (static gap) that you would want to adjust the size of your electrodes to reach as close to 120 bps as possible for maximum MMC discharge?

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