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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
I've tried Polypropylene Film Capacitors but they are made for DC first of all, they are far from optimum. Then I've tried Type 940C through 943C Power Film Capacitors & realized that you get what you pay for. I got the Polypropylene's for free and I used quite a few but they just aren't what is the appropriate item. I have seen them function but it's FAR from the CDE 943 series caps; like night and day. The problem is they (caps) have price ranges that are quite wide because if you have a business license you will buy them from a mfg, jack the price up several bucks and re-sell. Expect to pay about 2-3 dollars depending upon the size and specs. BUT you won't have to buy them again.
The other issue is that since your caps (that you have) still function....SELL them to someone who could use them (don't try to get every penny back) just get some money from them. Even if you get 1/2 of what you paid it's still a dent in the correct thing. My personal feeling is; don't ruin them, sell them especially as a group - just like you have them now (they look nice; you did a good job) be HONEST and MAYBE someone will have just what you need and help you out. IF you ruin them; you've wasted money. IF they are blown; that's that. ALWAYS be HONEST. You'd be surprised what happens to folks who are direct, open and honest.
Here is a nice little chart for 60Hz. It will help you save some money and buy just what you need. BUY the 943 series for your MMC; it will last a VERY long time. ]mmc_cap_chart2.pdf[/file]
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
MMM-
Your DC-rated caps are far from ideal for use in a spark-gap TC, but they're NOT the root cause of your wimpy 1" discharge.
Take your existing single-coil setup, and simplify it per the following instructions. As soon as you get a "basic" configuration tuned and generating decent arcs, you can incrementally add a second NST, larger toploads, open up the spark gap, and finally assemble it as a full bipolar-twin.
1. Disconnect one NST; start with only one. This will eliminate any potential problems with phasing, intermittently shorted secondaries, etc.
2. Remove the stacked 12" and 8" toploads; install a single 8" toroid, and make sure that you have electrical continuity from the toroid surface to the bottom winding of your secondary coil and through to your earth ground. Add a 2" long piece of bare wire sticking out horizontally from the toroid as a breakout point to aid initial tuning.
3. Reconfigure your MMC (disconnect strings) to yield a value of around .012uF. MEASURE the value if at all possible. Inspect ALL the caps in your MMC for signs of surface flashover, or capacitor-to-capacitor arcing. These caps have minimal external insulation, and will arc through the external film insulation very easily. They are NOT insulated well enough to lie in contact with each other.
4. Short two or three of your spark gap segments.
5. Keep your initial tap point around 6.5 turns.
6. Make sure ALL your tank circuit wiring is as short and direct as possible, and that ALL the connections are securely bolted or preferably soldered using at least 12 AWG wire. No "twisted wire" connections.
7. Make sure that your primary "tap" is making a solid, low-resistance (e.g. high clamping pressure) contact with your primary tubing, and that the tubing is clean and bright.
8. Make sure that your spark gap arcing surfaces are clean and bright. Run #320 or #400 grit silicon carbide sandpaper through each gap to remove any surface oxide.
9. Power up the system, and start moving yourprimary tap in 1/8-turn steps, buth "inward" and "outward". You should quickly which direction is yielding increased performance.
One other question: Are you using bleeder resistors anywhere in your MMC? Have you verified their value, and that there is no flashover taking place across the surface of the resistors?
Make the above modifications, and let us know the results.
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Did I waste money on these caps?
