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Wyatt

Wed Dec 29 2010, 10:24PM

Registered Member #3490 Joined: Wed Dec 08 2010, 11:55PM
Location: The Granite State
Posts: 34

Arcstarter wrote ...

I know that parallel tubes definately allow more average current. But, with a given plate voltage, the spark length does not grow. Well, it may grow some, but not 2x like you may expect.

I used such tiny wire on my primary with that 811a, I cannot believe how well it worked. I guess that is mostly because even though the wire is thin, the diameter is small enough that the overall resistance is low.

Just out of curiosity,do you know off hand what size wire you used on your primary? Out of cheapness I wound mine with something in the realm of 16 or 18 awg and had pangs of guilt when finished.:-)

Dr. Dark Current

Wed Dec 29 2010, 10:36PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

It does not grow, but you can tune the primary for bigger sparks (less impedance) than with one tube

Arcstarter

Wed Dec 29 2010, 11:01PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Dr. Kilovolt wrote ...

It does not grow, but you can tune the primary for bigger sparks (less impedance) than with one tube

That is very true, i have never done that. I think it is safe to say that the 102 degree fever made me forget

.

@Wyatt, i used 20 gauge

. It got hot, but didn't even melt the transparency 'coil form'. Man, it was so cheap and weak, and yet still got better sparks than any other setup i have used.

Dr. Drone

Thu Dec 30 2010, 01:05AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Herr Zapp

Thu Dec 30 2010, 10:14PM

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

U4R1A -

(Posted this yesterday afternoon, but it never appeared, so here it is again)

1. Your ceramic tank caps are of the type intended for DC, and are probably quite lossy carrying RF. Lowest losses will be with a mica transmitting cap, or even with a "mica MMC", using an array of the small 2" X 2" X 1" trapezoidal molded mica caps.
2. Wind your feedback coil on a separate coilform from the primary, and provide some means for adjusting the height relative to the primary.
3. Shorten the wiring between the tank capacitor, the primary coil and the tube. It looks like you may have several feet of unnecessary wire length between these components.

Herr Zapp

U4R1A

Thu Dec 30 2010, 10:56PM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

@ Herr I've been slowly rewiring/reducing the length of all of my connections. One question I do have is why I'd need to wind my tickler coil on a separate coil form then my primary? The one I currently have wound in the vid is on reversed masking tape so it slides up and down easily. would some caps like this -->CDM1000pf 1000v silver mica make for a decent mica MMC??

Herr Zapp

Fri Dec 31 2010, 12:15AM

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

U4R1A -

If your feedback coil is adjustable height-wise relative to your primary, you are OK. That feature was not apparent from your vid.

I'm not sure what kind of mica capacitor you are looking at. If it's the small "dipped epoxy" parts, they are not suitable as they are not designed to handle any significant current.

I was talking about the "intermediate-sized" molded mica caps widely used in WWII and Korean war vintage military equipment. These are typically rated at 2,500 or 3,000 working volts and have threaded female or tubular brass terminals, not wire leads. These are readily available at Ham radio swap meets, etc. I've bought scores of these over the years, and have never paid more than $1 each.

Herr Zapp

1293754508 480 FT104728 Molded Mica Caps

U4R1A

Fri Dec 31 2010, 01:08AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

Thanks a ton. I need to hunt around for a ham radio swap meet because ebay is a little more then a buck. Should I make the top end of the MMC 2-3 times the supply voltage like the MMC's for SGTC's?

Herr Zapp

Fri Dec 31 2010, 01:33AM

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

U4R1A -

Mica capacitors designed for RF service have been made in many different styles and packages. If you find a good Ham swap meet, you'll likely see a good variety if you keep your eyes open. There's nothing special about the molded micas I mentioned, it's just that I see them all the time at the Ham swap meets I attend. Any RF-rated mica cap will work if it will handle the tank current of your coil.

Take a look at Surplus Sales of Nebraska for examples of different types of RF-rated mica capacitors. SSN's pricing is outrageous, but it does serve as a kind of encyclopedia showing different styles and packages of mica caps.

See the molded mica's here:

and the ceramic-cased mica's here:

If you can find them cheap, the ceramic-cased micas are probably the best for VTTC use.

Herr Zapp

U4R1A

Fri Dec 31 2010, 01:55AM

Registered Member #3505 Joined: Sun Dec 12 2010, 06:03AM
Location: Albuquerque NM
Posts: 108

i use SSN for Litz wire and that's about where I stop. I wish those guys would get the clue that surplus is all about volume and having crazy prices doesn't do much for moving a product. Thanks a ton for taking the time to upgrade my knowledge level. I've seen a few of the ceramic-cased micas on ebay for decent prices. The last question would be how high should I go on the voltage rating?

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