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Marko

Thu Nov 17 2011, 04:40PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

teravolt wrote ...

Marko, I have seen pictures of thease caps as tank caps in rf tube amplifier equipment. they may not be as saperior as mica but I think thay will do at least for me. have you tried using them before.

Yes,I've tried, and failed miserably, they are actually the same as those blue ceramic HV caps... tried running a parallel tank circuit with them at huge disappointment, even with no load the Q was so bad it couldn't get to oscillate. Apart from high losses those are also very temperature variant and also display high piezoelectricity.

How well would they be for a tube coil, well, after all I've seen people using glass bottle caps but what you're really after are RF rated strontium titanate caps (which you'll find to have usually more conservative voltage rating than KVI's) or just go for a MMC of polypropylene caps.

Marko

teravolt

Fri Nov 18 2011, 04:12AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

hmm ok so your saying only poly's and mica's are good for MMC? I would prefer a nice mika for a QCW that I am working on around 500khz and I could aford something in the 100$ right now if you had any suggestions I am open. I have a cuple dozen of the russian doorknobs that Mads desribes Above and for one of my tests I plan to mount about 15-20 of those caps to a cuple of aluiminuim plates for a mmc.

Goodchild

Fri Nov 18 2011, 07:01AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

I would have to say MICA is by far the best choice for the QCW.

In my QCW I have tried poly,mica, and teflon. I will give a little insight on the three from my experiments.

The Teflon -> They work and they work HOT!!! They suck plain and simple... (but they do work... sorta...)

Poly -> Although they are great for high current pulsed applications like a regular DR they tend to get hot in the high RMS environment of the QCW. If you made an MMC you would need many caps in parallel for a good RMS rating. The plus side though is that because the peak current is low the voltage rating can also be very low, probably only two or three 2kv caps in series for the most applications.

MICA -> There awesome, most mica caps can have great RMS ratings and already have a nice high voltage rating. I use a single 10nF 10Kv MICA rated at 18Arms in my QCW and that thing don't break a sweat at 20pps 100Apk in the tank. I would highly suggest MICA for the QCW, that's if you can find them! hehe

teravolt

Fri Nov 18 2011, 03:10PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

Goodchild I definatly agree, it sound like there aren't many substitutes. I have a bunch of those big old shool mica transmiting caps on hand and some small ones to for experimental purpace but they are not very portable. I could buy a new mica right now but there are others like Grenadier hear who might have to save a bit. unfortunatly a QCW is not a pore mans game. I am building the SLR right now like Phillip Slawinski but with a cuple of tweaks of my own. With the stuff on hand I have spent 150 just for the water cooling. like Grenadier I am lerning what to do next. so far I have one yay and one nay for russian doorknobs if you include Mads Barnkob. have you had any expirence with them? the reson I picked them is because of there low series inductance. According to Marko they have to much series resistance and get hot. I don't know if his application is the same as mine so I hope to try it and report.

Marko

Fri Nov 18 2011, 03:39PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

Well I think russian doorknob caps are quite straight actually - they mostly look the same with the same green coating on them, but if they don't have a kVAR rating listed on them (such as the KVI3) they are the sucky type, while for example K15Y-1 look almost the same but have kVA rating on them.
There are also those tubular ceramic caps which have more surface area for cooling and better volume efficiency.

Still you will need quite a dozen of those to get the required capacitance as the biggest are 4.7nF or so.

It's somewhat bizzare that teflon caps end up being more lossy than mica counterparts, with mica being so much crappier dielectric than teflon. The teflon caps also seem very robustly constructed with whole aluminum foil electrodes.

On the other hand I've pushed 5 parallel strings of CDE caps to some 60-70A in CW and they didn't see even a tiny bit of heating (while the very wiring leading to them was actually getting hot!)

150nF CDE's would be too high capacitance for the purpose though; you would need some in 10-20nF range (group buy anyone?) to get enough parallel stacks.

Marko

Dr. Dark Current

Fri Nov 18 2011, 03:45PM

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

A question for you guys: When the coil is supplied from a half wave recified unfiltered mains without any modulator circuits (such as in my case), is there any "sweet spot" for the running frequency at which I get the longest sparks for a given input power? I always thought that lower frequency = longer sparks, because lower frequency sparks are not as hot so they might grow longer to lenght. Or is this not true?

Edit: This question should have went into my "CWDRSSTC" thread, but I think here it is ok too

Goodchild

Fri Nov 18 2011, 04:39PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Dr. Kilovolt wrote ...

A question for you guys: When the coil is supplied from a half wave recified unfiltered mains without any modulator circuits (such as in my case), is there any "sweet spot" for the running frequency at which I get the longest sparks for a given input power? I always thought that lower frequency = longer sparks, because lower frequency sparks are not as hot so they might grow longer to lenght. Or is this not true?

Edit: This question should have went into my "CWDRSSTC" thread, but I think here it is ok too

Well as I said before 300KHz+ is the best for making supper long strait sparks. Also if you could sync your interrupter to the mains voltage so the coil turns on right with the half wave pules it would operate in much similar way to the QCW using the half wave pulses as the bus modulation. This is very similar to how Steve Ward first experimented with the QCW.

Teravolt, I have not had a chance to work with the doorknob caps in my QCW so not sure if I can give any insight on them.

teravolt

Fri Nov 18 2011, 05:07PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I think you asked about this before but I have seen teslas that use rectified AC that have sword sparks and weren't VTTC. from what I have seen it is a combination of things like a ramped type of wave and secondary frequency. I think that VTTCs use half of a ac sign wave wich is 8.3mS to build sparks where all this started. tubes and small fets are ideal because of there frequency range. I believe that sword sparks just look longer because they are sreight. the sweat spot for frequeny has been stated to be 350k to 500k. I wonder what the sparks look like above that.

Dr. Dark Current

Fri Nov 18 2011, 05:26PM

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

Do you think the optimal frequency depends on input (peak?) power and spark length? Common sense tells me that the optimal frequency would be lower for longer sparks with more input power, or am I wrong here?

Herr Zapp

Sat Nov 19 2011, 06:24AM

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

Goodchild -

You mentioned that your QCW coil uses a single 10nF, 10kV mica capacitor. In the world of mica transmitting capacitors, .01uF at 10kV is a rather unusual rating, hinting at a physically very large capacitor.

Is this a G-series capacitor, with a cylindrical ceramic housing and cast aluminum end plates? Or a cylindrical CDE cap with cast-epoxy case? Or something else?

Can you post a photo of your cap?

Herr Zapp

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