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Need tips for a VTTC.

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LAN_403

Killa-X

Sat Jul 21 2012, 07:51PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

Okay, I own a GU-81M tube. my VTTC, i need help on. Never made one. SSTC, SGTC, DRSSTC, easy.

So. I used this as my schematic

Using the same rated capacitors and all.

Specifications:

* Primary
- 34 turns of 18awg stranded
- 4.5" diameter PVC pipe
- climbs up about 4 inches

* secondary
- 3.5" x 12" PVC pipe
- 493.83 turns of 24AWG heavy insulation
- 950Khz resonance

Now, earlier it was a 200khz coil and .. thats kinda low for a VTTC. So, i went to 900. and the change (left all the same) was 3" sparks to 1" sparks. and both times i was drawing 10A off mains, to the mot, to get 1" or 3" sparks.

Clearly my system is crap.

Suggestions? Also, with the primary windings, instead of climbing up 4 inches to get many turns, would it be fine to overlap?

Thanks...If you need more info, please ask.

Dr. Dark Current

Sat Jul 21 2012, 09:01PM

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

Please post some pictures of your coil. Do you use a topload? With the specs you gave, the unloaded secondary resonance comes to ~630 kHz, without a topload. Always calculate resonances with the primary in place, the resonant frequency can drop a lot.
Now your primary should resonate somewhere around 450-500 kHz. Fine tuning is absolutely required, often 1 turn changes the performance noticably. Few turns off and the output can change by a magnitude. This means a primary with taps on every turn, taps on half of the winding suffice (or even less if you have some experience).
The calculator says your primary resonates at around 510 kHz, so you might need to add some turns.

Your coupling seems too high, for VTTCs I usually select the secondary to have around half the diameter of the primary.

The "best" secondary unloaded resonance for me seemed to be around 450 kHz, but a VTTC can perform well in a really wide range of frequencies when tuned properly.

What type is your tank capacitor? It should be special RF ceramic, special RF mica or (if nothing else available) a MMC of MKP caps with enough voltage headroom.

How many feedback turns? Do you have the phasing right?

I suggest to connect all 3 grids of the tube together.

-Jan

2Spoons

Sat Jul 21 2012, 09:03PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

looking at the numbers on your schematic it seems to me the grid components will have a resonant frequency close to the primary resonance, but not the same. I'd try putting a lot more C on the feedback - maybe take it up to 10nF.

Killa-X

Sat Jul 21 2012, 09:13PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I got the 900khz off javatc. 90% thats never wrong but i guess according to you - it is.

So if cthe secondary needs to be around half the primary diameter, i guess i need to use 6" dia primary, as the secondary isnt changing

See the schematic for my tank caapcitor

25 feedback turns, thats not the issue, as I said im getting 1-3" sparks.

Yes phasing is correct, it shoots sparks.

Only got 2 grids together but i can connect all 3...

Either way i need to wind a new primary. i wasnt sure how much to do, since i guess when i did the math for resonance, i was told WAY too low VTTCs need like 40 turns, yet if it was a DRSSTC, my few turns were resonance.

Dr. Dark Current

Sat Jul 21 2012, 09:46PM

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

If it shoots sparks, the phasing is not necessarily correct.

The coil diameters are just a suggestion, you should be able to tune it like you have it.

When you were using Javatc to calculate the resonance, did you input real primary parameters as well? Intercoil capacitance decreases the resonance.

Edit: Just tried without the primary and got 930 kHz, but this is not the frequency you're looking for... Your coils are very close to each other, so a relatively large capacitance is between them. This has the same effect like adding a topload. I suggest you to re-calculate the resonance while inputting real primary parameters and position, I assumed the first turns are at the same height.

The schematic doesn't state the capacitor type.

Killa-X

Sun Jul 22 2012, 01:03AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

The capacitor type is ceramic. 15KV. 1nF. Single capacitor.

Is it fine that I go after this primary, or should i upgrade it. I do have larger (6.5") pipe if need be but that just seems way over. And at this time, my feedback is about half up.

Is there a typical average turn for feedback? As long as it fires...it works? I'm thinking of wraping 25 turns on a slightly larger diameter, then wrapping the wire so its all together and slides up/down easy for adjustment. The feedback i had, was just coiled around. not neatly like the secondary or primary.

And yes, i tried secondary 1" above, and equal to first turn. equal spit out the best.

Dr. Dark Current

Sun Jul 22 2012, 09:12AM

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

Is your capacitor the small "blue disc" type? If so, then this is absolutely unusable. You need a special capacitor or a MMC of MKP caps.

The feed back turns do matter of course. Too few turns and the tube will not turn on enough, too much turns and your conduction angle goes way up, with huge power draw and little increase in spark length. The feedback can be tuned with the grid resistor as well, but there will still be an optimal value of feedback turns.
Raising or lowering the feedback has a similar effect like changing turns, but I like to run my feedback as close to the primary as possible (because of coupling).

The first thing I would do in your place would be to get a proper RF resonant capacitor. Then you can try tuning it. The close coil spacing will become a problem when you get arc overs between the coils, then you will need to redo your primary or secondary for more spacing.

PhilGood

Sun Jul 22 2012, 09:28AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

There is an easy tip to make slidable windings : tight-roll a transparent plastic sheet on your primary form, then wind your wire on it. Glue together with transparent tape enough plastic sheets to have 3 layers when you roll them on your primary form. Use cyanocrylate glue to secure first and last turn, then apply one coat of polyurethane varnish. If the coil is too hard to slide, remove some of the plastic sheet from the inside (1 or 2cm at a time). Do this for both Primary and Feedback coils. Enjoy easy adjustments :) You can even try to put your feedback coil under your primary.

Killa-X

Sun Jul 22 2012, 07:39PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Thanks, will take that suggestion of easy-adjustable windings to design.

As far as the cap goes, its an orange, 8mm thick capacitor. ceramic.

I dont own any RF caps / doorknob caps. Considering the fact i have a 3.5" x 12" tube, and possible a 4.5" primary, is there a typical rule-of-thumb for what size cap to go at? Likely the 15-30+KV range, but right now i run 1nF, is this far too low?

Is it okay that i use them on the DC poewr from the MOT? The one that goes across the capacitor?

Thats the capacitor on my diode / primary.
Actual Picture:

Dr. Dark Current

Sun Jul 22 2012, 08:06PM

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

The cap you have is the wrong type, and it will not work well... I'm pretty sure Z5U is a very high permitivity dielectric with very high dissipation factor... I just checked and it is 4%, ouch! For example, polypropylene has a dissipation factor of 0.0005.
Edit: I just checked and your cap is Y5U sorry... But it will really be similar.

As for the capacitance, 1nF is all right, I used 6kV rated RF ceramic caps and they held well. If you build a MMC, I would go for ~20kV DC rating.

The standard ceramic caps should be OK for the rf bypass, but I'm not sure if the Z5U/Y5U type will work even here...

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