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Starting a new (first) VTTC First light!

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EDY19

Sun Jun 04 2006, 01:01AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I have some stuff to make a medium sized VTTC, but I decided to start small-which may be harder in my case, but oh well. I'm going to use a small triode tube- not sure what yet, and this is the coil and primary. Any suggestions so far? I want it to make a tiny spark, most likely smaller than .75 inches. I just want the whole thing to be really compact, just a brushy little discharge will do. Any suggestions on what type of tube to use? Also just got 8 pounds of 28AWG wire from essex, so a new big(ish) coil will be on its way soon- not sure waht topology yet though.

...

Sun Jun 04 2006, 04:28AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Don't worry about the size, vttc's are not like sstc's in the sense that you have to worry about the fets switching too slow, tubes will oscillate at just about whatever you want them to

That coil could put out 5" sparks with a 811a and a mot powering it...

I do worry about your primary... You do realise that with a vttc you need the primary and a feedback coil...

I would recommend that you dump the copper wire and use pvc insulated hookup wire (preferably double insulated)... For a single mot coil you could get away with 20awg for the primary/feedback if you want them to look the same, ideally I would use a 16wag primary and a 22awg feedback. (but just about anything will work). For a small one I would use 20awg for the primary and 22 for the feedback. The feedback just needs to be big enough that you have a good amount of insulation on it, the primay big enought that it can handle the rms currents in the tank circuit (which are pretty small with the small tank caps we use) You are probably going to need about 20 turns for each one... I would recommend that you rip out the existing copper, and if you have it use a piece of pvc pipe where it was. If you don't, just wrap a few layers of overhead transparencies over the existing supports and wind over that. See my coil if you don't understand what I am talking about...

If you are only looking for small sparks I have a few 809 triodes that I could sell you pretty cheap. One of those should give 1" sparks no problem, they gave 4" for about 30s before they overheat and start acting as diodes...

Good luck!

EDY19

Sun Jun 04 2006, 10:45AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

The reason I made the primary like that was so I could tune it easier- I knew i had to have a feedback too, but I wasn't quite sure where to put it yet

How did you go about tuning your coil with the PVC insulated wire? Also, what is the effect on the circuit if you have too many turns or too few on the feedback coil? Is it just lesser performance? I think I might wind a coil a little bigger than this one for my 811A coil, just so I can adjust things easier- maybe a 3 inch form 9 inches tall or so with 28AWG wire.

Dr. Shark

Sun Jun 04 2006, 03:42PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I have never build a VTTC, so take this with a bit of caution: For the compactness you are looking for, would it not be perfect to run the thing without a HV transformer, just from voltage-doubled mains? This way you could probably find suitable tubes much easier. You can easily get tubes good for 30MHz or so, so you dont have to worry about small physical size and corresponding higher frequency.
Heck, with all the hype about tiny SS and DRSS coils lately, I wonder why nobody had a go on a tiny VTTC yet.

...

Sun Jun 04 2006, 04:21PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Hmm, doubled mains... It would pe possible but I thtink you would have a hard time getting any sparks out of it...

I tuned my pvc primary 2 ways... The first rought test was done with a piece of wire that I had one big loop in and then put turns on the primary with until I got an idea of where it runs, then I built a tapable primary by taking the insulation off of the wire every turn (well actully about every 1.1 turns so the taaps were ofset) to do my fine tuning easily. Then once I was happy with the tuning I made my final primary, checked the tuning with +/- 1 turn to make sure it was still tuned, and then glued it on. The tuning is very important, as vttc's are not self tuning... You tune them by changing the pri cap/primary as you would do in a normal tc. Mine had a sweet spot about 2 turns, if you were more than 5 away from that there was very little output. The feedback isn't so critical, although it couldn't hurt to tune it. Also try messing with the posisition of the feedback, it does have an effect on the output. (with your really spaced out primary I think you may have problems with the feedback being in a non optimal position).

EDY19

Sun Jun 04 2006, 05:22PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Doubled mains is what i was going to do with that small coil- I was just hoping for around a half inch of output. But well see, I may move on to the bigger coil, then try a mini once more.

Philcogrump

Mon Jun 05 2006, 12:29AM

Registered Member #212 Joined: Sun Feb 19 2006, 05:42PM
Location: Texas
Posts: 20

Doubled mains could work, I built my first VTTC using a 6146 tube with a B+ (high voltage) supply of 300V from a transformer and a 5Y3 rectifier. That VTTC only gave out about 0.25" of sparks. With such a low HV supply it is very difficult to start oscillations so you'd need to have the coil tuned very well just to get some oscillations. Higher voltages like 1kV or more makes it easier for the oscillations to start.

Your coil in the picture looks almost the same size as my 6146 VTTC so it could work on doubled mains, but the current draw might require that you have larger capacitors in the voltage doubler, i.e. 200uF or so. Doesn't hurt to try.

EDY19

Mon Jun 05 2006, 12:42AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

That "Mini" is for later, I just wound a 2.5 inch by 9.25 inch coil with some new 28AWG GP-MR200 from superior essex. It looks great, just have to top it off with some epoxy or polyurethane. The circuit is almost ready to go- I have 880pf of primary capacitance, a ~2000V MOT, two MO caps/diodes for filtering (not going to double the MOT voltage), a 8kv .005uf bypass capacitor across the two MO caps, a 811A tube, a large 13k ohm resistor for the grid leak, and im still looking through my parts for a ~.001uf capacitor for the grid leak cap.

EDY19

Tue Jun 06 2006, 02:27AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Alright, making progress. Here is the primary, secondary, and feedback coils- the feedback can slide up and down. More to come... I think this one looks a lot better than the other one

...

Tue Jun 06 2006, 04:15AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

looks good!

I will add one thing... Make sure you have a sturdy topload before you fire it up for the firt time. On my mini I just left eh wire hanging out which worked ok until I got 1/2" of spark out and it melted down onto my pvc form. So I threw a bolt on top and once I got to 2" sparks th corona off the top carbonised the pvc and I started to get streamers off the top winding of the coil

So I dropped a washer with a sharpened bolt on the top and now it hasn't had any problems. For the start I will recommend that you put a few inches of small awg wire off the top (it has a hard time breaking out in the beginning)

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