Re: Starting a new (first) VTTC First light!
..., Sun Jun 04 2006, 04:28AM

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!

Re: Starting a new (first) VTTC First light!
EDY19, Sun Jun 04 2006, 10:45AM

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.

Re: Starting a new (first) VTTC First light!
Dr. Shark, Sun Jun 04 2006, 03:42PM

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.

Re: Starting a new (first) VTTC First light!
..., Sun Jun 04 2006, 04:21PM

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).

Re: Starting a new (first) VTTC First light!
EDY19, Sun Jun 04 2006, 05:22PM

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.

Re: Starting a new (first) VTTC First light!
Philcogrump, Mon Jun 05 2006, 12:29AM

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.

Re: Starting a new (first) VTTC First light!
EDY19, Mon Jun 05 2006, 12:42AM

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.

Re: Starting a new (first) VTTC First light!
EDY19, Tue Jun 06 2006, 02:27AM

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

Re: Starting a new (first) VTTC First light!
..., Tue Jun 06 2006, 04:15AM

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)

Re: Starting a new (first) VTTC First light!
Cesiumsponge, Wed Jun 07 2006, 12:47AM

Be careful and don't crank the power up too fast when first troubleshooting it. I was tuning mine and it had a breakout point, but it did not breakout and I had an arc leap from the feedback coil to the secondary and it scared the crap out of me.

I also concur on it having a dedicated breakout point. The secondary wire is just so thin, it has too little mass for thermal dissipation and I'm sure you could accidentally melt it even at low power levels and charring your secondary coilform is never fun.

You could simply take some solid 12awg wire, run it about 1" above the secondary coilform, bend it into a circular loop as field/gradient ring (or imagine it as a very anorexic looking minor diameter toroid), then bend it upwards into a sharp breakout point from the center.

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 07 2006, 03:22AM

Yes, I was planning on making a topload, I just hadn't gotten that far yet. But otherwise, power worthy ? I can but 3/4 inch flexible tubing from home depot, then cover that with aluminum tape and then add a breakout point for the toroid, and it should look nice. Updates to come!

Re: Starting a new (first) VTTC First light!
Cesiumsponge, Wed Jun 07 2006, 04:17AM

If you have some copper bus bar and flexible copper tubing handy, just solder some together. I got the idea from some ham's website as corona rings but I went an extra step and tinned mine so they won't tarnish. It's cheap and can be modified to fit any number of situations involving high voltage. The following image I attached should illustrate it pretty well.

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 07 2006, 07:59PM

Does the MOT core need to be grounded to the third prong ground along with the secondary coil?

Re: Starting a new (first) VTTC First light!
EDY19, Sun Jun 11 2006, 05:03PM

Heres an updated picture of the layout. The thing on the heatsink in the back is the grid leak resistor- made from 6 20Kohm resistors- 2 in series to increase the voltage rating, then 3 of these pairs in parallel for 13.3kohm. Each resistor is rated for 35W, and they are heatsinked, so I think this will work Suggestions welcome, I decided to use my home wound transformer for the filament, its heavy but if i want to i can add another tube to the coil, and add one more turn on the transformer to keep the voltage right around 6.3V, maybe a bit higher. One switch is for the filament, and one is for the MOT. And there is the secondary with the copper pipe fittings toroid and breakout.
EDIT: Also grounded prettymuch everything metal thats not part of the circuit including the filament transformer core, the on/off switches, and the MO caps metal body.

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 14 2006, 03:18PM

Ok, decided to make this a dual 811A coil, so I am making a few changes. First, the new secondary is 14 inches long. This should prevent flashovers to the nearby tubes Also, do you think that 4 inch PVC (4.25 o.d.) would be sufficent for the primary form? I was using three inch, but looking at other coils this seems way too small- even 4 inch may be to small. Im modeling this one after steve wards coil, and he uses 6 inch PVC- is this neccessary?

Re: Starting a new (first) VTTC First light!
..., Wed Jun 14 2006, 06:27PM

I found that with really tight coupling things didn't work (too tight of a feedback caused my tubes' grid to ark over) but I didn't really try too much... I use 6" for my single (or dual, I run it both ways) coil with a 2" by 5" secondary...

Re: Starting a new (first) VTTC First light!
EDY19, Thu Jun 15 2006, 02:02AM

I know "..." has had success with a doubled MOT supply and 2 tubes, but anyone else try this with success? I think I might go with just the MOT rectified and filtered unless there are HUGE improvements that can be made by level shifting the MOT.

Re: Starting a new (first) VTTC First light!
EDY19, Mon Jun 19 2006, 01:44AM

Ahh, awaiting some sockets and plate caps for first light, but everything is ready and waiting. I got my other tube yesterday in the mail, and everything is wired. The primary cap is almost directly under the primary coil, so there isn't much extra wire inbetween them. First light as soon as the other stuff arrives Suggestions welcome!

Re: Starting a new (first) VTTC First light!
EDY19, Tue Jun 20 2006, 08:27PM

First Light! It makes an aweful squealing noise, but it works! Sparks are about 4 inches long or so. Do all VTTCs make that horrible squeal? Im gonna try a couple things- removing filter capacitance (2uf on the output of the MOT) as well as lowering grid leak resistance. The feedback coil is the right polarity if I am getting these sparks, right? Other suggestions welcome!

Re: Starting a new (first) VTTC First light!
..., Tue Jun 20 2006, 09:50PM

it shouldn't squeal. It sounds like you are running off a rectified/filtered mot, which should make the output completly silent. If you run of unfiltered you should get a slight 120hz buzz

4" sparks probably mean the phasing is right, but try reversing it to be sure... They do spark when they are backwards

4" sparks sound about right though, I get 4" off my little coil running off hlafwave rectified 2100vdc witch a single tube, so if you are running cw...

Looks good!

Re: Starting a new (first) VTTC First light!
EDY19, Tue Jun 20 2006, 09:53PM

It arcs inside one tube now almost continuously after lowering grid leak resistance to 5.65K from 13k, but the horrible squealing is gone and the sparks look different. What happened? If the tube arcs internally, I'm guessing it is dead?

EDIT: I changed the resistor back to 13k, and one tube is dead but i changed the feedback leads and it acts a lot more like the VTTCs that I see here. It sounds a lot better too. The sparks are longer now on one tube than they were on two, but I killed a tube in the process, and it could be soo much better I don't know whether to buy another tube or to call it good with 4 inch sword-like sparks. I really dont want to waste money if it is going to arcover inside the tube again. would wrong feedback phasing kill a tube quickly? I also took off the filter capacitance so things should run cooler. I can't believe that it was working so well with the phasing off ..

Re: Starting a new (first) VTTC First light!
..., Tue Jun 20 2006, 11:21PM

Where was it arcing? Them 811a's seem to like to arc between the grid/filament connections on the bottom of the tube (outside of the vacuum) at the metal/glass seal. I lot a tube this way because it melted the grid connection clean off, so I was left with a diode If it was arcing inside the tube then you probably got a bad tube, a good 811a will take over 4kv on the plate (I have run a voltage doubled mot no problem).

Usually a coil that is tuned and working with a single tube will work just as well (but you can turn the voltage up higher with the second tube), if you parallel in a second. I would go for it, with a secondary that big you need some large sparks to make it look right

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 21 2006, 12:19AM

What type of tubes are you using? I am using Cetron JAN811A. Would taking off a couple of turns from the grid feedback help stop some of the arcing inside the tube? feedback to plate is usually what the arcing is, right?
Heres a picture of it working better:

Re: Starting a new (first) VTTC First light!
..., Wed Jun 21 2006, 03:43AM

I have used just about all of them

The jan's were the ones that I had arc over... But they are all just about the same

Taking turns off the feedback didn't really help, could you try moving the feedback arround on the coil a little? Perhahpse a smaller grid leak resistor (I use 4k)

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 21 2006, 03:57AM

using a smaller grid leak resistor is what killed it, but that could be because the phasing was off on the feedback, i guess. Moving the feedback up made little pinkish purple "speckles" on the plate, so i moved it back down, and they all dissapeared. I also removed the filter capacitance and the diode.

Re: Starting a new (first) VTTC First light!
Steve Ward, Wed Jun 21 2006, 04:36AM

You may have said this already, but were these new tubes or used? IF you saw *any* purple/pink discharge inside the tube, then its gassy and would also easily arc over inside. I wouldnt be surprised it you bought used tubes to find them in this condition.

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 21 2006, 02:26PM

They are new, bought from k5svc (military/gov't surplus) on ebay for 17.50 each. After one died, I had to give it an all out death, so i put it on my 6kv NST, and that would make a really cool nightlight! The whole thing glws purple I put the anode cap on the wrog direction the first time, and when i pulled it off, the metal part of the cap came off two, I glued it back on, but this was the tube that died, so i bet it broke the seal on the tube just a *little* bit when it came off.

Re: Starting a new (first) VTTC First light!
Dr. Drone, Wed Jun 21 2006, 03:36PM

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 21 2006, 03:53PM

Already bought another one, but if these don't work, thats an option. How important are matched pairs for TC duty? I didn't think it was too important, but i may be mistaken. Mine look like the marshall ones, but maybe i should try the taylors instead next time.

Re: Starting a new (first) VTTC First light!
Steve Ward, Wed Jun 21 2006, 05:30PM

Just FYI, i used the the taylor tubes from RF parts. They seemed to work just fine for me. I "accidentally" bought a matched pair, but when i called and specified that i only wanted 2 individual non-matched tubes, they refunded me the extra cost (and its true, i didnt want matched). I think for TC use, as long as both tubes are "good", they will be close enough to work fine. With my 833A coil, one of the tubes was severly low in emmissions, so the other tube was doing almost all of the work. If i used just 1 tube at a time, the good tube would throw a 20" spark, but the bad one only 10" . I dont think you will have that kind of problem though!

Re: Starting a new (first) VTTC First light!
Dr. Drone, Wed Jun 21 2006, 05:42PM

Re: Starting a new (first) VTTC First light!
EDY19, Wed Jun 21 2006, 06:00PM

What do you mean by baking them?

Re: Starting a new (first) VTTC First light!
Dr. Drone, Wed Jun 21 2006, 08:09PM

Re: Starting a new (first) VTTC First light!
EDY19, Sat Jul 01 2006, 08:26PM

Alright, slight troubles, the MOT died on me- nothing else seemed hot, and I have no idea what the priblems are. I have a 8kv .005uf RF bypass capacitor on the transformer, and one on the filaments of the tube, too. Whats the problem??

Edit: just saw that the Bypass capacitor may have been a cold solder joint and may have fallen off- bet this was the cause, as the transformer was barely warm when it happened, the insulation had just burnt off and melted some tape.

Re: Starting a new (first) VTTC First light!
Marko, Sat Jul 01 2006, 09:16PM

I didn't fully understand your post, can you give some more details?
Looks like you fried a cap somewhere or simply have a bad conact.

Re: Starting a new (first) VTTC First light!
EDY19, Sat Jul 01 2006, 09:17PM

You posted right while i was editing the post

Re: Starting a new (first) VTTC First light!
EDY19, Mon Jul 10 2006, 06:21PM

The new tube has arrived today, and right now I am in the process of burning them in- heating the filaments for a while- suggested by christopher to reduce any traces of air in the tube. "Second Light" soon to come!

EDIT:
Looks good, about 4 inch sparks first light, and then when the feedback coil was moved up on the form, they grew to about 5 inches. I could have pushed it further up, however the wires right at the base of the vaccum in the tube sparked a couple times a second. Would anything simple help prevent this so i can move the feedback coil higher?