VTTC Arcs From Feedback Coil

[MrScrOggs]

Hello everyone, I finished my dual 811a vttc and it works pretty well from what I can tell. I have an issue though. The streamers sometimes come from the the tap on the feedback coil. How to I stop this?

[MRMILSTAR]

I experienced the same problem on my 833C VTTC. This is a common problem when the grid feedback coil is located on top of the primary coil. There are two possible solutions. It looks as if the sparks are coming from a relatively sharp object. You can try covering that point with RTV or something equivalent. The second solution is what I did. I made a spark shield. This was made from acrylic and it sits on top of the primary coil form. It provides extra insulation between the secondary coil and the grid feedback coil. The spark shield consists of the acrylic ring and a 4inch section of PVC tubing. The PVC tubing section is glued to the inner ring of the acrylic. This provides shielding below the acrylic ring that extends downward about 4 inches. In the attached picture of my 833C VTTC the spark shield can be seen sitting on top of the primary coil form. You can't see it but the 4 inch section of PVC tubing extends below the acrylic ring.

Looking at your coil you may want to extend the PVC tube of the spark shield down below the level of your grid feedback coil to provide extra insulation.

[Mads Barnkob]

Too high coupling between coils, so either you need more insulation or lower the coupling.

You might need to redo your feedback bleeder circuit to accommodate for a lower amplitude feedback signal or add more turns to a feedback coil located further away.

I also had problems with normal copper magnet wire flashing over due to corona breakout damages, I fixed it by re-winding with distance between turns and keeping it as round as possible.

My VTTC: https://kaizerpowerelectronics.dk/tesla-coils/kaizer-vttc-i/

[SalinsLV]

I solved that issue by creating a ring that acts as a spacer for the feedback coil:



Previous replies already stated the solution, however i still wanted to include these photos as I had the same problem before and it was fixed.

[Mads Barnkob]

Hello everyone, I finished my dual 811a vttc and it works pretty well from what I can tell. I have an issue though. The streamers sometimes come from the the tap on the feedback coil. How to I stop this?

I see that you have another problem, one tube is red plating and the other is not. Your tubes are not matched good enough to operate in parallel. The red palting tube is pulling way more current than the other, this will also hinder spark output as you can not go to full power without burning the plate. I solved this by tapping the feedback coil, so one tube got 1 turn less than the other, that balanced out very well for me. Its harder to get right by changing the feedback bleeder circuits.

I solved that issue by creating a ring that acts as a spacer for the feedback coil:



Previous replies already stated the solution, however i still wanted to include these photos as I had the same problem before and it was fixed.

Thanks for posting your solution on how to lower coupling, as you cleverly shows it can be done by widening the coil instead of moving it!

[MRMILSTAR]

Congratulations on your solution! Another advantage of the ring is that you can easily slide it up and down to adjust the coupling if needed. My grid feedback coil is also wound on a ring of PVC which can be slid up and down.

[MrScrOggs]

I don't understand how one is red plating, I bought identical tubes from the same place. Imperfections I guess.

[johnnyzoo]

I don't understand how one is red plating, I bought identical tubes from the same place. Imperfections I guess.

I'm not a tube expert, but:

I think tubes can have significant differences in their characteristics due to the nature of the manufacturing process. Even more so than semiconductors, although this also applies to semiconductors, to an extent.

Unless you buy a matched set of tubes - tubes that have been specifically selected from a larger batch based on their measured properties - you may have to balance them yourself. Matched sets typically cost more than individual tubes, and the more tubes you want to include in such a set, the more you have to pay for each tube.

[MrScrOggs]

I don't understand how one is red plating, I bought identical tubes from the same place. Imperfections I guess.

I'm not a tube expert, but:

I think tubes can have significant differences in their characteristics due to the nature of the manufacturing process. Even more so than semiconductors, although this also applies to semiconductors, to an extent.

Unless you buy a matched set of tubes - tubes that have been specifically selected from a larger batch based on their measured properties - you may have to balance them yourself. Matched sets typically cost more than individual tubes, and the more tubes you want to include in such a set, the more you have to pay for each tube.

Honestly I think this is my first AND last vacuum tube coil that uses two vacuum tubes. Very frustrated with it.

[MRMILSTAR]

I would stick to just a single tube. There is much less trouble. Some time back I bought a matched set of 833C tubes although I only use one. They came with the measured parameters listed for each tube to guarantee that they are a matched pair. Many tubes are sold on E-Bay and other places as matched pairs but I bet that they are not. Its just two tubes of the same type.

[johnnyzoo]

With two tubes, a VTTC with a push-pull oscillator of some sort might be an interesting project too.