GU5B VTTC Flashover

[SalinsLV]

Hello!

I was experimenting with my newest build and while i was increasing power a flashover happened (i attached some photos in the attachments).
I am thinking of adding PVA glue (i currently can't get proper varnish because of the virus) on top of where the flashover happened, to isolate the burned area better. I also plan to decrease the number of turns on the feedback coil. Currently the feedback coil has 19 turns of 1mm wire and i have a tap point at 13 turn.

I also posted a video on how it happened:

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Before i turn on the coil is there anything i should check so that the flashover doesn't happen again?

[Mads Barnkob]

Looks like too high coupling between the secondary circuit and the feedback coil. Not so much the height of the coil but the diameters of the secondary coil, primary coil and feedback coil seems very close.

You could use a high voltage rated wire for the feedback coil instead of kinked varnished copper wire, it could be some cracks in the insulation that gives you problems. I had had problems with arcing between turns of my feedback coil in a dual 811A VTTC and I could only fix that with more spacing or another type of better insulated wire. The voltage per turn ratio on the feedback coil just seems too high for enameled copper wire.

[MRMILSTAR]

You could try rewinding the feedback coil onto a larger diameter plastic ring that will fit over your existing primary coil tube. This will put more distance between the feedback coil and the secondary coil. Done properly, it could also provide a means of raising and lowering the feedback coil to adjust the coupling. This is how I implemented the feedback coil on my 833C VTTC.

[SalinsLV]

Thanks for the replies!  

I will replace the feedback coil and report the results afterwards.

[SalinsLV]

I made a ring that increases the distance between the secondary and the feedback coil. Now i have no problems with flashovers. The sparks aren't that big. I'm using only one MOT at 90% power and the coil is partly tuned. I'm having a hard time with calculating the required capacitance for the tank circuit. That's why I'm experimenting with the capacitors that i have and looking for which makes the longest sparks at the lowest voltage.
Here are some specs if anyone is interested:

Secondary coil
Diameter: 13 cm
Lenght: 45cm
Wire diameter: 0,3mm
Turns: ~1500
Resistance: 126 ohm (measured with basic multimeter)
Secondary Fres: 197 406 Hz (with topload)

VTTC [primary coil]
diameter: 20cm
turns: 31
wire diameter: 0.1mm
height of primary: 7.1cm
inductance: 0,19mH +/- 0,18mH
resistance: 0.2 ohm (multimeter) but cheap LCR says 0,6ohm +/- 0,7ohm

Inductive reactance: 223.261 Ohms @ 197 406 Hz (secondary coil Fres)
Primary inductance: 0.18mH (measured with cheap LCR from ebay)
=
Currently used tank capacitor value: 3.6nF
Recommended capacitor: ?

[MRMILSTAR]

Do you have selectable tap points on your primary coil? Its hard to determine from your pictures. If not, then you need to add them in order to tune this coil.

You can use JAVATC to give you a good idea of the tap point with your capacitor.

[SalinsLV]

Currently i have no tap points. The way i tried to tune this coil was by changing the capacitor values to match with the primary coil inductance to oscillate at the secondary coil resonant frequency. The coil works and if i bring my hands closer, the sparks also get louder and bigger.

I am now upgrading the coil with another MOT and extra filter capacitors for higher power. I will also add tap points on the primary as previously suggested.

[SalinsLV]

 Should i also add tap points to the feedback coil?

[MRMILSTAR]

Some people do add tap points to the feedback coil. I didn't.

[SalinsLV]

The coil is working and it even plays music:

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I will design a new secondary coil because this one has a resonant frequency of 210KHz. At 3Kw the spark length is about 30cm.