VTTC on GU-81M

[Continuum]

Hello! I have been working on this project slowly for about 2 years now, and its finally coming together! Here are some specs: Power supply is two 700W MOTs wired in parallel with a voltage doubler(6uf cap). Its filtered with 2 russian 2.2nf caps connected in parallel, I honestly doubt they really do help in any way, but safety and common sence required me to put them in. The tube has a little filter on the anode, 22 ohms 15W wire wound resistor and about 23 turns of 1mm diameter magnet wire. The resonant cap is two 20kv 2nf caps connected in series, and the primary is wound with 2.5mm^2 wire, 39 turns with taps from the 24th turn upwards. The feedback coil is wound with 1mm diameter wire, 17 turns total with taps from the 12th turn upwards. Currently connected on the 15th turn. With the secondary I went a bit over board I think, its 2000 turns of 0.2mm magnet wire on 10cm diameter pvc pipe. It also has a little topload, about 9.4cm major diameter, 2.4cm minor diameter. Its a standard schematic with the grid leak connected to ground, and the tube is working as a triode. And the grid lieak circuit is a 910Ohm wire wound resistor and a 2nf 10kv cap at the moment. It was fine until i starded powering it with more and more voltage(Im not exceeding the limitations, its just that I previously powered it with half the line voltage). Something made a loud bang, there was a bright flash inside the chassey, and the poor tube died! I couldnt find the fault in the circuit. After doing a bit of research I found out that the tube I used (The Polish version, GU81 without the M) had a capacitor inside the bulb, so maybe that exploded? Later when cranking up the power there was a blue haze inside of the tube, it didnt seem to be gassy, but something was wrong clearly. I replaced the tube, and the same thing happened! The tube survived this time though, but I saw some blue discharges inside it for a second when tha fault happened. Only then I found out that the HV PSU transformer I was using at the time had some shorted turns on the secondary and there was clearly some arcing going on. But that couldnt have caused the loud bang or arcing inside the tube right? I replaced it with the 2 MOTs Im using at the moment, but Im still scared to turn the power up, I really dont want to kill the new tube! What could be causing these issues? What should I do to ensure I wont damage it?
I know the wiring looks messy on the photos, but in person it looks a lot nicer + of course I made sure no wires are touching for safety. Ive attached some photos for reference

[johnnyzoo]

Please take this with a grain of salt as I'm not very experienced with tube technology, and even less so with tesla coils...

Did you happen to see or hear where the bang happened?

Loud bang + transformer arcing later sounds like a breakdown somewhere (in the transformer, maybe?) followed by a rapid discharge of a capacitor and then steady arcing due to failed insulation.

What kind of transformer were you using before switching to MOTs?

[Continuum]

Thanks for the reply!
The transformer I was using previously was a big MOT, I found it on a local faire, it was really massive and rated for 1.4kW.
Sadly I couldnt find the place where the bang happened, so the only suspect is the transformer where there was clearly some arcing. But Im still not sure why did the transformer arc over in the first place(maybe it was damaged right out of the box actually, since I bought in used on a faire for cheap...) + why was there a blue haze in the tube during the fault? I know that happened because I was lucky enough to catch the fault happening on camera, but it was only pointed to the top of the machine, so still no clue what happened inside of the chassey.
I recall putting too many turns on the feedback coil when the fault first happened, I quickly noticed the mistake and reduced the size of that winding. The transformer was already broken by that point, so that explains why the bang happened again even after I reduced the feedback coil.
But why the blue haze inside the tube even after reducing the feedback coil? That indicates either a gassy tube or a significantly too large voltage in it... If only I knew the tube was good! It should be though, maybe I should consider just testing it out of the circuit? In the end, that too I bought on the faire, so I may have just gotten incredibly unlucky and both the transformer and the tube were faulty! Ill try powering the tube up out of the circuit and seeing if it is gassy, and Ill take a closer look to the old transformer that was damaged.

[johnnyzoo]

Are there any loose parts or metal chips inside the tube? Of course it will not tell everything, but it is easy to check.

You said there's a capacitor inside the tube. Can you locate it, does it look good or burned?

Good vacuum does not carry sound waves so an explosion inside an intact tube *might* not sound very loud... just my 2 cents.

[Mads Barnkob]

I have had problems with arcing inside tube before, seemed to happen between legs at the bottom if they were pushed beyond their voltage rating.

There is no capacitors inside of tubes, perhaps its just ceramic insulators.

Tubes are very forgiving on over-current and will glow red before white and into destruction, but over-voltage is harsh on it, if you have a flash-over between grid and anode, the grid could have been damaged, melted and short-circuit to something, quick flash+bang and its burned away.

Be sure its not just the anode voltage you have to keep in spec, but also the maximum grids to anode/cathode voltages!

[Continuum]

Sorry for the slow replies - havent had time to work on this project recently.
So in the Polish GU-81 there is a capacitor marked on the datasheet, and I actually can spot something that looks like one in side the glass bulb. Anyway, Im using a GU-81M which doesnt have that, so I guess Im safe for now. There is nothing flying around inside the bulb luckily, no metal or glass or anything
I took a look at the old transformer, and there is a spot that looks like it was very hot once - I suppose the flashover happened deeper inside the winding, causing the outer layers to be heated. Should i cut open the winding to check and see if there was a big flashover there? Just to test the hypothesis, Im getting desperate to figure out whats going on here.
Furthermore I looked at the video I took of the fault happening(link below), and slowed it down. The discharges inside the tube happen one frame before the bang happens! So that would indicate, there is a big overvoltage in the system, which first causes the tube to flash internally, and then the transformer follows! What could be the cause of such behaviour?
Link to the video:

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[johnnyzoo]

Just a note: The fourth attachment in the first post is corrupted. The thumbnail is fine but if you open the full picture, it shows garbage.

Delay of one frame between the flash and audible bang might also have been caused by the camera. I'm pretty sure that the bang was caused by the same event that caused the flash.

Do you have a variable supply transformer to slowly ramp up the voltage?

Measuring the anode and grid voltages with an oscilloscope might reveal something too, but it requires a special high voltage probe and great care.

[Mads Barnkob]

Flash-overs in tube for a VTTC has in my experience been from violating the maximum grid voltage ratings. Your feedback might be too hard, try to lower the coupling or number of windings of the feedback coil.