Hi! 811 A tesla coil

[jpvvv123]

Hi, I built a dual 811 A Tesla coil from plans and instructions I found on the internet.  However it does not work.  I checked to see if it was getting power and the transformer seems to be working fine.  I'm not really sure how to proceed.  When I first turned it on I got a very small spark about 1/4 long but now it doesn't even do that.

This is the schematic (also attached):
https://www.stevehv.4hv.org/VTTC1/dual811Aschematic.JPG

Thanks for any information which can help get this thing going.

[Duane B]

What kind of test equipment do you have? I assume the filament transformer is good or the tubes would be dark. The MOT is dangerous. Have you got fuses? Check the primary and secondary resistances of the MOT. I would not attempt to measure the secondary voltage unless you have a high voltage probe. I guess if you have it on a variac you can test it at lower voltages.

Double check your connections. Disconnect C1 and measure it to see if it is ok (not shorted, as a minimum test). Measure R2 and see if it is ok. Besides C1 and R2 there is not much else that would fail.

Make sure the phasing of the primary coil and grid feedback coil are correct. It will not oscillate if the phase is wrong.

Good luck, and let us know how it goes.

[jpvvv123]

I have been through it  checking the  values.

I have a high voltage probe that goes to 30kVAC.  Needless to say the resolution is not so good  down in the low kV levels.  This probe reds 3kV on the main transformer's secondary.

I need to find a high voltage probe for my oscilloscope, Does one exist?

How do I check that phasing? How can I control the phasing?

[Duane B]

Yes, high voltage probes exist. I don't think you need one to fix this Tesla coil though. If your tube is oscillating you should see some DC voltage on the grid (caused by grid current going through R2 and filtered by C2). You should see this voltage whether the secondary is present or not. If no voltage then the tube is not oscillating. The DC voltage on the grid tells you the average grid current, I = gridV/5k

The instructions for making the Tesla coil should have said what directions to wind the primary and grid coils, and what leads connect where. If it is not phased correctly the tube will not oscillate. You can try changing the connections on one of the coils. The phase is either correct or not, there is only two possibilities.

What bothers me is you said it gave 1/4" sparks for a moment, and then nothing. It's like something failed at start-up.

Going forward, I would try the system without the secondary coil and with just one tube. Make sure you get some DC voltage on the grid. Going from a 50-year-old memory on this Tesla coil, I think the grid voltage should be around -100 volts. Once you get DC grid voltage you can start adding the other tube and secondary coil.

Good luck.

[johnf]

Yes I was wondering why the grid RF bypass cap had to have such a high voltage rating

[Duane B]

The grid bypass cap has a high voltage rating in case there is an internal arc inside the tube or an external arc between the grid coil and secondary coil. All of those caps have a higher voltage rating than what they would normally see,

As for measuring grid DC voltage, measure the voltage across R2 and not directly on the grid. I should have said this before.

[jpvvv123]

I measured the voltage across R2. I read 4-10 volts, both AC and DC.

I do have a high voltage probe which is reading 3kV across the main transformer's secondary coil.

I checked C1 (a ceramic "door knob" type) and it was at something like 880 pF. it is rated for 30kV I believe.

When I switched the wires on L3 I started getting a small spark that gradually fades away.

@Duane B, what do you mean by secondary coil? L1, L2, L3?

[Duane B]

I would call L1 the primary tank coil, L2 the secondary coil, and L3 the grid feedback coil.

The DC grid voltage across R2 is way too small. This indicates that the oscillations are very weak, possibly cause by poor coupling between the primary and the grid feedback coil, or very week tubes. Here is a clip of typical operating conditions for a National 811A tube. Your grid voltage on R2 should be at least -50 volts with a maximum of -200. Low voltage means your HF AC grid feedback current is low.

Get a small neon light and tape it on the primary coil close to the plate connection. Do not wire the neon light. The RF oscillations from the tube should ionize the gas in the light (if the tubes are oscillating).

[jpvvv123]

Hi, I just tried the neon lamp thing and it is not glowing. 

I turned it on a few hours ago and got a little 1/4 in spark. Now there is nothing.

[jpvvv123]

I thought I'd check it one more time with the lights out. I can definitely see a little orange glow inside the neon Bub. It is very faint but it's there and seems to be in a steady state.

[304er]

Hi,

This is hard to troubleshoot remotely.

But double check you have all required grounds and make sure they are actually good connections. And check all other connections while you are at it. Make sure all correct.

Do you actually know if your 2 tubes are actually good? New or used? Could be weak. Are tubes just showing filament glow? If any other glowing like kinda  bluish, whitish, or purplish...tubes are bad... called gassy.

Tesla Coil could also be greatly out of tune.

There are just so many things.

How is secondary construction quality? Kinks and overlaps are bad.

Can you verify all different capacitors are actually good?

Some suggestions...

Chris Reeland
Ladd Illinois USA

[jpvvv123]

The tubes are brand new to the best of my knowledge. I will test them to verify.

[jpvvv123]

 I was ready to walk away and dismantle this thing. But the last thing I tried was replacement of the new 811A's with one old tube marked just "811". I now get a 1" spark and about -40 V across the grid resistor. The spark stays steady and does not diminish like it was doing.  I'm also getting a red hot plate as shown in the following pics.

All tubes test good but the new ones are weaker than the old 811.

Do you think tuning it might help? Can you use a variable capacitor? Won't a variable cap just arc and short out?

[304er]

Hi,

I guess a little progress.

Sounds like the single 811 is a bit overloaded by your power supply and something is still out of tune.
From the schematic you have really should be 2 tubes because of that transformer.

Your new tubes could be possibly still good...just something greatly out of tune elsewhere on coil.
Or the new ones are actually with weak filaments.

Not sure if you know...but never apply high voltage without filaments on or run filament voltage too low. This will render them weak in short order ruining them.

Currently can not see a picture...

If you are going to try a variable capacitor in the tank, I think you are possibly thinking of an air one...not sure by what you said...but it will work as long it is properly voltage rated... otherwise yes will just jump across shorting.

Keep at it...at least finally seeing signs of life finally.

Chris

[jpvvv123]

I figured out at least one problem: I wasn't driving the tubes nearly hard enough.  I had a 3 amp 6.3 v transformer and even with one tube was only getting about 3.5 v on the tube filament.

So I got a 10 A transformer.  Now I'm getting -270 volts on the grid and about 2" sparks.  And about 7 volts on the tube filaments.

The problem is now it's drawing too much current.  It trips the breaker on my isolation transformer which is rated for 250VA. 

I tried plugging directly into the power strip but I got arcing from the grid coil to one of the tube plates' wiring.  The base on one of the tubes shorted and burned and the grid resistor blew out one of the ends. Fortunately the transformer, tubes and colls were not damaged.  I don't know if the arcing caused the blowout because the wires were too close together but it seems the circuit can't handle any more current.

The designer of the circuit said I should be able to get a 6" arc.  My circuit may not be in tune, isn't tuning it going to cause even more current draw?  And I'm not sure what to do about the current draw?

I would attach a picture but can't figure out how to resize the image, it's a Mac.

[jpvvv123]

pic attached

[304er]

Hi,

Starting to show even more signs of life...

You are lucky you only ran for a short time with the very low filament voltage before...glad to see you have corrected this and tubes still seem usable. Your filament voltage is just a little hot now, but much better than very low.

I can not see whole layout of your coil...but yes watch distancing. Again sounds you got lucky overall with damage which could have been much worse.

From the schematic did you follow this very closely?
If you did you really need the top load toroid.
I see in the picture you have attached in the next post is just a long skinny extended secondary wire. This is bad... never will get good arc length this way, it just doesn't work this way...toroid with center straight up break out point about 3 inches long is required. The original plans have this toroid figured in... without your are most definitely likely way out of tune.

Also feedback winding needs adjustment most likely... your negative grid seems too high now.

Even with toroid, the original circuit will require some tuning, but you will be much closer to tune. There are always variables out of our control...no matter how closely you try to copy someone else's work.

It seems counter intuitive...but sometimes...high current draws happen when greatly out of tune. The extra power drawn is just wasted elsewhere in the circuit, instead of being put into the desired spark length.

And actually... when all is in "harmony" current draw will go down to reasonable expected levels when in properly tuned and it will have decent spark length also.

Going from past memories/experience on a dual 811 tuned up real good will see maybe from 12-14A @ 120VAC (I am in the USA) when using a MOT output only...just like your current schematic (NO level shifter). I achieved 14" arcs. (But this was a much different coil form setup and running frequency.)

They are not really efficient to begin with and are even worse from my experience with newer MOTs on tube coils.

Tube coils are actually "tricky beasts" at times.
Keep at it and be careful with your components with changes and you too... remember... even though lower high voltages here...they can be deadly if a mistake is made. Treat it with the same respect as a spark gap type coil.

Chris Reeland

[jpvvv123]

Well, I got a metal dome type :toroid".  After I installed it I powered it up.

3 different arcs appeared on my wiring: tickler coil to secondary grounded end, the main transformer is arcing somewhere I can't see plus arcing to tickler coil to secondary grounded end

The main transformer is now arcing into the wood base, as far as I can tell.  There are now several blackened pathways conducting.

And, no spark at top.  All the sparks are in the wiring.

This part has made things a lot worse. I don't know the damage done yet or what to do about it.

[304er]

Hi,

Sorry to hear about more problems...

You have mentioned you put on a toroid ? ...but no real details of this ?
Not really sure what you are talking about for sure on the top load... what is it actually?
What size is it?
Did you put a break out point on as I said?

I will quote again what I said:  "toroid with center straight up break out point about 3 inches long is required".
Your schematic you are trying to copy says what was used for a toroid.
Not sure if you know...but a break out point is a MUST on top loads on VTTC's. They are not like SGTC's at all.

Details from you are needed and important.

On the various arc shorts... definitely could be related if you DID NOT have a break out point.
Also...if you go too large a top load...can be a problem too...goes way out of tune the OTHER direction, causing possible problems.

Or your wiring layout work needs to be redone.
You have mentioned some problems related to this already previously.
You must have sufficient clearance between various parts of the VTTC circuit. There is high frequency voltage/current on various wires...it will take "short cuts" on ANY weakness in wiring. Certain wires need separation... insulation on these wires...does next to nothing...high frequency will just pass right through short arcing.

Hard to make out much from your one picture...but your MOT is looking too close to the primary and feedback winding coil form...this is bad in a few different ways. And if you are on a wood base... from what you are saying about burn tracks.. if you are just bare wood...even more clearance is required between components when bare...wood absorbs any humidity in the air... again ANY weakness.

Chris Reeland

[jpvvv123]

I got  a breakout wire with the toroid and rearranged some wiring. Rerouted the output wire from the MOT. Raised the coil form up to get some distance between it and the grounded end of the secondary coil.  Now there are no more fires!

Getting 3" sparks. It is still tripping the breaker on my isolation transformer though. It runs about 30 seconds before tripping.

How do I go about tuning it? Could I add some capacitors in parallel to C1? That would make the capacitors add...how would I lower the capacitance. I tried to attach photos, but I can't get it to work.