Author Topic: Hi! 811 A tesa coil  (Read 2238 times)

Offline jpvvv123

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Hi! 811 A tesa coil
« on: September 29, 2021, 10:14:36 PM »
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.

Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #1 on: September 30, 2021, 03:34:33 AM »
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.
Duane Bylund

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #2 on: September 30, 2021, 11:29:14 PM »
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?

Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #3 on: October 01, 2021, 12:09:59 AM »
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.
« Last Edit: October 01, 2021, 12:27:59 AM by Duane B »
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Offline johnf

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Re: Hi! 811 A tesa coil
« Reply #4 on: October 01, 2021, 08:50:32 AM »
Yes I was wondering why the grid RF bypass cap had to have such a high voltage rating

Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #5 on: October 01, 2021, 05:42:32 PM »
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.
Duane Bylund

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #6 on: October 22, 2021, 11:52:23 PM »
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?

Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #7 on: October 23, 2021, 12:48:40 AM »
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).

« Last Edit: October 23, 2021, 01:36:36 AM by Duane B »
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #8 on: November 24, 2021, 01:20:15 AM »
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.

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #9 on: November 24, 2021, 01:33:58 AM »
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.

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #10 on: November 24, 2021, 01:48:21 AM »
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
Chris Reeland
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #11 on: November 24, 2021, 05:34:02 AM »
The tubes are brand new to the best of my knowledge. I will test them to verify.

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #12 on: November 27, 2021, 11:09:43 PM »
 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?


Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #13 on: November 27, 2021, 11:32:12 PM »
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
« Last Edit: November 27, 2021, 11:47:09 PM by 304er »
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #14 on: December 12, 2021, 08:40:54 PM »
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.

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #15 on: December 12, 2021, 08:48:29 PM »
pic attached

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #16 on: December 13, 2021, 04:10:51 AM »
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


« Last Edit: December 13, 2021, 05:03:24 AM by 304er »
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #17 on: December 18, 2021, 01:33:09 AM »
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.

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #18 on: December 18, 2021, 08:00:26 AM »
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





« Last Edit: December 18, 2021, 09:01:19 AM by 304er »
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #19 on: January 06, 2022, 03:40:43 AM »
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.

« Last Edit: January 06, 2022, 03:43:31 AM by jpvvv123 »

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #20 on: January 07, 2022, 05:56:28 PM »
Hi,

Getting better.

Before making changes to primary capacitance... let's deal with other things first.

Your top load is smaller than what the plans you were following from what I can see. Plans had a 6" one. But this is okay right now. Since this is a bit smaller of a top load...cut the breakout point/wire to 1.5"-2" instead of the current 3".  Tube coils do not like this too long or too short for best performance and this is directly related to top load size. May not initially right now see any big change...but this will really help later as this coil get refined for better performance...this does make a difference eventually.

Next... your MOT is too close from what I can see in these updated pictures. Need to move away. The iron core of the MOT is most likely in the magnetic flux field of the primary winding of the coil. This will "suck" energy out of the primary windings... instead of getting as much as possible to our desired secondary.

I would say also possibly affecting your primary to feedback winding transfer of energy also.

So before any changes to capacitor, I would suggest these things first.

Chris
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Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #21 on: January 07, 2022, 10:20:10 PM »
As shown in the picture there is nowhere to move the MOT, nor is there any open area on the board, which is a 12 X12 X 0.75 piece of plywood.

The only choice I have is to strip everything off and rebuild it.

I have to think about it and decide if I want invest that kind of time into it.

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #22 on: January 08, 2022, 09:05:41 AM »
Hi,

Sorry about the bad news of being too close.

I went to Steve's website to get a better look at what your are trying to copy.
This was an early first one he did. His MOT is too close having the same flaw. But his MOT happens to be turned 90 degrees by chance I guess. I suppose if you try this...it can help some I think. Still too close from what I know...but it is your best option since you do not want to redo whole mounting board to larger.
This will maybe get you closer to the 6" you are hoping for eventually.

I did say much earlier, tube coils are "tricky beasts at times". Efficient proper layout of certain coil components is necessary also. This is one of several reasons I achieved 12"+ on a dual 811 on a straight MOT only.

Adjustment of primary capacitance is the next important thing to do, while kind of the same time...experimenting with the number of feedback turns and changing grid leak resistance values and grid capacitor values are necessary also for best performance. My good running T200 VTTC took dozens and dozens of tries to get this area at it's best. Even though I usually from experience can "tell what a coil wants" it still can take me a while at times myself, especially if I want the best possible.

Otherwise you need to look at what Steve did again in his pictures and REALLY COPY what he did and do not stray at ALL if you can on ANYTHING. I can see several things you did different. These things which may not seem that big of a difference in reality can be much larger than you think.

Again really sorry to give bad news here and you will have to decide.

Chris



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Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #23 on: January 09, 2022, 05:52:11 AM »
You mentioned that the circuit breaker on your isolation transformer is still tripping. If it is only rated at 250 watts then the isolation transformer is way too small. I wouldn't be surprised if the voltage reaching the primary of the MOT is below 100 volts. Like Chris mentioned, the current draw from this VTTC should be 10-14 amps. This equates to 1000 to 1500 watts. Why are you using an isolation transformer anyway? Get rid of the isolation transformer. Use a 15-amp fuse in the primary wiring. The MOT and filament transformer provides isolation without using another isolation transformer.
Duane Bylund

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #24 on: January 09, 2022, 11:04:25 PM »
If I turn out most of the lights in my kitchen and garage I can run the coil from the main power directly. It's a 20amp breaker. It trips if I have any other lights turned on than the garage light.

The plates on the tubes are glowing red hot.  I'm getting about 5 in sparks.

I do have a toroid coming in the mail that is a lot closer in size to the one mentioned on the schematic.

Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #25 on: January 10, 2022, 03:59:09 AM »
Hi,

Getting little by little better output...but unfortunately way out of tune being the high current draw type...

Did you at least also turn your MOT 90 degrees like I said earlier?

This should help some. Even better would be just put this on a small separate piece of wood about a foot away at least and wire back in to coil. Just a suggestion before trying further tuning. I feel this is currently a problem and can contribute to high current draw, which you are obviously seeing in your red hot tube plates and your 20amp breaker easily tripped.

Question for Duane:
Do you agree the MOT core looks too close robbing primary energy?

I am sure you possibly know...but do not run real long this way...your tube plates can melt holes in them! They can possibly tolerate very small holes...but once too large they are ruined.

Various tuning needs to be done now which I have mentioned already. You can wait for your bigger toroid first if you want. But I will say... some tuning will still be required from the still differences I can see between yours and Steve's coil. Steve admitted looking back at this coil...in the description of this coil, that the output should have been much better than the 7" he got. And yes it should...12-14" approximately.

You have earlier mentioned 6" desired...you are getting close at 5"...but as you see it is not running correctly at all for this output.

Keep at it...this is a learning experience...only you ultimately can fix this...we can only try to help...we can not see or hear what is happening...and if you do not tell us what your level of electrical high voltage knowledge is from the beginning...it makes this much harder. We are assuming some knowledge. Your various electrical shorts already are an example.

Okay... please do not get discouraged... keep at it.

One more thing...can you please get pictures of the whole coil?
I need to get a look at it better... especially the board/base components.
I want a better look also of your grid leak components.
So please better complete whole pictures.

Chris


« Last Edit: January 10, 2022, 04:24:36 AM by 304er »
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Offline Duane B

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Re: Hi! 811 A tesa coil
« Reply #26 on: January 10, 2022, 09:38:53 PM »

Question for Duane:
Do you agree the MOT core looks too close robbing primary energy?

I think the core is close and probably does have an energy robbing effect. However, the high frequency energy may not penetrate the iron core that deep. It may not be causing too much of a problem. I would have to experiment with it to see if it is that much of a factor.

To reduce power draw until the coil is/can be tuned better, the 5k grid resistor can be increased to 7k or 8k ohms. This would increase the negative grid bias and reduce average plate current. The 811 coil I built many years ago had a variable rheostat for the grid resistor and it could be adjusted while the coil was in operation.
Duane Bylund

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #27 on: January 22, 2022, 12:53:04 AM »
I tried removing the transformer-I moved it outward and rotated almost 90 degrees. I wanted to try it quickly so I didn't have to rewire the transformer.  It was about 7in away from the outer coils at that point.  I did not measure any appreciable effect on arc length.

Leaving in place and trying to turn it 90deg also requires rewiring.  Plus I prefer not to have the hot side of wire exposed on the transformer terminal on the outside of the device.  If I. try to turn it the other way the hot terminal is on the inside but it gets really close to the outer coils.

I've tried to attach a few more pictures.  I can't remember how I got them to work before, but now I can't attach them.

Still waiting on the new toriod-not surprisingly, it has to come from China.



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Re: Hi! 811 A tesa coil
« Reply #28 on: January 23, 2022, 06:18:13 PM »
Hi,
Okay on what you said on your MOT.
Currently on your VTTC since so way out of tune, yeah probably not much noticable when you tried moving some. If trying to achieve a maximum of about 14" arcs this MOT being close will most likely prevent this from happening. That said...if you just want to get your 6" arcs you said earlier but good running with a reasonable amp draw...it should be achievable with it close...since you will not move.
MOT's are actually pretty horrible and the newer that they are, they get even worse on their power draw even at idle (no load). I would say from experience this one is about 7 amps at idle. A so called bigger better older MOT still has a idle draw of 5 amps, that I was recently using in my 304er VTTC. So if you can get this VTTC tuned up properly for 6" arc output... probably see about 10-11 amps total power draw for this shorter output.
Again proper tuning is required. Your bigger toroid coming will not correct all other problems, but it is factor of what you were trying to copy from the beginning. I highly doubt you will get lucky just putting it "on". You may achieve possibly 6+ inches, but coil will most likely still be drawing way too many amps.
The whole feedback circuit most likely still needs to be adjusted/changed.
The number of primary turns possibly need to be changed or a change of capacitance in the primary circuit is required.

We still need good pictures of your grid leak section of your coil that I have already asked for previously and we need more than just the feedback coil which is already seen. Your just posted 4 completely identical pictures are not useful at all and this area was already seen in your earlier pictures.



Again not useful at all if you want any suggestions from us and I am close to staying away from this thread topic real soon if you do not try harder at this overall on everything. This thread I feel is becoming/already a hard to follow mess. Really sorry to say this.
I have worked really hard to get where I am now in my knowledge for many, many years and I still have a lot to learn on these Tesla coils.
 
You never stop learning...

Just trying to only copy the work of someone else is not learning.

Chris

« Last Edit: January 23, 2022, 07:19:47 PM by 304er »
Chris Reeland
Ladd Illinois USA
Former Printer

Offline jpvvv123

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Re: Hi! 811 A tesa coil
« Reply #29 on: January 24, 2022, 04:13:06 AM »
I have tried to send in more photographs.  I uploaded 4 different ones but got 4 identical ones on the actual post.  I don't think it wasn't anything I did, but maybe I did it wrong.

The pictures don't show up until you actually make the post and I don't see how to tell if they have uploaded successfully.

I appreciate everyone's help on this.  I will try to attach more pictures again.


Offline 304er

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Re: Hi! 811 A tesa coil
« Reply #30 on: January 24, 2022, 09:05:40 AM »
Unfortunately, as you may know, the same 4 pictures again have posted.

Here is some information near the bottom of this, read closely what it says:

Site rules/forum guide, must be read before posting! (Updated August 13th 2021): https://highvoltageforum.net/index.php?topic=31.0

I am not sure what is happening at your end, again can not see what you are trying/doing.

Maybe someone else will chime in with more maybe.

You can just also just post a link only to the picture(s) from a host site if it is already uploaded to the internet somewhere.

Chris


Chris Reeland
Ladd Illinois USA
Former Printer

High Voltage Forum

Re: Hi! 811 A tesa coil
« Reply #30 on: January 24, 2022, 09:05:40 AM »

 


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