Author Topic: Hi! 811 A tesa coil  (Read 2002 times)

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
Chris Reeland
Ladd Illinois USA
Former Printer

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



Chris Reeland
Ladd Illinois USA
Former Printer

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 »
Chris Reeland
Ladd Illinois USA
Former Printer

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

High Voltage Forum

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

 


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