Author Topic: Hi! 811 A tesa coil  (Read 493 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 »
Duane Bylund

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.

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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
Ladd Illinois USA
Former Printer

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.

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Re: Hi! 811 A tesa coil
« Reply #11 on: November 24, 2021, 05:34:02 AM »

 


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