Rebuilding VTTC / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Rebuilding VTTC

1 2 next

Move Thread

LAN_403

Terry Fritz

Mon Dec 28 2020, 02:01AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hello,

I am rebuilding and older duel 811 tube coil to a dual 572B coil.
The coil is actually in good working condition but I am changing out the weaker parts to best of bread ones. The older 811 Svetlana tubes have gotten holes burned through the plates and stuff. ;)
I am not very knowledgeable about tube coil things so I have run into some questions.
Here are the pictures, schematic and parts list (last two links).

Questions:
1. The original microwave caps out of the microwave transformer were 1.786uF (C1) but I am changing them to two new caps I had to 2.381uF at 2500VAC total. I am thinking the exact value of these caps is not critical at all?

2. There is that C2 cap at 10kV 1nF. Does it really do anything useful at all or is it just eyewash and can be removed?

3. The plate transformer (T3) has two ~2nF caps across the heater winding. Do they really do anything at all either? If such parts really are useless I will remove them.

4. The grid leak thing has a 8.1 kOhm 30 watt resistor in parallel with a 2.2nF 2kV ceramic cap (C6 R1). This was original to the dual 811's. Should it be changed for dual 562Bs?
Any other suggests are welcome and thank you for your consideration!

Terry

RoV IW3IPD

Mon Dec 28 2020, 03:40PM

Registered Member #64746 Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27

Hi Terry, my thoughts:
1. you will overload a bit the transformer, hopefully getting a bit more power. Shouldn't be a great problem
2, 3, 4: all those caps are necessary to bypass RF, but the voltage rating for C4,C5,C6 can be lowered. Since 1 nF has a reactance of 447 ohm at 356 kHz, I would suggest a higher capacitance for C4/C5, like 10 nF, but a 250 V rating is more than adequate for them: they are required, because the cathode RF current flows on both heater wires and must be bypassed to GND. I would also at least double the capacitance of C2, but in that case the voltage rating is ok.
I would try a smaller number of turns for the grid winding, after all you need a peak grid voltage around 1/10th of the plate voltage. Besides, I would wind them in the bottom side of the primary. You could try with a separate temporary winding with - say - 5 turns and see what happens.
Changing the grid coil will require adjustment also to the grid resistor, where I would expect a lower dissipation.
Nice job, thanks for sharing!
Roberto

Terry Fritz

Wed Dec 30 2020, 12:14AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Roberto,

Many thanks for your input!

I will leave all the caps in. I won't change any of the values yet since it works right now and I don't want to break it. :)

I now see what the caps are doing. The same things those NST filters were doing to filter the RF out of the outer windings of NSTs.

I should have remembered what "that guy" was doing there! 0:D

I have the ability to directly measure currents and voltages so I might play around seeing what all is there.

I bought this coil like 20 years ago on Ebay and it has always worked very well. But I really don't know how it works so I thought I would fix it up and figure it out now.

I actually have a computer simulation of it going now.

It's very preliminary but it sort of runs and allows me to fiddle with things while I learn. After messing with the model I refined the values and it should actually be running at 407kHz.

Antonio's INCA program was super useful!

I got the secondary measurements all wrong since it was installed in the coil and very hard to measure in place. It is actually 800 turns of #27 wire and the INCA program found this error right away!

So I think I am off to a good start figuring this coil out. :)

Best Regards,
Terry

RoV IW3IPD

Wed Dec 30 2020, 09:21AM

Registered Member #64746 Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27

Hi Terry,
I think the simulation model lacks a diode with a resistor in series from grid to GND, in order to model grid rectification on the positive side of sinusoids.
I have read your notes on NSTs... When I was young (I suppose it was 1979), for my first TC I used an ignition transformer coming from an old centralized boiler. I made the mistake of connecting the secondary in parallel to the cap, with the spark in series with the coil and in this way I burned one of the two secondaries. I had to drill through the resin to remove the damaged winding and save the remaining half transformer... fortunately it was still usable.

RoV IW3IPD

Wed Dec 30 2020, 09:43AM

Registered Member #64746 Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27

I have found one photo with my first TC

, from a demo of a few years ago. It still works.
It's the second from the left and the missing secondary is still visible. There is also a paper-oil capacitor to limit the primary current and avoid a resistor on the HV.
1st and 2nd from the left are mine, the others belong to a friend.

photo

Terry Fritz

Thu Dec 31 2020, 06:40AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi Roberto,

Thanks for the tip about the grid diode! I never thought about that before.

The models are starting to look sort of real now.

I am sort of guessing about the grid and anode resistors. Any idea as to if that is close or not?

I tried to make real tube models using current sources and all to match the real tube specs. But I got overwhelmed and it would not run yet. So I bailed out back to using just the simple relay for the moment now.

I just got all the parts in to fix the coil so I will work on getting it all running nice.

Once it is running I can use voltage and current probes all over to get real actual readings and data to study! !:)
Then I can work on real tuning and all that.

This is great fun! China still makes tubes and can supply them quick (10 days over Christmas direct from Shenzhen) and not super expensive ($70 with shipping). Amazon sells all kinds of microwave parts very cheap!
Neon sigh transformers are getting very hard to find and super expensive. Not sure what the solid state guys are up to these days but tube coils are very practical right now.

Thanks so much for your help!

Terry

RoV IW3IPD

Thu Dec 31 2020, 10:05AM

Registered Member #64746 Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27

Terry, the grid diode should be reversed.
You can find tube spice models around, specially in diyaudio forum. There are models
ready for the LTSpice free simulator, for which there are also builtin tube drawing symbols

Terry Fritz

Thu Dec 31 2020, 10:46AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

"Terry, the grid diode should be reversed."

OH!!!!! Shows how much I know about tubes :DD

I have run Linux for the past 20 years now so Window's LTspice is odd... QUCS was the next thing on google that runs everywhere...... :)

Qucs as been in the beta software development stage now for 16 years so that is a giant plus! :D

Of course, all the simulators are just the same. Same s***, different decade... :D

What really matters is getting the real readings to match the simulators. That's what really counts.

Simulation are fun time wasters while waiting for parts to arrive, but the parts are here now!!!

Stay tuned :)

Terry

Terry Fritz

Sun Jan 03 2021, 03:35AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I have been playing with my toob coil things.

Mostly working on the primary circuits.

This is the latest schematic model.

I went through 4 circuit simulators nowâ€¦â€¦... and LTspice is working the best. All the other suffered â€˜bazaarâ€™ failuresâ€¦ :p
I run in Linux Windows simulators, so yeah...

Working on the C2 1nF original filter cap thing. Without any cap there is like 100Vac riding on the rail (blue trace, ignore the yellow stuff). The microwave transformer seems to have about 170uH of reactance.

So I simply stuck a 10nF 2000V MMC cap on it and the noise is vastly reduced!

The inductance of these transformers makes them worthless for draining RF and also makes then susceptible to damage from RFâ€¦.

MMC type caps have like a rating of 2.5 Arms for decades and even though I am doing low power testing the current is use trivial for poly caps.

Just some pics for reference and for those that think I just make this stuff up. :)

So I have figured out the C2 rail cap thingâ€¦ I think the caps across the plate transformer will be super easy to figure now too. ;)

So I looked at the grid voltages in the low power caseâ€¦.

That looks like a mess in over voltage and the computer models for the tube (so far) do not track it at allâ€¦.
The model suggest that 2.5nF might be a far better tuning for the Primary capâ€¦..
Of course this coils likes to blow things up so adjustments are expected. :D

The computer model track things like the Fo frequency perfectly (357kHz)! Even with all those mutual inductances. But other things need a lot of workâ€¦

I run at very low power right now with old tubes in case it all messes up. =:D

T.

RoV IW3IPD

Sun Jan 03 2021, 10:26PM

Registered Member #64746 Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27

nice!
But the last figure (the grid voltage) doesn't seem taken on the grid: it seems taken on Cfeed, where it would make sense. Very high voltage, according to me due to an excessive number of turns on the feedback coil.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob