UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 12th Jan 2017, 11:19 pm   #81
Bufo Bill
Heptode
 
Bufo Bill's Avatar
 
Join Date: Jan 2016
Location: Wolverhampton, West Midlands, UK.
Posts: 777
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Thanks Al, just what I wanted to know
Many thanks from Bill.
Bufo Bill is offline  
Old 13th Jan 2017, 2:07 pm   #82
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by Bufo Bill View Post
Thanks Al, just what I wanted to know
Hey Bill, you're welcome. Here, as promised, is a photo -- 100A rated earthing braid from the metal screening of the GU81M valve holder to ground.
Attached Thumbnails
Click image for larger version

Name:	IMG_1115.jpg
Views:	178
Size:	53.6 KB
ID:	135854  
__________________
Al
Al (astral highway) is offline  
Old 13th Jan 2017, 2:14 pm   #83
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Good to see your back on track Al, looking forward to seeing it running.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 13th Jan 2017, 2:42 pm   #84
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by Dr Wobble View Post
...looking forward to seeing it running.
Cheers, Andy! A few more careful steps to go, lots of cross-checking etc, but shouldn't be that long!
__________________
Al
Al (astral highway) is offline  
Old 13th Jan 2017, 3:46 pm   #85
Bufo Bill
Heptode
 
Bufo Bill's Avatar
 
Join Date: Jan 2016
Location: Wolverhampton, West Midlands, UK.
Posts: 777
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Cheers Al, that's very instructive, I was imagining something more like soldering braid in my head!
Remembering back a page or two, I think you mentioned that your home has lightning rods for grounding? Did I get that right? So can you use this to your advantage with your build? If so, would you mind explaining the differences with the grounding in say, an older building? This would be of interest because I was fascinated the other weekwatching a faraday cage on the RI Christmas Lectures. I believe your home could be described as a faraday cage, with the Lightning conductors?
Bufo Bill is offline  
Old 13th Jan 2017, 7:38 pm   #86
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by Bufo Bill View Post
Remembering back a page or two, I think you mentioned that your home has lightning rods for grounding? Did I get that right? So can you use this to your advantage with your build? If so, would you mind explaining the differences with the grounding in say, an older building? ...
Hello Bill,

My home is a new-build flat. What I think you're referring to is my note on the construction - the frame of the building is large steels bolted together and then there is brick in-fill. So this framework is going to make it impossible for electromagnetic waves of this frequency pass through - they are 300m long and the steels are metre apart.

The safety earth of the building is conventional but the large earth busbar is in a small electrical intake room nearby. Hope that helps ?
Cheers
__________________
Al
Al (astral highway) is offline  
Old 13th Jan 2017, 8:34 pm   #87
Sean Williams
Dekatron
 
Sean Williams's Avatar
 
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Al, while the fundamental frequency is low, a Tesla coil is a really efficient spark transmitter, and will be rich in harmonics well into HF and VHF - you will be heard!
__________________
Engineers make things work and have spare bits when finished
Sean Williams is offline  
Old 13th Jan 2017, 9:17 pm   #88
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by Sean Williams View Post
Al, while the fundamental frequency is low, a Tesla coil is a really efficient spark transmitter, and will be rich in harmonics well into HF and VHF ...
Hey Sean, this one isn't mechanically excited, it's a valve design, so it isn't like a spark transmitter in that sense. There are very careful RFI measures throughout and crucially, the Q of the main inductor in my design is really high -600 - so the bandwidth will be very narrow.

I will also be using a phase-locked loop (my idea - I have never seen this in a valve design before, only in solid-state designs) so that its operation remains tightly fixed around 1.3Mhz

And as noted before, nobody listens to AM in crowded areas like mine - there is just no point -, so I am unlikely to be heard at all!

Subwoofers vibrating the superstructure and pirate FM are by far the biggest nuisance where I live
__________________
Al

Last edited by Al (astral highway); 13th Jan 2017 at 9:26 pm. Reason: Extra note..
Al (astral highway) is offline  
Old 13th Jan 2017, 11:22 pm   #89
Bufo Bill
Heptode
 
Bufo Bill's Avatar
 
Join Date: Jan 2016
Location: Wolverhampton, West Midlands, UK.
Posts: 777
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Many thanks for the clarification Al, you have helped sort out some of the mysteries I have in my addled little head!

What I love about this forum is the fact that help s given freely and with no hint of snobbishness.
Bufo Bill is offline  
Old 19th Jan 2017, 3:39 pm   #90
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quick update:

I ran the set up with a new primary coil wound on Litz wire and with the new EHT power supply and grid leak circuit. This time , no oscillation!

I scratched my head and re-did all the calculations. There is one critical value that is completely unknown , just a guess. It's the capacitance of the metal sphere that is a top load for the secondary coil.

I may have way overestimated this and so I am now re-modelling for a self -resonant frequency that is far higher, nearly 2MHz. This means re-winding the primary and re-designing the grid-leak, too.

The grid leak R values are not only influenced by the DC characteristics of the tube and by its grid current handling capacity, but are also an ideal ratio to one RF period defined by the operating frequency of the main inductor (secondary coil).

I will do this rebuild next week and hopefully report back with success shortly afterwards.
__________________
Al
Al (astral highway) is offline  
Old 6th Feb 2017, 4:02 pm   #91
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Progress may seem slow, but this thread [URL="https://www.vintage-radio.net/forum/showthread.php?t=133397"] about my DC tests of the big GU81M should link here.

At the moment I am trying to converge on the correct inductance for the primary coil (which depends in turn on measuring the inductance of the secondary coil - the primary is in a parallel resonance tank circuit) and then to re-wind the primary accordingly. Experienced builders of valve Tesla coils advise tuning the tank circuit a few tens of KHz below the secondary. More on the reasoning behind this this in a future post.

I will be using a home-made inductance tester and another, more elaborate method to achieve better results than any of the formulae for air-cored coils could. None of these allows for the permettivity of the coil former substrate.

I haven't even built the home-made tester yet, but parts are on order to complement what's in my junk box.

Once I have the correct primary wound - this is the third attempt - I will need to attend to the correct coupling coefficient between the grid feedback coil and the primary, which means making the grid coil adjustable in position. The waveform of the grid oscillation is important and may have unwanted artefacts if it is too closely coupled or had too many turns. There are many variables here, all critical.

It really is much, much simpler to follow and off-the shelf 'as built' design (-"as for my first three Tesla coils) , but there's far less satisfaction, too.
__________________
Al

Last edited by Al (astral highway); 6th Feb 2017 at 4:29 pm. Reason: Typos
Al (astral highway) is offline  
Old 2nd Oct 2017, 6:49 pm   #92
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Hi folks, it's been a while. For those not familiar, this thread explains why! Now I'm back in action on this project - although I can still barely move the beast - and have an update.

If you're expecting to see video of wild plasma arcs, turn away. It is time for a rethink and a little bit of maths to explain why.

Before I put the thing away last time, I had made a pretty bombproof EHT power supply and was twiddling with the tank coil to match the main ('secondary coil') inductor. I'd hand wound this and it was very well insulated/potted beast with what I thought was a nice high Q.

Things didn't work out so promisingly. I got an instantaneously huge flashover between the grid feedback coil and the primary, which again spiked out the meter in the power supply, bending the needle - touch sad, as it took me hours to remove it from the set, take it apart and mend it, on two previous occasions. I couldn't tell what sort of breakout there would be if this hadn't happened as it devastated the whole primary, which isn't trivial to wind.

But as I'd built similar Tesla coils in the past with rather fewer, or no problems whatever, I began to suspect that things were not well with the basic design.

And here's what I found. The secondary coil, which is self-resonant at around 1.97MHz, isn't a great fit with this particular valve and power supply. Now of course, any of us can make any inductor resonate at any frequency below the limits of a valve or transistor. But with Tesla coils, things are rather fiddly.

Here are some calcs showing why I need to redesign the main inductor. Turn away if you don't like this kind of thing. Stay tuned if you do!

Parameters:
Current in GU81M anode: 600mA

Voltage (under load) in power supply: 4,000VDC (It's half-wave doubled from 2200VAC - I'm taking an informed guess at the average after some experimentation).

now the important bit...

Class C load/ impedance calculation

R (anode) =V/4*I, where V=4,000
So, R=4000/4 *0.6 =1,667R

which is all well and good, right?

Fres= 1.97MHz.
L (pri) =45uH. X= 2* pi * f * L
=6.28 * 1.97e6 * 45e-6.
=556R

which looks ok to begin with, but is actually rather too high. We want it between 80-160R.

Now let's look at Q.

Q=R/X by one method that will do us nicely.
Hence, 1,667 (anode)/556=2.994
...which is very low.
We want it at least 10-20 for decent breakout.

Next thing, the capacitance. Using a simple parallel resonance formula, my 150pF capacitor looks fine. But using the Q calculation to reach the ideal capacitance yields another figure altogether, like this:

C = (Q / (2 * pi * f * R)) * 10^9 (nF)
which yields 1.44nF instead of 150pF!!!

The Q of this matching tank capacitor is derived as follows:

Q = 2 *pi * f * R * C
= 6.28 *1.97e6 *1,667 * 1.44e-9
=10

Which is ok. But the Q of the main inductor isn't- see above. Long story short, it's better for me to redesign the secondary to have a lower self-resonant frequency. Typically, they are indeed usually below 1MHz, often substantially so.

The main constraint stopping me at the outset was that I wanted a table sized model, and now the main inductor will be quite imposing.

I'll just have to replicate a method of detaching it to put it away and then re-assemble it - I've already got such a design in place for the current one, so will just repeat this.

Exact dimensions are still on the drawing board, as it will partly depend on what's out there in terms of materials.

Hope that's not too unwieldy!

Cheers,

Al
__________________
Al
Al (astral highway) is offline  
Old 2nd Oct 2017, 7:09 pm   #93
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

I thought Q = X/R ?

Lawrence.
ms660 is offline  
Old 2nd Oct 2017, 8:16 pm   #94
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by ms660 View Post
I thought Q = X/R ?
Good spot, as usual. I make transcription errors the whole time at the moment -very frustrating.

The resulting conclusion is even worse.
__________________
Al
Al (astral highway) is offline  
Old 2nd Oct 2017, 8:20 pm   #95
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Must admit I'm a bit lost with this sort of stuff but I still follow the thread with some interest.

Where did the Class C load/impedance formula come from?

Lawrence.
ms660 is offline  
Old 2nd Oct 2017, 8:31 pm   #96
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by ms660 View Post

Where did the Class C load/impedance formula come from?
It came indirectly from a well-known US Tesla coil builder who has a few medium sized builds to his name.

Maybe some RF power engineers here will be able to corroborate.
__________________
Al
Al (astral highway) is offline  
Old 4th Oct 2017, 6:52 am   #97
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Good to see your back to fettling again Al. Would drain pipe be useful for winding your main coil on? You can get all sorts of flangers and fittings, so with a female to male flange screwed to your box, a 4" pipe should slot on. You'd then have to have your coil terminated in flying leads with plugs on to plug into the "doings".

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 4th Oct 2017, 10:42 am   #98
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Quote:
Originally Posted by Diabolical Artificer View Post
Good to see your back to fettling again Al. Would drain pipe be useful for winding your main coil on?
Hey Andy, great idea, very ingenious! The last one I built embodied a similar idea only using brass plumbing fittings at the earthy end.

I think PVC is ok for small secondaries but somewhat lossy at higher voltages - IIRC it slightly softens. It is however fine for the primary coil.

Thanks for your encouragement and enthusiasm!
__________________
Al
Al (astral highway) is offline  
Old 7th Nov 2017, 1:43 pm   #99
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

Hey folks,

I'm back in action on this. I want it finished well before the Winter Solstice, when it will make a great light display.

A few modifications - I'm rebuilding it entirely since the prototype.

1) New, lower operating frequency. Only 300KHz instead of around 2MHz. Forum member Richard (Mole42UK) is currently winding one of the main inductors on his lathe, very kindly - it should be back in the next week or so, I understand. This puts the third harmonic (dominates after the fundamental) at around 1MHz. Thank you, Richard!

2) A single, but more efficient transformer. The weight of the paralleled microwave oven transformers was one factor pushing the weight of the prototype towards 35 kilos - far too much for me to heft around safely.

Another forum Richard is kindly investigating whether he can source me a commercial microwave oven transformer - these are up to 1800W capacity at 2300V. Thank you, Richard! Also, Richard is looking into another source of 1uF (ish) caps rated at this voltage. I hope to have these before the end of November, if things work out.

3) More robust power supply. That means ironing out current surges, and making it flashover and short-circuit resilient. It's absolutely true that hours of work can be destroyed in milliseconds by these kinds of power supplies going wrong! Also, doubling the voltage rating of capacitors, as all have previously failed after short, experimental service.

4) More open design to enable easier troubleshooting and disassembly of faulty components.

5) More robust, higher Q tank circuit, including flashover-proofed primary coil. I kept getting shorts between a feedback winding and the tank circuit, even across 2.5-3cm of air. Volts in here are supposed to be 15kV peak, so clearly this is in practice exceeded.

6) Anti parasitic measures.

7) Phase-locked loop operation - I'm building the electronics for this now.

That's all for now, people, thanks for looking.
__________________
Al
Al (astral highway) is offline  
Old 7th Nov 2017, 3:17 pm   #100
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Design-and-building a large valve Tesla coil (class C, Armstrong oscillator).

You must have a very understanding insurance broker. 2.4kW input to an RF power oscillator in an ordinary flat (and a PSU with no overload protection) seems to be an application for a Darwin award. Mains and radio HT safety looks trivial by comparison.
G8HQP Dave is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:18 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.