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Barkhausen Effect  

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turretslug
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My grandmother, a physiotherapist, had a 400W 50MHz (ish!) diathermy set made in 1930, an enormously expensive high-tech investment at the time. It had 2 large DH triodes (grid cap about a third way up the envelope) working in push-pull directly from a 1500V transformer (which had a 150V primary tap as well as the expected 200-250V settings.) As well as being notorious causes of interference to the later Band 1 TV service, I wonder if there would have been a degree of Barkhausen oscillation going on as the self-rectifying valves had their anodes surge down to -1500V and back again? Didn't matterat the time I guess, but I warned her not to switch it on in the eighties, in case she got a knock on the door...,

Colin.

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Posted : 07/10/2012 1:23 pm
Anonymous
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I doubt it.
There have been plenty of high power RF gear. Barkhausen oscillation or more commonly Barkhausen noise more likely to occur when magnetic materials are "hit" with a pulse.

It's almost impossible in a valve or with typical Air cored High Power RF P.A.

Maybe Chris was thinking of Cernekov Radiation, though in practice any mysterious purple glow at very high power is more likely due to a very tiny amount of gas and/or phosphorescence of impurities in the glass getting hit by secondary electron emission from the Anode. I doubt that "real" Cernekov radiation is possible in a valve. Bremsstrahlung Radiation is technically possible in higher voltage valves.

In any case the reason for flaking of metallisation is likely combination of thermal cycling and oxidisation. It's interesting that while on most valve designs they added internal metal shield (Solid, mesh or perforated) from late 1930s that some European versions of US designs continued to use external metallization (lower capacitance to Anode?). Of course the very sub-miniature "tubes" used in Military applications (walkie talkies to Bomb/Shell "fuses") or hearing aids and Russian Rod Pentodes have no space for internal screen so even though produced in 1950s (and to 1970s for military even in West) they use external screening or metallization.

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Posted : 07/10/2012 1:48 pm
crustytv
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The thing which was most striking and I guess the photo fails to relay this, is the strange wave effect to the metalisation.

It had not fully come off the valve, it was extremely rippled and ballooned from the valves envelope suggesting a wave. I've seen many a metalisation flaked but never in this striking way before. Which leads one to think some effect had taken place to cause it to appear like an oscillation of some sort might have been responsible.

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Posted : 07/10/2012 2:19 pm
Anonymous
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The distance of the waves is too large, it's a thermal effect, maybe too much solvent originally. Think scumble and crackle / hammerite etc and underlying surface heated when paint not fully dried out.

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Posted : 07/10/2012 2:49 pm
turretslug
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I can well see how "Barkhausen" might have occurred to Chris (oldticktock), though- if you put something like a Lurpack wrapper in a microwave oven, it gives a pretty "blue cobweb" tracery over the surface (before it catches fire!- probably not good for the oven, either, done too repetitively) One can imagine that a powerful high-UHF parasitic might cause nodal heating. I think that Barkhausen-Kurz is fairly low power and inefficient, generally, though doubtless a specific oscillator valve would optimise it.

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Posted : 07/10/2012 2:53 pm
Brian Cuff
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My grandmother, a physiotherapist, had a 400W 50MHz (ish!) diathermy set made in 1930 .................. notorious causes of interference to the later Band 1 TV service

It was these diathermy sets which were the answer to Germany's Knickebien bomber guidance system which fortunately, worked on about the same frequency as the diathermy sets. This meant that there was a plethora of small transmitters which could be modified to interfere with the beams! Such a simple solution.

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Posted : 07/10/2012 3:14 pm
Anonymous
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if you put something like a Lurpack wrapper in a microwave oven, it gives a pretty "blue cobweb" tracery over the surface (before it catches fire!

800W of 2500MHz does all sorts of exciting stuff. Students have probably tried most of the possibilities and made videos. But the visible wrinkling is thermal. Sparking is high electric fields.

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Posted : 07/10/2012 3:33 pm
Refugee
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I retrieved some NOS EF37As back in the 1980s and the screening was stuck to the inner wrapping of the boxes due to rain.
I would go for the set having got damp at some time.

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Posted : 07/10/2012 4:26 pm
turretslug
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Yes, as techie students in a shared house about 25 (gulp) years ago, we were only too keen to try all sorts of metallic/metallised objects in the expensive new toy! I think that taco shell wraps gave best results but Lurpak wrapper was dependable. I had the idea of trying a fresh coconut but none of us dared... Around the same time, my grandmother comprehensively burned out the microwave we'd recently bought her, the diathermy set was obviously made of tougher stuff. Sadly, it went to land-fill in a house move, I found out just hours later but it was too late. :roll:

I'm intrigued by http://www.r-type.org/addtext/add006.htm- though I don't know how I'd have found transient B-K if it had been present. This powerful P-P oscillator had a common grid resistor like a chunk of bar fire element

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Posted : 07/10/2012 4:36 pm
Anonymous
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Barkhausen-Kurz isn't anything to do with Barkhausen noise or Barkhausen anything else. It needed specially constructed valves and massive power to get a tiny amount of RF. You wouldn't get it on a 1930s Push Pull amp.

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Posted : 07/10/2012 5:03 pm
turretslug
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Ah, voila. In the past, I'd (incorrectly) assumed that when people referred to "Barkhausen effect" or "Barkhausen oscillation" causing a thin, maybe patterned line in a TV picture that they meant "Barkhausen-Kurz oscillation" or an RF spurio as a result of LOP circuit resonance driving LOP valve anode negative. So it results from changing magnetic stress in the transformer, rather than a transient UHF parasitic in the valve.

Well, you do indeed learn something every day!

Thanks, Colin.

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Posted : 07/10/2012 6:56 pm
Anonymous
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Wheatstone means something very different to musicians and electrical engineers. Same guy.

My Dad had a Wheatstone Concertina.

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Posted : 07/10/2012 7:31 pm
turretslug
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Brian, I think my grandmother's set was of too low power to be of interest (it was trolley-mounted for mobile physio use) but it seems that higher power hospital sets were comandeered for this purpose. The sets did not have HT rectifiers, the valves simply burst into oscillation when the supply went positve to the anodes- heating limb joints doesn't need "clean" or even continuous carrier. Indeed, it was said that pulsed power works better but, to me, that sounds suspiciously like a salesman's excuse for not spending money on rectifiers! This results in a harsh 50Hz pulsing with plenty of FM. I don't think that they worked very well as jammers but it warned the Luftwaffe that they had been runbled.

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Posted : 07/10/2012 9:04 pm
turretslug
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Thanks for the pointer, Jeffrey- I have a hunch that my father may have that book somewhere. I hadn't realised that use of requisitioned diathermy sets had an official term, I thought it was a very hasty and not very effective countermeasure- shouting boo at a goose, so to speak- until real jammers had been made for the job,

Isn't this forum great? This thread went from glueing on valve top-caps via the different flavours of "Barkhausen" to jamming enemy guidance systems. That would have earned a fusillade of tetchy warnings in other places,

Colin.

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Posted : 08/10/2012 12:29 am
Refugee
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If you want to see a photo with a non rectified valve in it i have got one and put it on for us all to ponder over.
I had to look for this thread again and the internet is bad this evening.

viewtopic.php?f=15&t=3094

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Posted : 08/10/2012 2:35 am
turretslug
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The Ally Pally TX must have well and truly shouted it out.

I recalled that the afore-mentioned diathermy set used STC 3C150A triodes and- yay!- found them at:

http://www.radiomuseum.org/tubes/tube_3c150a.html

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Posted : 08/10/2012 7:00 pm
Terry
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Ah, voila. In the past, I'd (incorrectly) assumed that when people referred to "Barkhausen effect" or "Barkhausen oscillation" causing a thin, maybe patterned line in a TV picture that they meant "Barkhausen-Kurz oscillation" or an RF spurio as a result of LOP circuit resonance driving LOP valve anode negative. So it results from changing magnetic stress in the transformer, rather than a transient UHF parasitic in the valve ...

If that is the case, why was a common 'cure' (it may only have shifted the frequency* to a non-interfering one ...) for Barkhausen-Kurz interference to take an ion-trap magnet and adapt it to fit around the line output valve ...?

* I did read a suggestion once that the valve geometry was later modified to move B-K oscillation from Band III to UHF. Of course, ten years later ...

When all else fails, read the instructions

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Posted : 08/10/2012 7:31 pm
Anonymous
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Maybe it wasn't "Barkhausen-Kurz oscillation" in the valve or "Barkhausen effect" on LOPT core but simple parasitic oscillation and the extra metal was simply screening or changing the capacitance, a primitive "neutralisation".

Usually oscillation is incorrect loading or lack of neutralisation. As the voltage varies on the scan, the gain and phase will vary. So you will tend to get parasitic oscillation at a particular area of the line time.

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Posted : 08/10/2012 7:47 pm
turretslug
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Not sure on this one at all now, Terry- and geting hazier by the minute...

I'd always taken it as read that when people talked about a thin vertical line towards the LHS of picture as being a result of Barkhausen "effect" or "oscillation" that it meant "Barkhausen-Kurz" oscillation. This fitted in with my understanding of LOP anode potential swinging negarive during scan, creating a possible circumstance for a burst of parasitic UHF oscillation. This was further flagged in my mind by the phenomenon being a vertical line, i.e. occurring at the same point in the line scan each time. However, it's all too easy to build supposition/confirmation bias on a house of cards and I can be as guilty of this as the next man.

I'm not a professional engineer, merely a dabbling hobbyist but I'm always aware that there are many on this forum who could knock my knowledge and experience in the field into a cocked hat. So it seemed with my assumption on the "Barkhausen" question, once it had been outlined that Barkhausen effect and Barkhausen oscillation were very different phenomena from Barkhausen-Kurz oscillation, it looked like time to change my assumptions,

Colin.

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Posted : 08/10/2012 8:11 pm
turretslug
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Michael, I was interupted by work (tut) while typing, our posts crossed!

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Posted : 08/10/2012 8:18 pm
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