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Old 7th Jul 2020, 9:14 pm   #1
AdrianH
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Default Cold cathode voltage stabilizers, OA2 and the like.

I picked up a couple of OA2's to play with, some work OK, some I am having an issue with. At the moment I am just trying them in the home made valve tester using it to provide voltage and adding an anode resistor to drop from 220 ish volts down to the 150 Volts. The ones I have problems with will fire, but not hold voltage, some repeatedly firing about every second some after several seconds when you think it has stabilized it will fire again.

So the question is really this do these cold cathode devices wear out just as normal valves do, is it a gassing issue or something else, perhaps I have a problem using the buck converters I use?

Lastly are there a range of other standard voltages, looking for 175 Volts?

Adrian
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Old 7th Jul 2020, 10:10 pm   #2
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

What size resistor?
You do realise they have a minimum operating current?
5mA when new - but I guess this may rise with age.

Also the strike voltage is 185V (when new) and again this may rise with age.
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Old 7th Jul 2020, 10:19 pm   #3
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Look at the spec for the likes of the OA2: it needs to be passing something between 5 and 30mA when it's 'lit'.

http://www.r-type.org/exhib/aaa0696.htm

so your series-resistor needs to be sized accordingly, If you use a high-value series-resistor this - coupled with stray capacitance across the OA2 - can produce a 'neon relaxation-oscillator' effect!

You also need "enough" voltage to fire the thing - they typically need something around 200V to fire reliably. This can be a problem in some circuits where the 'load' starts drawing current at the same time as the HT supply starts supplying: there were a few tricks involving a diode and additional resistor to provide an initial 'shove' to get the regulator lit. I can post an example of this if you're interested?
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Old 7th Jul 2020, 10:31 pm   #4
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

The resistor was a 3K0 Ohm, 3 Watt. I generally left the buck set for 220 Volts before inserting the OA2 in it's holder and then adjusted the voltage of the buck converter to provide a steady current of 15mA and left them for a while. On the good ones I could adjust the voltage down and up to cover the full range of 5 to 30mA but some had a propensity to keep striking, so this is why I am asking if the cold cathode devices can wear out and how?

When striking, it obviously hits a peak current so I did add a larger cap on the converter output to try and make sure the supply voltage was not dropping passed cut off but it seemed to make little difference. I do not have any issues with anyone just wish to understand if they fail or if it is me being daft!

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Old 7th Jul 2020, 10:53 pm   #5
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

You could use 2 85A2s in series for 170volts. You'd need to include a high-value resistor from the anode of the top valve to the anode/cathode junction of the two valves to make sure that the lower valve strikes. 85A2s are usually pretty reliable and only need between 1 and 10mA for operation. No capacitors of more than 100nF to be used in parallel or you will get oscillation.
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Old 7th Jul 2020, 11:19 pm   #6
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

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Originally Posted by barretter View Post
No capacitors of more than 100nF to be used in parallel or you will get oscillation.
Now that bit I do not understand as to why, it is the same on the OA2 data sheet one can not use more than 0.1uF or it may oscillate, just wondering how that happens.

When using these regulators to stabilize screen volts how can one not use extra capacitance to provide the capacitive shunt at g2?

Also the OA2 sheet I have has a circuit for two in series, only one resistor though?

Cheers

Adrian.
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Old 8th Jul 2020, 12:08 am   #7
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

It takes time to strike and extinguish ionisation

Considering the size of resistor it will normally be used with and typical current consumption downstream, that capacitance limit should put the oscillation frequency higher than the tube can support.

Despite the wet-finger assumptions, it seems to work.

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Old 8th Jul 2020, 9:13 am   #8
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

I am trying to picture how it could oscillate not having any feedback system and the only thing that comes to my mind is with a large'ish cap, on first switch on the cap is at 0 Volts and charges up exponentially through the anode resistor, the striking point is reached and it shorts the cap discharging it taking the stabilizer past its extinguishing point so the cap has to charge up again to striking point etc.

Not sure if that is why but I see the action just being faster without the cap, with my 3K resistor taking 75 mA on strike would drop the anode voltage to around 0 Volts.

I guess what you mean by the 'It takes time to strike and extinguish ionisation' is that the anode volts has to be back above 150 Volts before the ionised path dies away therefore keeping some current flowing.

Must test the faulty units again with more light on them or higher potential.

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Old 8th Jul 2020, 9:49 am   #9
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Quote:
with my 3K resistor taking 75 mA on strike
On strike the current is zero until ionised! (The figure of 75mA in the specification is a limit, not a characteristic)

Gas filled valves exhibit dynamic negative resistance - and that is where your feedback comes from by the way. Also, there is always some capacitance around - it can never be zero.

If you are finding any weird problems I would run them for a while at a reasonable current to see if they improve, but like any valve they can go low-emission. But also check that your power supply is not doing something funny when the current abruptly starts to flow.
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Old 8th Jul 2020, 2:42 pm   #10
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Yes, the oscillation depends on the 'hysteresis' between striking and operating voltage.

See https://en.wikipedia.org/wiki/Pearson-Anson_effect

The capacitor charges through the resistor, until it reaches the striking voltage. The neon then strikes, drawing a pulse of current from the capacitor but there is insufficient continuous current available through the resistor to sustain the ionisation in a steady state so the neon de-ionises, goes out, and the capacitor-charge starts to increase again... Wash, Rinse, Repeat as they say.


You get a nice sawtooth: the idea was used to provide a timebase in some early oscilloscopes (they generally used a rather high voltage supply coupled with a high-value resistor to majke the 'ramp' part of the sawtooth as linear as possible). A later evolvement of the same idea was the Thyratron, a 3-electrode gasfilled valve, that was used in some early TVs.
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Old 8th Jul 2020, 3:32 pm   #11
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

I did some playing with this effect some time ago https://www.vintage-radio.net/forum/...ad.php?t=31105


D
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Old 8th Jul 2020, 3:38 pm   #12
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Quote:
Originally Posted by AdrianH View Post
Also the OA2 sheet I have has a circuit for two in series, only one resistor though?
You don't have to use the second resistor but it may help if the second 85A2 fails to "strike".
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Old 8th Jul 2020, 4:46 pm   #13
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

As for gas-discharge tube oscillators, my Agaphone wire recorder has a little neon bulb that is used as a recording level indicator (I think you adjust the level so it just lights on the loudest parts). There is also a button that records an audio tone on the wire, this uses the same neon with an RC circuit as an oscillator.

I spent the afternoon looking at the power supply module in an old analogue XY plotter. This has 4 stabiliser valves in it (2 off 150B2, 2 of 85A2) to supply +ve and -ve voltages to the slidewires, position controls, etc. There are small-ish (0.5uF or so) capacitors in parallel with some of them, but I guess the designers knew what they were doing and it doesn't oscillate. We shall see when I get the rest of the unit working.
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Old 8th Jul 2020, 7:00 pm   #14
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Well thanks for the answers to my questions, it gives me more to think about, the OA2's will be retried again, but today I have been working on a PSU. Which when finished could be supplying one of the OA2's and a 25 Volt, 5 Watt Zener until I find a better alternative without spending more money.

On the subject of the OA2 having a negative resistance, I would not have thought of them like that. I recall an old style gun diode being negative resistance over a certain operation voltage range.
But as we are taking of the cascade of electrons at breakdown, as I have been calling the strike, I can follow.

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Old 8th Jul 2020, 7:07 pm   #15
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Quote:
Originally Posted by dominicbeesley View Post
I did some playing with this effect some time ago https://www.vintage-radio.net/forum/...ad.php?t=31105
D
Dom that looks like a very interesting project, not one I am about to jump onto at present but did you ever take it to it's end as you said you were thinking of progressing further with it. It has brought more questions about syncing oscillators to external references but that must be for another thread.

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Old 8th Jul 2020, 8:10 pm   #16
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

When I was in the Services, we were warned about breakages, especially the OA5. If one was dropped we had to rope the area off and obtain a kit from stores which comprised a heavy metal cylindrical container with screw on lid. Inside were tissues, gloves and tweezers and the procedure was to put on the gloves and use the tweezers to pick up all broken parts and place into the container. Next was to cover the area with a dampened tissue and carefully attempt to lift the small particulars into it then place into the container. Finally, we had to place tweezers and gloves into the container and firmly secure the lid. The newer OA5's at that time came with an Orange band around the base to warn of radioactivity, I still to this day handle any regulators like eggs and always keep them securely wrapped in tissue and bubble wrap. Has anybody else heard of this?
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Old 8th Jul 2020, 8:25 pm   #17
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Certainly there were some voltage=regulators made immediately before and during WWII that used a small amount of a radioactive isotope such as Krypton-85 to 'assist' the ionisation of the gas in the envelope.

https://www.orau.org/ptp/collection/...ctrontubes.htm
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Old 9th Jul 2020, 11:04 am   #18
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

I just noticed that OA2 uses radioactive cobolt-60 to promote clean striking.

That might be a problem because the half-life is only 5.3 years - so it will be very depleted resulting it less predictable operating (but on the other hand, much safer!)
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Old 9th Jul 2020, 11:42 am   #19
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

Quote:
Originally Posted by AdrianH View Post
Dom that looks like a very interesting project, not one I am about to jump onto at present but did you ever take it to it's end as you said you were thinking of progressing further with it. It has brought more questions about syncing oscillators to external references but that must be for another thread.

Adrian
I didn't I'm afraid I got sidetracked into magnetically deflected stuff. All my CRT stuff is still boxed up in a big pile in my garage somewhere. I really needed a higher voltage PSU to progress but that would have required a more safe setup than my usual mess of croc clips and chock blocks.

If your OA2's have gone tired due to their needing a radioactive source you might try putting a UV led near them to see if that tickles them into responding? I know in my experiments some neons didn't always start with the lights out! Or maybe dismantle a smoke detector and use the source from one of those (with appropriate care!). I never did try my geiger counters on the soviet regulators...no idea what they contain!

D
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Old 9th Jul 2020, 11:54 am   #20
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Default Re: Cold cathode voltage stabilizers, OA2 and the like.

The smoke detector idea is 'creative' but it wouldn't work I'm afraid. They use Am-241 which is an alpha emitter and the alpha particles wouldn't be able to penetrate the valve's glass envelope.

EDIT: If the radioactive material in the stabiliser really is Co-60 then that's primarily an emitter of relatively low energy beta particles, which would be somewhat absorbed by the glass envelope too. Unfortunately in the process they would generate bremsstrahlung X-rays and these are much harder to stop. So it's not impossible that a significant fraction of whatever radiation there is could be escaping from the device.

Cheers,

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