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Old 31st Mar 2018, 7:00 pm   #1
eddie_ce
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Default Question about cathode poisoning

Hi and happy Easter to you all,

I hope I have got this the right way around:

Leaving a valve running with a heater supply and no anode voltage over a prolonged period can lead to cathode poisoning.

My question is;

Is running a valve with no heater supply but with anode and screen voltages present (approx. 250V) harmful? If this is OK it would make the switching arrangements for my current project a lot easier.

Thanks as always for your advice.

Regards
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Old 31st Mar 2018, 7:59 pm   #2
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Smile Re: Question about cathode poisoning

Hi,
Personally. I can't see a problem with that. If the heater is cold, then there won't any thermionic emission. Therefore, the valve will just appear as an open circuit.
Cheers, Pete.
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Old 31st Mar 2018, 8:25 pm   #3
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Default Re: Question about cathode poisoning

That is my thinking, I was just wondering whether there were any pitfalls I had not thought about.
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Old 31st Mar 2018, 8:25 pm   #4
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Default Re: Question about cathode poisoning

I am sure that Pete is right.
Didn't some portable valve sets just switch the heaters off with the on/off switch? I'm not sure about TV sets with a TV radio switch, on my Dynatron all of the TV only valve heaters and the CRT heater are switched off when on radio certainly but I think maybe the HT is switched as well.

Rich.
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Old 31st Mar 2018, 8:28 pm   #5
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Default Re: Question about cathode poisoning

Most battery sets switched only the heaters off. There was then no HT current flowing and I've never heard anything said of resulting problems. An awful lot of sets have waited in this state for an awful lot of hours.

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Old 31st Mar 2018, 9:07 pm   #6
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Default Re: Question about cathode poisoning

Maybe the issue would not be cathode poisoning (interface resistance) but more one of possible cathode stripping when full HT current is switched on and the cathode is at a cold temperature and not emissive?
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Old 31st Mar 2018, 10:56 pm   #7
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Default Re: Question about cathode poisoning

The valves might well be fine. But if you leave the HT on with no current flowing through the valves then the DC voltages across the passive components will change.

If you imagine an HT rail consisting of a series of dropper resistors followed by smoothing capacitors, for example, then under normal operating conditions the current flow will mean that the voltage after each dropper resistor will be lower than before it. So at the 'far end' of the rail you could normally get away with using a lower voltage smoothing capacitor. But if there is no current flow then there will be no voltage drop and the full HT voltage (maybe even increased because there is no load on the HT power supply) will appear across each and every smoothing capacitor. Woe betide any 350V capacitor which suddenly finds it has 480V across it. The same can be true of interstage coupling capacitors which, without any voltage drop down the anode load resistors, will now see the full HT too.

If you want a concrete example then look at the circuit for the Radford STA25 amplifier http://www.ampslab.com/SCHEMATICS/RadfordSTA25.gif. On load the HT before the smoothing choke is 445V so you can be sure that it will be higher than that with all the valves cold. With no HT current flowing, except for the few mA in the R9+R32 and the R5+R13+R14 chains, pretty much the full HT will appear across any capacitor which is connected, directly or othwerwise, to HT at one end and ground at the other. This includes C2 which is only rated at 350V and which will have lived most of its life with just 100V across it.

This also explains why it can be a daft idea to run any piece of kit with some of the valves pulled out.

Cheers,

GJ
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Old 31st Mar 2018, 11:48 pm   #8
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Default Re: Question about cathode poisoning

Quote:
Originally Posted by GrimJosef View Post
This also explains why it can be a daft idea to run any piece of kit with some of the valves pulled out.
Or, conversely, why it would be a good idea to rate all components so that they remain in their ratings with all or any combination of valves pulled-out.

Mind you, in this age of toob-rolling, some wannabe pundit is going to try anything in any socket he can get the pins to fit. I'm not sure how we can prevent that being damaging. I know they only want to hear how it sounds, but sometimes the sound they hear is "PHUT!"

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Old 1st Apr 2018, 11:48 am   #9
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Default Re: Question about cathode poisoning

Thanks folks,

my circuits is:
230V Transformer primary; B300C125 selenium rectifier; 47R surge limiter; 2 x 32µF @ 500/550V (1 reservoir and 1 smoother) so I wouldn't expect any problems with excessive off-load voltages exceeding ratings.
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Old 1st Apr 2018, 12:21 pm   #10
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Default Re: Question about cathode poisoning

What's your transformer secundary?
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Old 1st Apr 2018, 12:49 pm   #11
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Default Re: Question about cathode poisoning

Quote:
Originally Posted by Radio Wrangler View Post
... Or, conversely, why it would be a good idea to rate all components so that they remain in their ratings with all or any combination of valves pulled-out ...
There's a good case for that, not least given that valve heaters do sometimes blow. But money talks I suppose.

VB
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Old 1st Apr 2018, 12:58 pm   #12
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Default Re: Question about cathode poisoning

sorry, a senior moment

my last post should have read:

transformer secondary 230V - doh
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Old 1st Apr 2018, 1:13 pm   #13
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Default Re: Question about cathode poisoning

There was also a phenomenon called cathode interface that Tektronix discovered back in the day. This is different to cathode poisoning. Described in Tektronix Service Scope, August 1960 http://w140.com/tekwiki/images/4/4a/...3_Aug_1960.pdf

The big valved Tek scopes also had a delay of 30 seconds or more between firing up the heaters and turning on the HT. This was done with a thermal delay device in a valve body.

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Old 1st Apr 2018, 8:16 pm   #14
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Default Re: Question about cathode poisoning

I think a bit depends on the valves and your switching!

If they are power valves, and you switch off the LT but leave HT on, and you do this switching a lot, then there will be a lot of occasions when the cathode is cooling down while current is still being drawn.

A cool or underpowered cathode has very little space charge, so positive ions get to bombard the cathode directly rather than being neutralised by the space charge while still a safe distance away. Only when current has virtually ceased, so no ions are being created, will this cease.

Doing this occasionally would be one thing; but often is another. The cooling down is more prolonged than the warming up!
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Old 1st Apr 2018, 10:49 pm   #15
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Default Re: Question about cathode poisoning

Tommy Flowers persuaded Bletchley to go ahead with his design of Colossus. Their initial objection was the lack of reliability of a machine that contained so many valves. Flowers replied to the effect "they are only unreliable if you switch them on and off. If you leave them switched on all the time, it will be just fine".

The Colossus rebuild at the National Museum of Computing is brought up slowly on huge variacs to prevent thermal shock from damaging the now rather rare valves.

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Old 1st Apr 2018, 10:59 pm   #16
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Default Re: Question about cathode poisoning

Surely cathode poisoning is not the same as cathode interface? The interface layer occurs when a cathode is hot but no cathode current flows. Poisoning occurs when the cathode is warm enough to emit some electrons, but not enough to create a sturdy space charge to protect it from positive ion bombardment due to remaining gas. These two effects take place in different places: interface is between the underside of the oxide layer and the underlying metal (usually nickel); poisoning is on the outer emissive layer of the oxide.

Anyway, cold cathode with full anode potential etc. is usually fine. The conditions to avoid are hot cathode with no anode potential (causes interface), and lukewarm cathode with anode potential (causes poisoning). Safest of all is all off or all on.
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