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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Jun 2008, 3:45 pm   #41
kalee20
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Join Date: Feb 2007
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Default Re: Cathode poisoning in valves.

Thanks also, Duke.

I'm also convinced that damaging ion bombardment is going to take place principally with anode current flowing (my post 30 above) and not with zero current, because of there being no high energy electrons to clobber gas atoms and ionise them.

Your theory about the emissive layer evaporating and, in case of zero cathode current, not being replenished because there's no current within the cathode bulk to drive the diffusion towards the surface, makes sense, but it would be great to have some numbers to back this up. I can't see a milliamp or so of current making a great deal of difference here. But, I've got nothing better to offer!

Anyhow, if you are right, then a valve with heater energised but no cathode current
will deteriorate more than one with current flowing. But, at some time in the future, it will potentially be capable of being rejuvenated better than the valve which has had current flowing - because, in the first valve, the active ingredients will still be dispersed within the cathode material, whereas the second, they will already have been drawn to the cathode surface and evaporated out to a greater extent.

Incidentally, I do recall a rejuvenating procedure for both thoriated cathodes and barium/strontium oxide cathodes. Unfortunately, I don't have the book any longer, but I'm sure it was published by the RSGB. The procedure was:

1: Energise the heater at 2 - 2.5 times rated voltage for 15 seconds with no other electrodes connected (this evaporates off the surface layer, together with any impurities caused by combination of the surface with gas ions)

2: Run the heater at 1.5 times rated voltage for 1 minute, with normal electrode voltages applied (this encourages diffusion of active material to the surface)

3: Run the heater at normal rated voltage for 5 minutes with no other electrode voltages applied (this stabilises and consolidates the newly-formed surface layer).

The precise times and voltage multipliers above were different for the thoriated cathodes and the oxide cathodes, but frustratingly I can only remember the above, which could apply to either!

There was a caveat that if the cathode is depleted of active material, the above process will evaporate the existing surface and there won't be anything left to diffuse from the bulk material, so the last state will be much worse than the first.
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Old 13th Jun 2008, 10:34 pm   #42
pentode12
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Default Re: Cathode poisoning in valves.

Isn't cathode poisoning caused by a valves heaters being under run !!??
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Old 14th Jun 2008, 9:44 pm   #43
kalee20
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Default Re: Cathode poisoning in valves.

Well, cathode damage certainly occurs, whether we call it poisoning is just a question of terminology.

If the cathode is under-run (ie below the specified heater voltage, the temperature will be low, and this massively reduces emission. So, the space charge will be a lot smaller than normal.

If anode current is flowing, any gas in the valve will be ionised (as described in earlier posts), and the positive ions will be attracted to the negative cathode. But, whereas normally they would be neutralised by the cloud of electrons surrounding the cathode, and then stop being attracted, with the reduced space charge they will get a lot closer, accelerating all the time. So cathode bombardment increases lots!

This is why it's a bad idea to draw anode current with an under-run heater. And, for some gas-filled valves (mercury vapour rectifiers), it's absolutely essential to delay applying electrode voltages until the cathode is thoroughly up to temperature.
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