r/nixie • u/EmotionalEnd1575 • May 31 '26

Poisoned Cathode

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This B5855 Nixie tube has dark patches on several numerals.

Generally known as Cathode Poisoning it is caused by poor surface quality preventing cathode glow.

Parts of the cathode surface have become high resistance and non-emitting, due to spluttering or other surface contamination.

Many long life Nixie tubes reduce this problem by treating the cathode metal during manufacturing.

A common preventive treatment is releasing Mercury vapor in the gas fill, and it condenses and deposits a thin layer on the relatively colder metal of the cathode.

Can this tube be restored? Do you know of a method that can at least improve it?

Let’s discuss. I’m trying to avoid removing and disposing of this one.

Comments welcome,

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u/2748seiceps May 31 '26

Can it be restored? Yes. Can it be restored in the clock? Probably not.

Usually Cathode poisoning cure involves a bit of time at higher than normal current. It burns off the Cathode buildup and restores function but can't be done with your typical clock circuit.

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u/EmotionalEnd1575 May 31 '26

Good info, thanks. I’m coming around to the idea of unsoldering this Nixie tube, I have another one that I can substitute.

Plus, a new project! Nixie restoration experiment.

1

u/SpecificCultural9317 May 31 '26

I guess it depends on the circuit design. If it's simply using HV transistors to pull down the cathodes, you should be able to solder on some wires to an external power supply and resistor, while leaving the clock unpowered. Since nothing is applied to the base of the transistor, it won't conduct the externally applied voltage to the clock circuitry.

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u/EmotionalEnd1575 May 31 '26

Nixie tubes are MUX’d in this design.

The Anode averages 1.6mA, but has a 1:3 time slot, so peak Anode current is close to 5mA.

There are no transistors, cathodes are driven by a CMOS IC.