Dieter's Nixie- Tube Pages
Tutorial: Cathode Poisoning Reversal

This Page describes my experiences with cathode poisoning,
using a Rodan CD47 tube as an example.

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What is cathode poisoning and how does it occur?

Most nixie tubes have more than one cathode. If a cathode is illuminated, material is sputtered from this active cathode onto the glass envelope and the inactive cathodes. You've probably seen darkened nixie tubes with a gray or silvery coating on the inside of the glass. This is the extreme case of sputtered material on the glass at or near the end of tube life. If a cathode is not used for a long time while others are lit, the coating on the inactive electrode(s) gets thicker and thicker. This coating is highly resistive, so if it gets thick enough, it prevents the inactive cathode from glowing. Then if the inactive cathode gets switched on, parts of it will be dimmer or totally dark. Different tube types have different vulnerability to cathode poisoning, but normally it takes thousands of hours until it becomes a problem.

Example: A CD47 nixie tube with cathode poisoning on the "6".

The picture shows the illumination at rated current:

How to reverse cathode poisoning

Drive the poisoned cathode with the rated current.
Then turn up the current (for example, using a variac as your power source) while watching the regions where the cathodes are darkened.
If a formerly-dark region is completely illuminated (even if still dimmer than the rest of the cathode), the current is enough.
If the lead-in wire glows, go back to a lower current until it stops glowing, or it will also glow later at normal current. Even if part of the cathode remains dark at this current level, healing can take place - the dark region(s) will gradually shrink.

Here we see Dieter's tube at the start of the regeneration process:

Use values for the cathode current from about 2 to 10 times higher than the maximum rated current for that tube type.
The larger the tube, the lower that factor.
For the CD47 shown, Dieter used approx. factor 2 (rated: 32.5 mA max., he used 50 to 60 mA)
The darkened region(s) will become smaller and/or brighter, the longer you let the tube bake in this condition.
This is because now the coating is being sputtered away.
It can take minutes to hours, depending on the thickness of the coating. For example the "6" of the CD47 shown took 4 hours until it was completely regenerated.

Here are incremental images of the healing process:

20 mA after 1 hour:

going back to "bake" at 55mA:

20 mA after 3 hours:

going back to 50mA:

after 4 hours, the "6" is fully illuminated at low current (20mA):

at high current also (35mA):


Now you can go to the next digit.

How can you prevent cathode poisoning?

Exercise inactive cathodes (occasionally cycle the tubes through all cathodes).
This will keep cleaning the coating off before it becomes too thick.
Example- Nixie Clocks: Rotate the numbers like in a slot machine (a.k.a. one-armed bandit or fruit machine).

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