Eliminating Nixie Tube Cathode Poisoning, Bi-Quinary Digit Ghosting, and Heavily Oxidized Leads

IN-4 Nixie tube with odd and even anode connections for bi-quinary operation

I’ve been surprised how complicated driving IN-4 Nixie tubes with good results could be. The datasheet seemed simple, just apply 170v between to the anode through a bias resistor, and then short the cathode of the corresponding digit to the ground to get that digit to display. Isn’t that simple? The IN-4 also has a unique option to drive the digits in a bi-quinary fashion. This method minimizes your high voltage serial shift registers because you only need six registers to represent all 10 digits in a single Nixie. With a six-segment display, this equates to 36 registers versus 60. However, this also adds to the complications. It turns out that cathode poisoning, bi-quinary digit ghosting, lead oxidation, and weighted burn-in were all problems to solve. This blog is about the tips and tricks I learned getting these inexpensive IN-4 bi-quinary nixie tubes tuned and tweaked for good performance and longer lifetime.

Reversing and detecting cathode poisoning

Nixie tube cathode poisoning can be detected by monitoring the on-voltage and current through the Nixie tube while igniting the digit. As described by www.tube-tester.com, if a nixie tube is operated without exciting all the digits, the unused digits will often become coating by a highly resistive sputtered material. This material keeps the cathode from emitting electrons and thus shows up visually as a digit that has missing sections.

IN-4 with cathode poisoning on the “four” digit

Fortunately, this cathode poisoning can be reversed. By igniting a digit with this poisoning, the sputtered material that is keeping portions of the digit from displaying will break down and gradually the complete digit will be displayed.

IN-4 Nixie tube digit “four” without visable cathode poisoning

However, once the digit is fully displayed, the digit may still have cathode poisoning, not visible with the eye but detectable by monitoring the voltage and current. For the IN-4, experiments with approximately 20 IN-4 Nixie tubes have shown that a digit will still have some signs of cathode poisoning if the on-voltage is above 130v with 2.1mA of bias current. This is vastly different from the voltages shown on the typically found datasheet from the web. The datasheet shows on voltages of 150v and current at 2.5mA. I did not find this to be true for the course and fine grid IN_4 tubes I purchased.

In the image below is an IN-4 Nixie tube that shows a fully excited digit 3 with approximately 2.8mA of current. In this case, the on-voltage is 169v, well above the nominal 130v value and thus still shows cathode poisoning.

A fully formed IN-4 Digit “3” with sensed Cathode poisoning

The quickest way to remove cathode poisoning is to bring the bias current for a particular digit up to double the maximum operating current for a limited time and monitor the anode on-voltage. Under this higher current condition, the ignition voltage will be higher than 130v, even if no cathode poisoning is present so you can’t go by the absolute value under this condition. However, as long as the voltage is being reduced, then the cathode is getting more efficient. Do this for 30 minutes per digit, rotating between all the digits. After testing all digits, return the bias current to 2.1mA and check the ignition voltage. If this voltage is 130v, then repeat this process until the ignition voltage drops below 130v.

Eliminating all the poisoning from a Nixie tube may take a while, possibly many days. The 30 minutes is somewhat arbitrary but worked well to improve the poisoning for all the digits at once. When increasing the bias current on one digit to remove the contamination sputtered material will continue to contaminate the other digits, so a gradual 30-minute cycled process is recommended.

The IN-4 can be driven in a bi-quinary fashion, with a separate anode for the odd digits and a anode for the even digits. As a result, the procedure above can be done using these anodes.

For other types of Nixie tubes, the indications that cathode poisoning is eliminated is that the on-voltage voltage is approximately the same (i.e. within 10 v) for each digit.

Picking the optimum Nixie bias resistor

With a Nixie tube eliminated from cathode poisoning, the bias resistor is chosen so that all the digits are fully ignited. For the IN-4 tubes, digits 3,4,0, and 8 tend to need more bias currents than the other digits.

Slowly decrease the bias resistor so that the minimum current to display the complete digit is found. For the IN-4 and with the poison eliminated this should be around 2.0mA. As the bias current increases, you may notice that there will be a ghosting character also being driven. Driving the digit “3” will often get small ghosting of the digit “2”. Don’t worry about this ghosting for now, but continue to increase the bias current until all digits are fully displayed.

Circuit to fix bi-quinary digit ghosting

Driving the digit “3” with the Odd Cathode shows ghosting on Digit 2

The even and odd anodes for the IN-4 (pins 10 and 13) are placed on the front and back of the Nixie Tube. All the odd digits are located on the same side as the outer odd anode and all the even digits are located on the other side with the even anode. Since both an odd and even cathode is driven low for each digit, it is only up to the location of the anode and distance from the opposite digit that causes it not to light. With excessive bias current, as shown above, both the odd and even digits can be excited. When the “3” digit is being driven from the odd anode, the “2” digit may be excited. This extra digit is what is coined as bi-quinary digit ghosting.

Even, odd, and center anode locations in the Nixie tube

By adding a low impedance path for excess electrons to flow through the center, unused anode (pin 4), this ghosting can be eliminated. This center grid will screen and split the Nixie tube and isolate the odd and even digits. With the proper choice of the bias resistor and elimination of cathode poisoning, only one or two odd/even pairs may show ghosting. For the IN-4, the 3 often shows a “2” ghosting and the 4 often shows a “5” ghosting. The following circuit can eliminate ghosting for the 2. The value of the resistor is just enough to drop its voltage below 110v so the even digit does not ignite. A similar resistor can be used for other ghosting digits.

Adding a resistor to the center anode to eliminate ghosting on the “2”.
Elimination of the “2” ghosting by using a center connected anode resistor

Weighted burn-in for improved lifetime

A weighted burn-in procedure keeps the IN-4 Nixie tubes cathode poison free. For the iN-4 Nixie tubes, the 0,4,3, and 9 are susceptible to cathode poisoning if the digits are displayed even the same amount. In this case, it is important to adjust the display time to offset this nonsymmetrical behavior. This is done during a burn-in procedure each evening. During the ETA Nixie Tube Clock operation, every evening from eight to midnight and when no movement is noticed, the Nixie tubes will go into a burn-in procedure where the digits will be displayed continuously for a given number of minutes. Digits 0,3,4 and 9 have the longest times, digits 6 and 8 will be the next longest, and 1,2,5,7 are burned in the least. Since the ETA Nixie Tube has a motion sensor, this burn-in procedure is not often noticed by the user since it resumes normal operation as soon as a movement is detected.

# Burnin Times
BurnInMinutes = 20
# STart time Hour (8pm)
BurnInStart = 20
# stop time hour (1am)
BurnInStop = 1
# Digit Test Order.   
DigitsToTest = [0,3,4,9,1,2,5,6,7,8]
# The test time is BurnInMinutes when weighted at 1.0
DigitsTimeTest = [.8,1.5,1.5,.8,.1,.1,.1,0.3,.1,0.3]
DigIndex = 0
SecIndex = 0
BurnInSec = BurnInMinutes*60
TimerStopped = False
ETATimerStopped = False
TimeForBurnIn(BurnInStart,BurnInStop,DigitSec)

Steel wool rescues oxidized Nixie pins

Steel wool rescues Nixie leads

For one of the ETA Nixie Clocks I recently built, I purchased fine grid IN-4 Nixie tubes manufactured in 1971. They looked great but were impossible to solder. The cause was that the leads seemed to have 48 years of oxidation on them. After trying several methods, simple steel wool worked the best. If you don’t have steel wool, individually scrapping each lead with a wire cutter worked, but is slow and time-consuming. Rubbing the leads with steel wool was vastly quicker. Simply take the steel wool in one hand and the Nixie tube in another hand and rub the leads through the wool. Concentrate on both sides of the leads and watch as you see the lead color change from dark gray to less dark gray.

IN-4 Nixie Tubes with a 1971 date code and shiny leads after using steel wool

Recommended improvements

Bi-quinary driven Nixie tubes have nice features by requiring a low bias current and minimized shift registers, but also are sensitive to digit ghosting and require a good level of cathode poison elimination. With the procedures and circuit tricks described above, this was successful for the ETA Nixie Tube Clocks. Good luck with your testing. Let me know if you have any comments or your own improvements.

References

https://wp.me/p85ddV-Ka

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