Nixie tubes that are run with too low driving current are very susceptible to cathode poisoning. Glowing cathodes sputter. The sputtering is what causes the gas to glow. The reason for running within spec range is that when the cathodes are properly driven, the cathode that is operating will be driven hard enough so that it will drive off impurities that have been deposited on the surface of the cathode. Driving the cathode at too low current will not allow the normal operation to drive off cumulative deposits from sputtering of other cathodes.
These cathode deposits can occur from: · Long term storage when impurities in the tube will contaminate the cathode surfaces. This is why tubes that are NOS and in boxes since 70’s and 80’s will often need to be run for some time in order to get them to completely glow on all cathodes. · Running at too low current means that the cathodes cannot properly “clean” themselves from deposits of adjacent cathode sputtering. Higher currents cause higher excitation of the gas and “heating” of the cathode in operation. The higher excitation and heat cause the contaminants to be driven off of the surface of the cathode in operation. Running a tube at currents higher than the normal ratings will cause more overall sputtering but cathodes can clean themselves better. That is why higher currents are used to “de-poison” or “clean” a cathode which does not glow completely due to the contamination on the surface from sputtering or impurities in the gas. The downside to high current operation is that the tube life will be shortened significantly if driven too hard for long periods of time. The conclusion is that reducing the drive current below the tube spec is NOT the way to prevent cathode poisoning, it will actually cause the problem. Also, pulsed current drive requires higher current levels during the pulse than continuous drive to achieve the same brightness. The debate is in the physics of operation and whether the pulsed current times duty cycle has an equivalent effect to constant current of the same computed value. There is a case to be made that a pulsed tube actually is less susceptible to cathode poisoning but there is insufficient data regarding the long term life of the tube for pulsed vs constant current. I have read conflicting opinions over the usable life of the same nixie using the different drive techniques for operation. My experience is that a nixie tube operated in spec, with good seals AND mercury doping can run for more than 30 years, as evidenced by a clock that I built in the early 70’s which ran continuously for over 35 years without tube failure. The brightness was lower after 35 years and some of that was attributed to silvering on the inside of the glass envelope after many years of operation. From: neonixie-l@googlegroups.com [mailto:neonixie-l@googlegroups.com] On Behalf Of Tomasz Kowalczyk Sent: Thursday, June 1, 2017 3:45 AM To: neonixie-l Subject: Re: [neonixie-l] Pulsed DC vs direct DC in cathode poisioned There is no "economical" model of a true oscilloscope. You might use some sort of audio card oscilloscope, but with such voltages you have to be very careful not to burn your PC - for example, use a voltage divider made of 330k and 1k resistors (1k between GND and audio card input and 330k from audio card input to HV). This + some software (there are many audio card oscilloscope programs) should do the work for frequencies from audio range (20Hz-20kHz). I would also place two standard diodes in series, anode to audio input and cathode to ground - cheap and rather reliable way to produce a ~1,4V voltage limit. Datasheet of IN-18 is hard to understand for me - I can read cyrillic, but barely understand language alone, so I make alot of assumptions from my native, also slavic language - but as far as I can read, allowed continous current is 4-7mA and average current for pulsed operation should be 2-4mA (specified for 50Hz pulses). If the current you have measured in your clock is a true average value of 5mA, then it is out of specs. Can you check clock PSU voltage, anode resistor value, anode voltage drop (avg) and number of tubes used in clock? I bet doubling anode resistor value would fix the problem, as this starts looking like it isn't a problem with pulsed vs direct operation, but a design which is out of datasheet values for the tube. This will of course change the brightness, so you might want to use some other resistor - 30% bigger value than the original should be also okay. Normally pulsed operation doesn't do anything bad to the tube, unless it is out of specs (too high or too low current*). Many smaller tubes in later designs were multiplexed, for example calculators - I've seen displays of one east german and one russian calculators and they both were multiplexed (Z574M and IN-14 tubes). *I am not sure if too low current can hurt a nixie, but I believe it might increase sputtering - I have no data to back it up, it is just my strong feeling. W dniu wtorek, 30 maja 2017 16:15:18 UTC+2 użytkownik Trumpeter napisał: Slot machine effect every 10min. Tubes were NOS but I wonder if I got a bad batch. I guess I should buy a scope, any reccomendations for an economical model that will get the job done? -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/0e39db9b-b7f3-4370-8f70-0c22c72d5981%40googlegroups.com <https://groups.google.com/d/msgid/neonixie-l/0e39db9b-b7f3-4370-8f70-0c22c72d5981%40googlegroups.com?utm_medium=email&utm_source=footer> . For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. 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