My tubes were over driven by 2X-4X compared to the maximum values in the B7971 data sheet: HV=-195V, cathode resistors=430ohms, MUX duty cycle = 12%. After running 24/7 for 40+ years, the tubes are very slightly dimmer than the tubes I had kept as spares with a slight color shift. (You can see this color shift in the PEOPLE LIKE NEON movie at the top of this news group, where tube #3 has some pink in addition to the orange glow. My spare tubes also glow slightly pink, while all the old tubes are uniformly orange.) After 40 years, the only significant change was darkening on the inside of the glass and on the back plate under the segments that I used (in a seven segment display mode). There were two clocks, and the darkening on the glass in each set was uniform, but it was more pronounced in one of the sets; I suspect this may have been exacerbated by the amount of overdrive.
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