IN18 (1992) tube changing color in Blue Dream - on its way out?

[Piano Forte]

I've had a set of NOS 1992 IN18 tubes in a Blue Dream clock for about 10 months with the anti-poisoning cycle increased from 5 seconds, every 5 minutes to 10 seconds every 3 minutes. Since the Blue Dream has rigid "cup" style sockets, I loosened them up a bit via a tool of the same diameter as an IN18 pin I made, and I have not rotated the tubes, based on the rationale that they are late production and cycled very frequently (also shut down 7 hours a night). Nevertheless, tube #1 (which activates only the "1" at 12,11,10) developed cathode poisoning on the used digits. I also noticed during the Blue Dream test cycle, that the 5, for example (which had poisoning) would light partially at first, then fully). I removed the tube and applied 10ma at 170V and cleared all digits. To my surprise this tube (now back in the clock) is noticeably more red in color than the others. Is this a sign of outgassing? (possibly due to the rigid cup design of the Blue Dream?). Finally, many of the digits of the problem tube are also less crisp looking than the other 5 tubes.

Here is a photo, note how red the top of the 5 is, and how "fuzzy" the lower portion is of the left tube compared to the right.

[gregebert]

Based on your info, gas-leaking theory sounds reasonable to me.

I have a fourteen-tube clock with IN-18's, and they are similar date-code (1991), and similar age (started service Feb 2016) so it's a good baseline for comparing. I start the clock around 7AM in he morning and it runs all day. At 11PM, it goes into a depoisoning run for 1 hour, then turns off (same as your 7-hour nightly shutdown). The 'static' digits (day, month, year) cycle all 10 cathodes. Hours digits also cycle all-10 positions. Tens minutes and tens seconds cycle 6-9 because those are never displayed during the day. The units minutes and units seconds are turned off because they already cycled 0-9 during the day.

My tubes are socketed, but I use socket-pins with very low insertion force, and they are prefitted to the tube then soldered to the PCB to minimize stress. I dont rotate the tubes; they are serial-numbered and assigned to specific sockets.

So far, none of my tubes show any noticeable degradation.