NO!!!! NO and NO!
If a blue spot appears, it has NOTHING(!!!) to do with a bad tube or outgassing or something.
Read again my description where it comes from.
The low voltage drivers are the problem.
Also if tube x work with the driver, tube y may not work.
Of course the amount of mercury is different in the tubes.
But the more mercury, the longer the tubes life.
So your tube with the blue spots IS BETTER(!!!!) than the others you have and it will work perfect without blue spots, for a long long time (if driven correctly)
So it is as I said:
IF A BLUE SPOT APPEARS IN YOUR IN-18 TUBE THE CIRCUIT IS WRONG, NOT THE TUBE!
Sorry I was distracted when I wrote that up... Should have re-read it.
Makes sense that the tube has a differnt Mercury content. I'll compair the tube dates...
Maybe this is a good reason to move to my TaylorEdge kit. And move off the Tubehobby kit. TaylorEdge kit has HV driver on all the pins. Tubehobby has a Multiplexed setup and the clock does hum so maybe that is what I'll do to resolve that issue.
I'm reviving this topic. because it comes up over and over again, and also has important contributions from old masters.
The blue spot, as mentioned earlier here, is caused by undriven cathodes floating at a voltage above what the drivers can withstand without breaking down, causing leakage and resulting finally in the current flow being altered. This alteration of current flow causes "hot spots", and the blue glow.
The "blue dot" is not a defect in the tube, it is a defect in the driver!
The correct solution is to use a driver that can handle the floating voltages without breaking down! The 74141 and K155ID1 are not really suitable for driving IN-18s. Instead use more modern high voltage drivers (e.g. HV5622), or a discrete transistor circuit.