Well, I'm sick and tired of assumed 'self-evident' conclusions, which on closer investigation turn out to be lukewarm only.
Such assumptions are currently quite popular here in Germany, e.g. concerning climate protests and effectiveness of CO
2 reduction measures.
So instead of such speculations, I made some real physics experiments on the worn out and new neons.
Setup is simple, a variable 0.. +/-200V source, 100k current limiting/measuring resistor, across that the Dave-O-Meter, in series with the neon bulb, either exposed to the illuminated room, or inside a light-tight box.
I cranked up the voltage, until either the neon turned on, or the DMM showed a voltage drop.
The old and new neons were both specified to have an ignition voltage of < 135V
dc. In addition, the old NE2U contained some radio active substances to mitigate the dark effect, that is to let the neon ignite even if these are used in total darkness, like in this chopper application.
The old neon at its worn out bar ignited at about 120V, likewise inside the box, but the emission was really bad and irregular.
Using the pristine bar, by reversing the voltage, it also ignited at about 117V with light shining on it, and it really showed a very regular glow around the bar, but at > 150V / 1mA only.
Inside the box, the neon did not ignite before 170V, so very probably not inside the 845AR any more.
Therefore, these bulbs were really worn out, probably the neon gas, and/or the radioactive substance.
Now I tried that same experiment on the Barthelme neons.
With lights on, they ignite already at ~ 65V, independent from polarity, and sample. Only 500µA @ 110V were needed to get a nice and full glow.
To my big surprise, they even ignite inside the box at exactly the same voltage as outside, so they show absolutely no dark effect.
That's maybe due to new gas mixtures, like adding some Krypton gas.
So that's the reason, why this replacement really works, and why I now really love such optical choppers with neons inside.
Frank