My vote is to play it safe and find or make a supply that gives you 2.5kV.
I was going to recommend a series dropping resistor, along with the 3kV
supply, but I dont think that is a good idea. The reason is that while the
resistor will provide a voltage drop at *nominal* current, it wont do
anything at low or zero current. Given NIMO tubes are expensive and rare, I
wouldn't risk applying higher-than-spec voltages, just in case it might do
something unexpected. For example, I've seen tin whiskers develop in laptop
CCFL driver boards. Literally a microscopic metal hair that grows in a high
electric field. Even at lower voltages, I've seen evidence suggesting this
happened in my IN-1 nixies.
High voltage zener diodes (~160V) might work but be very careful about
leakage current; you may end up with the same paranoia I have about using
resistors mentioned above. If you do use zeners, I would use a 'bleeder'
resistor from NIMO-anode to GND to drain zener leakage current, such as a
few 22Meg in-series.
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