The IV-17 (“NB-17”) is a medium-sized sixteen-plus-two segment VFD tube manufactured in the Soviet Union by Reflector, known today as Sovtek. Unlike the IV-12 seven-segment tube, the IV-17 can display all letters of the English alphabet in addition to the numbers zero through nine. Sixteen segments are dedicated to displaying characters, while the remaining two are used for the left and right decimal points. The character set used in the photos below is based on the one suggested by the datasheet of the Burroughs B-7971 sixteen-segment Nixie tube.
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VFD tubes usually contain multiple phosphor-coated segments, each positioned in front of an anode. Electrons emitted from the cathode are accelrated towards positively the charged anodes, striking the phosphor and causing it to light up. By combining multiple segments, digits and various other characters can be displayed.
The IV-12 (“NB-12”) is a large seven segment VFD tube produced in Soviet Union by Reflector (known today as Sovtek). It closely resembles the IV-11 but lacks a decimal point and has solid pins instead of flying leads. This allows the tube to be installed in a common 10-pin socket still being produced in China. Like most VFD tubes, the IV-12 includes a grid that can be negatively charged to interrupt electron flow without disabling the filament. This capability allows for multiplexed operation (see schematic shown below) and effectively turns the tube into a triode.