Transistor switch for VFD tube grid
I did some tests, and I found the main weak point of this design:
the voltage on R_GRID depends on the current flowing through the grid. And unfortunately, this current is not always 0.85mA, as stated in the documentation. It can vary a lot (yesterday I measured 0.22mA for example) and also depends on the filament voltage and current. So although this design is valid from a theoretical point of view, it isn't very compatible with the real world.
I opted for a more flexible solution, which uses a 12V zener at BJT the emitter.