Isn't the intention of the bias to bring the non-lit elements to a potential where they will not glow (60v giving 170-60=110v, not enough for a glow), if left to flap in the breeze then they adopt a potential which reflects their nearest element?
Here's an interesting graph... http://www.decodesystems.com/re-how-figure-5.jpg
And the full treatment of how much glow is objectionable etc! http://www.decodesystems.com/re-how-nixies-work.html
Is the term "pre-bias" used for two different concepts? I found this one, which Dave is using, too, for protecting the FET, because the voltage can't go higher than 50V:
But when the FET is turned off, the cathode for the digit is floating. I guess the best solution would be a push-pull output, with the full 170V when on and 0V when off? Because if the FET has some leakage, a current can flow, like the website NivagSwerdna cited describes and this could cause some glow of the digits that should be off. If I understand it correctly, the 50V pre-bias in the circuit above doesn't help with the glow, because of the diodes and the FETs can still leak and pull-down the voltage lower than 50V even when switched off.
I did some tests with a Z560M Nixie tube (was cheap on eBay, but doesn't look as nice as other Nixie tubes, which can cost a lot) and a BSS131 MOSFET (nice chip, allows to control the gate with 3.3 V logic level voltage). I connected two digits, 5 and 3. When digit 5 is on, I can measure 54V at the drain of the turned-off MOSFET for digit 3. But maybe this is my multimeter? It has 10 megohm input impedance (a BM257S) and I measure the same voltage when I disconnect the cathode from the MOSFET. When I add a 1 megohm pull-up resistor from 170V to digit 3, I can measure 130V (resistor at the anode side is 22k).
I don't know what this means. Do I need a pullup-resistor?
This is the datasheet of the MOSFET. I think I don't need the bias for protecting it, because Vds is 240V (and it still works after some time testing it), but I can't find the leakage current in the datasheet when it is off. I think it is very low, because the voltage doesn't change when I connect the MOSFET drain to the cathode. But maybe a high pull-up resistor like 10meg would be still useful, just to make sure that the cathode has a clearly defined high voltage level when the MOSFET is off?
Sorry for dumb questions, I don't know much about analog electronics, but these Nixie tubes are fascinating.