pwillard:
The 74141 is an open collector output with Zener protection diodes. When the outputs are OFF... no current flows through the TTL IC gate output transistor though the zener protection diode will happily work independently of gate status. When the TTL Gate is ON, it supplies a ground path to the NIXIE segment, closing the circuit and allowing it to light. A standard neon lamp operates at 60-80V, a Nixie operates at 170V. using the same ttl gate IC for the two devices is impractical.
If it is true that a standard neon lamp operates at 60-80v (and I have no reason to doubt it), and the zener diodes in the TTL IC are conducting at 70v, then it follows that 170v is far too much voltage to be supplying. So I think the solution is to run the nixie tubes from 170v and the neon lamps from something much lower, in the region of 100 to 120v.
If you only have 170v available, then one way of doing this would be to connect the neon lamps like this:
TTL driver output ----- one side of neon lamp
other side of neon lamp ----- resistor to +170v AND second resistor to ground
If 200K in series with the neon lamp is about right when you run the combo from 170v, then try 180K to +170v and 330k or 390K to ground.