Thanks Emblazened,
Um, but  I'm afraid I don't know how to do that. And actually I don't even want them to flash.
Sorry, I'm pretty new to all this. In fact I'm kind of surprised that I got as far as I have pimply simply using the code that you wrote.

I get that I need to declare the variable of of the fourth anode:

Code:

int ledPin_a_4 = 13

...and I assume determine that that anode will be firing along with all the others:

Code:

case 3:    anodPin =  ledPin_a_4;    break;

But then where and how do I say to turn a cathode on the fourth anode? I'm guessing that I can still do this in the setup if don't want the decimal to blink.

So what modifications would I have to make to be able to drive a pair of decimals?

I'm guessing that I'd have to set up the fourth anode which would be int ledPin_a_4 = 13; but I'm kind of at a loss beyond that.

Thanks for the new driver chips. That solved the problem that I was having.  Yay!

Well it looks like that's it.
If I swap the two driver chips the tubes that have the ghost lit cathodes follow along. I either have an issue with the first three didgits (from one chip) or with the last three (from the other chip) but not both.

Well I've had some new success. I have all the cathodes firing now, but I'm still getting faint "ghost" cathodes lighting up.

I changed capacitor c4 again for one that's 5.6pf (rather than the 7pf that I had in there) just to make sure that wasn't a problem snce the 7pc capacitor was one I found that didn't have a voltage rating either.  And  when I was done I did a pretty vigorous cleaning of both sides of the arduinix board with Acetone. I also added a bit more heat to some solder points that looked less than perfect. It's possible that a solder point was preventing one cathode from firing and it does look a bit cleaner but I don't actually think that did anything. So, your suggestion about multiplexing of the anodes might make sense.

I've also played with the anode resistors a bit. I'm using a 5.1kohm resistors on the anode of each tube (8 in total). I wasn't able to get every cathode to light up before changing these from 10k to 5k although I think that that may have to do with these tubes. The in-4 tubes may require a bit more current/voltage than some other tubes. The tube-tester page has the most information that I can find on them anywhere (http://www.tube-tester.com/sites/nixie/data/in-4/in-4-sh1.htm).  

I've got my driver chips in sockets. Do you think that I should try swapping them for one another and see If that affects the faint ghost cathodes?

A short would make sense. I've tried a few other tubes but keep getting the same thing. But a short in my cathodes or anodes?
I always get a faintly illuminated cathode that matches the same number cathode of a the fully illuminated tube that's running off the same cathode output but a different anode beside it.
A quick diagram would be something like this:

Although I've also got three more bulbs on the second set of cathode outputs and they share the same three anode outputs.

I can take a photo of the board that I've mounted the tubes on if you think it would help. I've checked it visually and tested for any shorts that I could figured that I could find with my multi-meter but didn't find anything. Would dirt or solder flux residue be enough? I've cleaned it with solvent too.

Here's a pic of what I've got set up now. The loose wires on the far right are for setting the time.



Most cathodes are working fine, like this



but some just don't wont to turn on



or even weirder is that I get a ghostie of a cathode that is not supposed to be lit lighting up. It'll be a cathode the same cathode as that of a different tube on the same cathode output.



I hope that these aren't too many photos. I just can't think of how else to describe it. In the bottom picture the fourth digit was supposed to be a 3 when the photo was taken.
I suppose I should mention that I've tried changing the update speed of the bulbs. It doesn't seem to make too much difference. 7 is about the longest I can stand before the flickering gets to me though.
I'm thinking that maybe it's either a short on the circuit or in my wiring because the barely lit cathodes. For a while the 2 cathode on my last digit just wouldn't quite go out.

Well, It looks like I'm almost there.
I'm using a power supply that's rated for 1A at 9v but it's actually putting out a voltage near 12v.
I've decreased c4 to 7pf which gets me a reading of up near 220v without any tubes plugged in. But my Russian in-4s aren't lighting up all the time. I've got a configuration like that in the 6 tube layout with three tubes per cathode and two tubes per anode (anodes 2, 3 and 4). When I hook up my tubes the voltage tests at about 140v from the test point to the grounds pins on the arduinix board. Does that sound about right?

I've reduced the resistors on the anode outputs to about 2.2kohms which made the tubes a bit brighter when they're glowing but I'm still not getting every cathode to light up. Actually cathonde 4 if the second didget of the seconds display only occasionally lights up. I've checked my connections to the tubes though. Could it be the nixie controler chip?

I'm also curious if I should have individual resistors for the anode of each tube or if a resistor on each anode output is fine. That's how I have it configured now because I didn't think it would matter. I've tried a resistor to each tube's anode but it doesn't seem to make a difference.

It seems to be working for me.
It's great to have a nice simple time setting function using the analog input.
Thanks.

Now if I could also get a count up timer function then I'd be set.
I'm trying to replace a timer that counts up, holds and resets. Plus I'd like to be able to switch back and forth between the timer and a clock...
Anyone using the arduinix for a timer?

I just got a board with some IN-4 tubes together the other day. It looked great with a single tube but when I started plugging in more tubes I though I had shorts or had just done a shoddy soldering job. It looks like I just wasn't getting enough juice to drive them with that 46pf cap so I upped it to a 7pf cap that I had hanging around and that got things working. I'm still not getting quite what I'd like for these tubes (although it drives some B-5750S http://www.tube-tester.com/sites/nixie/data/b5750s.htm tubes great). I suppose I'll change it again to the suggested 5.6pf cap and see if that does it. Unfortunately I have a set of 6 tubes but one's bad so I'm waiting for another tube to arrive from Russia before I'll really be able to finish this project.