Here the task is to decode binary numbers and enter their decimal counterpart. The binary numbers are shown on a ИВ-25/IV-25 tube. The idea is to then enter the number on a keypad and the decimal number will be shown on three ИВ-11/IV-11 tubes. Whenever you think that you've successfully translated the binary number to decimal and entered it you hit the "CONVERT" button. If you we're right the next number will show, if not, you'll have to try again.  You'll have to enter a number of numbers to complete the level. 

First I found a suiting brass pipe part to mount the first tube in, a real piece of steampunk in itself. 

I then soldered a bunch of wires to the tube, made a hole in the box and dry-fitted it to see how it looked. 

Now more junk-box-diving! I had an old keypad from some vacuum pump controller that I wanted to use. To be able to make a square hole I decided to remove the "0" and have only 1-9. This will limit my options a bit but never mind. After taking away some components from this board I soldered on new wires to the board and made a nice square hole using my dremel as a router. 

Next up I prepared mounts for the other tubes and the button. 

To make sure all the tubes were working I tried them with 25V supply for the grid and 5V and a 100 ohm resistor in series with the filament. They worked just fine after 25+ yerars in a box! 

Now I made a test circuit with 4 shift registers (SN74HC595) and 4 darlington arrays (ULN2803) to se if the circuit was feasible. It was, yay!

As you can see the middle tube is not very bright. The brightness depends on the current through the filament, and here I'm using fixed resistors and the resistance for the middle one should be lower. In the perf-board version below I've replaced the filament resistors with 500 ohm pots in order to make the brightness adjustable. 

Next it was mounting time! Everything fits as seen below. The only thing left is to figure out how to hold the tubes in place. They are now twisted a bit. The glass surface is very slippery so I think I need some sort on glue. Maybe epoxy. That, however, makes it very non-repairable. I'll try some less permanent solutions before that. Anyway! 

Here is also a video of the game being played. The correct numbers are: 8, 21, 127, 2 69, and then they restart. When the game is done it won't restart bur unlock the next level. When my guess is wrong it simply resets the digits. Right now the LSB (least significant bit) is on the top of the binary tube. Maybe it makes more sense to have it in the bottom. I'll ask my beta testers, if you have any input please say so in the comments!