The Adafruit design drives the IV-18 filament with ~2.5v of direct current, and the IV-18 filament was designed to be driven by ~5v of alternating current. On some tubes, the Ice Tube Clock is unable to sufficiently heat the filament, and the result is a final and/or initial digit that is noticeably dimmer. This dim digit issue can be resolved by replacing Q3 with a transistor capable of providing more power to the IV-18 filament.
I suggest the ZVP2110A, available from Digi-Key (
ZVP2110A-ND) or Mouser (
522-ZVP2110A). Another excellent alternative is the is the PN2907A/resistor combination
described by Russell27. Both solutions will provide additional power to the IV-18 filament and even out the display.