With R3 jumpered can you see the filaments glowing in a dark room?
With no segments lit, I can see the filament glowing even with the resistor installed--but only after my eyes have fully adjusted to the dark. But the filament definitely glows brighter with R3 jumpered.
I would expect some drop across the junction of Q3 but you're measuring around 1.5 V, no?
I didn't test that, but should have! In any case, your estimate based on the numbers I reported is correct. The voltage going into Q3 is 4.7v and the voltage coming out of Q3 is 3.3v (with the tube plugged in). So the voltage drop across Q3 is 1.4v. On a second clock, the voltage coming out of Q3 is 3.4v, so these numbers are probably typical.
According to one source, the current draw is highest with no segments lit, but I can't confirm that.
I can confirm that, but the difference was only ~1mA on the tube I tested. Nothing to worry about.
It would seem that the lower filament voltage is the real cause of the dim digits people are seeing.
That seems to be the case. At the other extreme, driving these tubes at higher currents tends to produce a brightness gradient across the entire display, so I'm guessing that R3 was selected to minimize that effect for most tubes. But R3 seems to be too large in some cases.
The easiest hardware solution seems to be replacing R3 with a jumper--something I've tried on two different clocks. In both cases, current running through the filament was well under 70 mA--safely below the 95 mA maximum. On one of the tubes, jumpering R3 introduced a very sight brightness gradient across the entire display.
To maximize display life, I wanted to run the tubes at the minimum current to extend display life, so I replaced the jumpers with 50 ohm potentiometers (Digi-Key, part number CT6EX500.) On one clock, the potentiometer is underneath the circuit board; the other, above the alarm switch.
At high resistance, the rightmost and leftmost digits are extremely dim, replicating the issue I observed. At medium-high resistance, only the rightmost digit is noticeably dim, replicating what others have observed. At medium-low to low resistance, the display is almost perfectly even. On only one of the clocks was there a slight gradient across the entire display at zero resistance.