According to the b7971 datasheet, the maximum allowable current is 21mA. 
Yet when you add-up the currents for each cathode to produce the  '8' or 
'*' characters, they add up to 40mA, which is way over the spec limit.

For those of you who design their clocks, did you scale-back all segment 
currents so they never exceed 21mA ?

I'll probably current-limit each cathode individually, and put another 
current-limit on the anode. Even though that adds up to a lot of 
transistors for a direct-drive clock, these tubes are getting pretty scarce 
and expensive so I'm not taking any chances.



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