On Friday, May 15, 2020 at 9:51:45 AM UTC-4, gregebert wrote:
>
> For each of my expensive tubes (b7971, R|z568m) I measure and plot the I-V 
> characteristics. There aren't any serial numbers on b7971's so I just write 
> one on the backside.
>
> If the tube fails later on, I check the I-V characteristics to see if 
> anything changes; so far only 1 b7971 has failed when segments shorted 
> together. None of the unaffected segments showed any measurable changes.
>

B7971s are amazingly robust. The MOD-SIX in the room where I'm typing this 
reports 76,571 tube-on hours (nearly 9 years) and is on its original tubes 
(the CPU, PSU and case have been replaced over the years, with the tube-on 
hours copied to the newer CPUs). I've never had one that worked when I got 
it fail on me. In around 100 I've purchased over the years I've had 4 or so 
DOAs - 2 cracked, one with an unfixable short and one with a poisoned 
cathode I couldn't clean up.

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