Tom,
You've just provided 'Reliability 101'.
MTBF is a figure often bandied about, but without the correct assumptions,
numbers and inputs to the equations, can be very misleading. For example, the
MTBF of a tube amplifier with tubes changed on a time basis can be very
different to the MTBF if tubes are run until they fail - unless you count tube
change as a failure, in which case the MTBF is much lower. Back in the days of
aircraft radios using tubes, one company at Heathrow paid the service techs on
a piecework basis - they got so much per radio fixed. So the first thing they
did when a radio arrived on the bench was to change every tube, whether it
needed it or not. Thus those radios showed a horrendous MTBF on tubes....
Another complication with electrolytics is the 'memory' - use a 450 volt part
long enough at 250, and putting 450 on it gets an enormous leakage until it's
reformed at the higher voltage.
73
Peter G3RZP
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