I don' want to enter deep, but I've seen a number of 8877 working along
their life with much bigger grid currents than 50/60ma (2-3 times more).
Those tubes were only replaced after the scheduled "on" hours passed,
and then discarded. Most of them later "arrived" as a gift into amateurs
PAs, and they (the tubes...) are still doing the job.
Years ago (10), while developing an HF amplifier using a pair of 8877 in
Gg, a number of "accidents" occurred and the grid current peaked, for
some while but a number of times, over the spec. max ratings.
Those tubes not only survived but they are still alive and in good health.
On the contrary, a number of brand new 8877 had failed repeatitively in
the mid 90s' but grid current had totally nothing to do such problems.
73,
Mauri I4JMY
E-mail: i4jmy@iol.it
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