A significant failure mode of some gridded vacuum tubes is cathode
damage from ion bombardment during discharges. Another indication is
holes in the grid from the pass thru energy. I have a photo of a 7835
grounded grid triode, under autopsy, showing a quarter inch hole in
the wound grid. Down the hall at work, someone has a planar triode
that was working, although a large percentage of the grid (very
fragile thing) has holes.
If anyone wishes to see the photo, I can ship the JPEG of the photo,
about 100 K in size.
73
John
K5PRO
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