For what it is worth dept. A while back I bought a used Ameritron AL 800H
that uses a pair 3cx800. About a minute or so after I first turned it on
[nothing hooked to it but the power cord] I heard a very loud bang. So I
shut if off and inspected for damage. Nothing found.
While reading the manual imagine my surprise to see the bang
mentioned, said to not be that uncommon, and not to worry about it unless it
starts happening often--then replace the tube.
It never did it again and the amp worked fine business untill I sold
Thinking back many years, I remember my National NCL 2000 doing the
same thing [ just setting on the bench in standby] except several times over
a period of several months. Scared hell out of me but again I could find no
evidence [not even an arc track] of external damage and the amp worked fine.
The old National was particulary loud--sounded like a .38 sp snub
> Rocky Pt effect, arcs in vacuum tubes between grid and plate, cathode
> and grids, cathode and plate, are commonly seen, heard and found
> through autopsy.