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Ralph Mowery

In article ,
says...

T fuses were not always easy to buy, so it was common to use an oversize F fuse instead - labelling often encouraged you to do so.

Whatever the combination of dumb ideas, it is not hard to do a number of on-off cycles while keeping a close eye on an F fuse. If it bends severely at switch on it's gonna fail soon.

I had a solid state amplifier for my ham gear that used a 20 or 25 amp
3AG type fuse. While the fuse was rated fine for the current ( about 5
amps more than the current drain), after about a month it would do what
I call a 'sag out'. The element would just be bent to the bottom of
the glass and quit making connection.

I guess that the contacts for the plug in with a twist lock cap was not
making contact well enough to keep the heat down.

The current would cycle from almost no current to full current about 20
or more times a day on the average.