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Originally Posted by Chad Hauris
The 12DT8 or 12AT7 FM convertor tube seems to be a common failure point on AM/FM sets...somewhere I read that because many sets switch off the B+ to the FM tube but leave the filament on when the set is on AM, that is bad for the tube...people probably listened to more AM on the sets in the 50's/60's and the FM tube deteriorated.
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I've heard of tubes failing that way--it's called "sleeping sickness" according to one technical book I read on the subject. Supposedly it even happens with oscillator tubes that are used but don't see a high enough duty cycle. Sometimes they'll fail for certain frequencies but be fine in others.
In my set, the situation is now opposite of the 50s/60s. I mostly use it in FM mode and only rarely tune in the AM stations. Mostly because I work in an office with lots of computers around, so there's a little more static interference on AM than I prefer to hear.