Tube Sets & single failing tubes

[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.

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.

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.

[wa2ise]

Originally Posted by joe_tbird

I've heard of tubes failing that way--it's called "sleeping sickness" according to one technical book I read on the subject.

Have heard that you only need to draw a few microamps of current from an otherwise heated but unused tube to keep it from getting sleeping sickness. In an AM/Fm radio, I've connected a 470K resistor between the AM B+ and the FM B+ (at the switch is a convenient point) to draw small amounts of current from the unused tubes. This wouldn't be enough current for local oscillators to run so no am<->FM interference should occur.