DuMont 108A - Mansfield FOUND!!!!!

[N2IXK]

Originally Posted by Kevin Kuehn

Maybe as long as there's no beam current, the cathodes not taking a beating, and leaving the filament lit may actually save it from premature burn out?

Not sure about that. Operating tubes for long periods of time under cutoff conditions contributes to a particular form of degradation called "cathode interface formation" (aka "Sleeping Sickness"), in which the active emitting oxides interact with the cathode base metal to form a resistive layer which in turn begins to "insulate" the remaining active oxides from the base metal. This caused all kinds of problems in vacuum tube computers, which might have flip-flops and gates that stay cutoff for hours at a time. Eventually, special cathode chemistry (ultra high purity nickel alloys) developed for computer service to combat the problem, but ordinary TV types (including CRTs) aren't likely to use them.

Probably the reason that sets with "instant-on" almost always have dead CRTs....

[Kevin Kuehn]

Strange that the DuMont engineers would have stood for that, or possibly they didn't really understand the long term consequences at that time. I suppose with a TV CRT, that the useful life span of the CRT possibly catches up with any cathode interface formation - based on a hunch that most folks, used primarily the TV function.

[wa2ise]

Originally Posted by Kevin Kuehn

Strange that the DuMont engineers would have stood for that,

Maybe the boss didn't want to pay for the heavy duty switch the heater circuit would call for, or a separate heater transformer and regular switch for the TV only tubes. But as few people listened to FM radio back then, maybe you're right that most everyone watched TV. And that the feature of FM radio was thrown in as a selling point, and it wasn't expected that anyone would actually use it.

There was an Emerson portable TV with AM radio, using series string tubes, so you couldn't kill the TV tubes heaters.