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The ones I've dealt with usually simply work fine one minute and burn up the next.
I've seen others that exhibited low or no B+ on their output.
I've been told, by the old repairmen that I've talked to, that some of these devices failed after only a few years. Then, there are those, like what's in your radios, that go on for 50 years without trouble. How long will they continue to last? Who knows. Maybe 5 minutes, maybe 5 months, maybe 5 years, maybe even another 50 years.
I usually replace them with a modern silicon diode and a resistor, even if the old part is still good.
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