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I have never heard of using a rectifier tube as protection in a car radio. The only purpose I ever saw was simply rectification of the square wave produced at the secondary of the power transformer. The job of the vibrator in such sets was simply to create the square wave from the battery dc.
With a self rectifying vibrator, there was no need for a rectifier and one was never used. But self rectifying wasn't a good solution; the vibrator was expensive and a tube was perhaps more reliable.
They never worried that much about current drain. If the battery ran down, too bad. When they did care about drain they might use a cold cathode rectifier such as the 0Z4 but those weren't such a great solution either.
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