Quote:
Originally Posted by bob91343
That's called permeability tuning. Many designs used it but the vast majority were variable capacitors. I doubt that it had anything to do with scarcity of materials.
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Yep, it's also sometimes called inductor tuning. Most car radios had them before the late 50's. I've never read that they were used because of a shortage of metal (copper was scarce too). Instead, I think they were cheaper to make than the gang capacitor. Their main drawback is that they are very delicate and even the smallest deforming of the coil will throw off the tuning.
By far the biggest user of them was a company called Detrola. Look up Detrola 571 on the internet and see if any look like yours. Also, a lot of early FM radios used them too.