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1930's Vacuum Tube Hearing AidVacuum tube (that is, electronic) hearing aids were becoming popular, but not yet a serious competetor for carbon aids. They were still too large and consisted of two parts (a battery pack and seperate amplifier). They did not become small and wearable in a practical manner until late 1930's. They required one battery to warm up the vacuum tube filament and a second battery to power the amplifier. |
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