It was quite awhile before vacuum tube hearing aids caught on. Although a much better sound there two big batteries, a very fragile tube and large size that kept it out of the market for quite awhile. Although invented in the 1919 by Earl Hansen, it did not begin to sell very well until the late 1930s….World War II made the devices even better. check out the development of these antique hearing aids in my story at Hearing Health and Technology Matters using the following link:
The link goes to the story of The Road to the Vacuum Tube Hearing Aid, published July 21, 2015 by Hearing Health and Technology Matters, LLC, Tuscon, Arizona. Linked by permission.