Unlike the carbon instruments, these had adequate power for severe hearing losses but were also usable by persons with a lesser loss. The first one appeared in 1921, but this type did not become practical until the early 1930's, and did not appear in a wearable version until 1934. Vacuum tube aids required two batteries, so costs were rather high.
A.) Ear Receiver
B.) Microphone/Amplifier
C.) Batteries
Tel-Audio
Table model vacuum tube hearing aid. Has a large external microphone and earphones. This is probably the first hearing aid with a telecoil! It plugged into the wall, or had optional battery power. Made in 1936.
Vactuphone
Globe "Vactuphone". VACuum TUbe TelePHONE. Made to order by Western Electric Company for Globe. The first hearing aid to use a vacuum tube! Case was 18.4 cm x 10.0 cm x 18.3 cm. Fist made in 1921. Model shown is the commercial model. The engineers' model (not shown) was not leather covered and did not have a name and patent plate. Also shown is the single vacuum tube used in the hearing aid.
1930s Vacuum Tube Hearing Aid
Vacuum tube (that is, electronic) hearing aids were becoming popular, but not yet a serious competitor for carbon aids. They were still too large and consisted of two parts (a battery pack and separate 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.
Mono-Pac Melody
Beltone Mono-Pac "M" (Melody). One piece vacuum tube hearing aid. Still needed two batteries, but no longer external. Available with the optional broach styled microphone shown here. Made in 1950.
Sonotone 533
Sonotone 533, made in 1942. This aid contains 3 vacuum tubes. This is a very flexible hearing aid! It is changeable between air conduction and bone conduction (bone shown). Two battery options were also available, with the dual battery option shown. Also shown is the additional cord available for air conduction receivers. Case measures 5" x 2 7/16" x 1".