A man tuning a Grebe Synchrophase vacuum tube radio receiver in 1925, made by the A. H. Grebe Co. This type of radio, called a tuned radio frequency (TRF) receiver, was popular during the 1920s. It had several stages of amplification, each with a tuned circuit which had to be tuned to the desired radio station with a knob on the front panel connected to a variable capacitor inside. Tuning a TRF receiver to a new station was difficult because all the knobs had to be tuned to the correct point before the station could be heard. The knobs were not marked with frequency, due to large frequency drift of tube receiver. So all the knobs had to be rotated in unison to keep the receiver in tune.

This receiver had 3 tuned stages. In order to make the process easier, the controls are thumbwheels instead of ordinary knobs, which can each be rotated with a finger. This picture shows the tuning technique; the operator is turning all 3 thumbwheels simultaneously, Once a station is found, the index numbers on the dials above the wheels are written down so that henceforth the dials can individually be turned directly to the station without this elaborate procedure. The radio's speaker (right) is a large horn loudspeaker as modern cone speakers were not prevalent. The radio is powered by a square 45 V "B" battery, visible behind the cabinet.

Caption: "TWO HANDED OPERATION OF THE THREE TUNINC CONTROLS - Here the operator demonstrates the method of tuning the three dials simultaneously, thus eliminating the necessity for a third hand"