Beginning in the 1920s, the first amplifiers and speakers designed for gigging musicians became available. From the 1920s to the 1940s, upright bass players who wanted to strengthen the acoustic sound of their instrument had to use small portable PA systems or guitar amp combos designed for acoustic guitar or archtop guitars. Since these systems were not specifically designed to amplify bass instruments, their utility was limited. The only speakers available during the early 1920s were "radio horns of limited frequency range and low acoustic output," and the modern cone speaker was not available until 1925. The first amplifiers and speakers were PA speaker setups, and were powered with large batteries, which made them heavy and hard to carry around.
Engineers invented the first loud, powerful amplifier and speaker systems for public address systems and movie theaters. These were bulky and very expensive, not intended to be readily transportable, and so they could not be used by most touring and gigging musicians. After 1927, smaller, portable AC mains-powered PA systems "quickly became popular with musicians"; indeed, "...Leon McAuliffe (with Bob Wills) still used a carbon mic and a portable PA as late as 1935."
During the late 1920s to mid-1930s, small portable PA systems and guitar combo amplifiers were fairly similar. These early amps had a "single volume control and one or two input jacks, field coil speakers" and thin wooden cabinets, and generally did not have tone controls or even an on-off switch.