the string vibration, a potentially dangerous approach that did not become popular. In 1934 Dobro released a guitar amp with a vacuum tube rectifier and two power tubes. By 1935, Dobro and National began selling combo amps for Hawaiian guitar. In 1934, Gibson had developed prototype combo amps, but never released them. By 1935, Electro/Rickenbacher had sold more amps and electric guitars than all the amps and electrified or electric guitars that had been made from 1928 through the end of 1934. The first electric instrument amplifiers were not intended for electric guitars, but were portable PA systems. These appeared in the early 1930s when the introduction of electrolytic capacitors and rectifier tubes enabled economical built-in power supplies that could plug into wall sockets. Previously, amplifiers required heavy multiple battery packs. People used these amplifiers to amplify acoustic guitar, but electronic amplification of guitar first became widely popular in the 1930s and 1940s craze for Western Swing and Hawaiian music, which extensively used amplified lap steel guitars. In fact, the very first recording of an electrically amplified string instrument was the September 1933 recordings of Milton Brown and his Musical Brownies, featuring steel guitarist Bob Dunn In the 1920s, the earliest combo amplifiers had no tone controls. The first tone controls were simple, mainly providing treble adjustment. The limited controls, the early loudspeakers, and the low amplifier power (typically 15 watts or less before the mid-1950s) gave poor high treble and bass output. Some models also provided effects such as an electronic tremolo unit. In confusion over nomenclature, Fender labeled early amplifier tremolo as "vibrato" and called the vibrato arm of the Stratocaster guitar a "tremolo bar" (see vibrato unit, electric guitar, and tremolo). Some later amplifier models included an onboard spring reverb effect, one of the first being the Ampeg Reverberocket amp. In the 1950s, several guitarists experimented with producing distortion by deliberately overdriving amplifiers. These included Goree Carter, Joe Hill Louis, Elmore James, Ike Turner, Willie Johnson, Pat Hare, Guitar Slim, Chuck Berry, Johnny Burnette, and Link Wray. In the early 1960s, surf rock guitarist Dick Dale worked closely with Fender to produce custom made amplifiers, including the first 100-watt guitar amplifier. He pushed the limits of electric amplification technology, helping to develop new equipment that was capable of producing "thick, clearly defined tones" at "previously undreamed-of volumes." Distortion became more popular from the mid-1960s, when The Kinks