Engineers invented the first loud, powerful amplifier and speaker systems for public address systems and movie theaters. These PA systems and movie theatre sound systems were very large and very expensive, and so they could not be used by most touring musicians. After 1927, smaller, portable AC mains-powered PA systems that could be plugged into a regular wall socket "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; remarkably, these early amps did not have tone controls or even an on-off switch. In 1928, the Stromberg-Voisinet firm marketed an electric stringed instrument and amplifier package. There are no records as to how many—if any—of the amps were ever built and sold, beyond marketing materials. Stromberg-Voisinet still launched a new idea: a portable electric instrument amp with a speaker, all in a transportable wooden cabinet. In 1929, Vega electrics launched a portable banjo amplifier. In 1932, Electro String Instruments and amplifier (this is not the same company as Stromberg Electro Instruments) introduced a guitar amp with "high output" and a "string driven magnetic pickup". Electro set out the standard template for combo amps: a wooden cabinet with the electronic amplifier mounted inside, and convenient carrying handles to facilitate transporting the cabinet. In 1933, Vivi-Tone amp set-ups were used for live performances and radio shows. In 1934, Rickenbacker launched a similar combo amp that added metal corner protectors to keep the corners in good condition during transportation. In 1933, Dobro released an electric guitar and amp package. The combo amp had "two 8″ Lansing speakers and a five-tube chassis. Dobro made a two speaker combo amp that was on the market over 12 years before Fender launched its two-speaker "Dual Professional/Super" combo amp. In 1933, Audio-Vox was founded by Paul Tutmarc, the inventor of the first electric bass (Tutmarc's instrument did not achieve market success until Leo Fender's launched the Precision Bass). In 1933, Vega sold a pickup and amplifier set for musicians to use with existing guitars. In that same year, the Los Angeles-based Volu-Tone company also sold a pickup/amplifier set. Volu-Tone used "high voltage current" to sense