Combo amplifiers, like amplifier heads, are devices that increase or amplifiy a small electrical signal (that comes from an instrument pickup) so that the signal is large enough to operate a speaker. Unlike stand-alone amplifier heads, combo amplifiers combine a power amplifier and speaker (or speakers) in a single enclosure or cabinet. The wire connections between the amplifier and speakers are permanent and internal to the cabinet. Combo amps generally have smaller power ratings and speaker sizes than amplifier systems in which the power amplifier and speakers have separate cabinets.
Combo amps are available for electric guitar, keyboard, acoustic guitar and bass. While an electric guitar combo amp will work for a keyboard or acoustic guitar, it will not serve as well as a combo for the instrument being played. The enclosure for an electric guitar combo is usually (not always) open in the back, revealing the speaker cone, supporting an edgy sound that is more desirable for electric guitar than it is for acoustic guitar or keyboard. The enclosures for acoustic guitar, keyboard and bass are usually closed in the back, supporting a richer, mellower sound. The style of the enclosure, however, is not the sole reason for using the right kind of combo for the specific instrument.
In addition to the style of the enclosure, combo amps differ with respect to the frequencies that they support. Amplifiers for guitars and other instruments that have mostly mid-range and treble tonality have electronic circuits and speaker designs that support those mid-range and higher frequencies. Amplifiers for basses have electronic circuits and speaker designs that support bass frequencies. Amplifiers for keyboards have circuitry and speakers that support a wide range of frequencies. The differences in design to optimize combos for specific instruments makes it inadvisable to use a guitar amp for a bass; the amount of motion that bass frequencies create in the speaker can damage a speaker designed for mid-range and treble frequencies. The best choice is to use electric guitar amps for electric guitars, acoustic amps for acoustic instruments, bass amps for basses and keyboard amps for keyboards. Following this practice, players get the best sound and the longest life for their gear.