The Ampeg SVT is a bass amplifier

Instrument amplifier

An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...

 made by Ampeg

Ampeg

Ampeg is primarily a musical instrument amplifier manufacturer headquartered in Woodinville, Washington, though they also manufacture guitars to a small extent...

. The SVT, which stands for "Super Valve Technology", was designed by Bill Hughes and introduced in 1969. The SVT bass head produced 300 watts at a time when most amplifiers made less than 100. The SVT has been through many design changes over the years but is still in production today. The SVT head was initially coupled with a pair of sealed 8x10" speaker enclosures because one cabinet could not handle the power of the SVT. Later on Ampeg updated the speakers in the enclosures so that one cabinet was sufficient.
There are three types of original SVT amps. The first are the "blue line" SVTs. These are called that because of the engraved blue text and lines that surround the tone controls. Early 1969-70 "blue lines" used 6146B power tubes. The 6146B proved to be unstable in the high-voltage amp, and was switched to the more robust and reliable 6550 tube around mid-1970. The second version of a vintage SVT is what many call the "black line" SVT. The lines and text are black instead of blue, and like the later run "blue lines" use 6550 power tubes instead of 6146Bs. Later 70s models have the same features as the "black line" SVTs except the lines around the tone controls have rounded corners and curve into the tone controls. They also came with 3-prong power cables, and no polarity switch.

In the early 80s, Ampeg was bought by MTI in Japan, and the SVT resumed production there. Functionally identical, its primary cosmetic differences consisting of: white lettering on a black faceplate, black grill cloth, "elephant hide" or rougher textured tolex, and rack case-style spring-loaded handles, updated from the previous(and painful) rubber covered metal strap handles. Other notable differences of this era of SVT are found on the back panel, including a toggle switch for 2 or 4 ohm speaker impedance loads and a longer and thicker gauge 3-prong power cable. Apparently the transformers used are also of Japanese origin as opposed to the previous manufacturer in Chicago.