"Plexi" is a slang term for the Marshall 1959SLP Super Lead amplifier, which was produced from 1965 to 1969. The amp is known for its high-gain, overdriven sound, which was popularised by rock and blues guitarists such as Jimi Hendrix, Eric Clapton, and Jimmy Page.
The plexi style amp derived its name from the clear plastic panels that were used on the front and back of the amplifier head. The panels were made of Plexiglas, a brand name for polymethyl methacrylate, which is a type of transparent plastic. The plexi panels allowed the user to see the inner workings of the amplifier, including the vacuum tubes and transformers.
The Marshall 1959SLP Super Lead was a 100-watt amplifier that used four EL34 power tubes and four ECC83 preamp tubes. The amp had a simple control panel with just a few knobs to control the volume, treble, middle, and bass. The amp also had a switch to select between normal and vibrato channels.
The plexi style amp was a very loud amplifier, and it was often used in large concert venues. The amp was also very popular for recording, and it can be heard on many classic rock and blues albums from the 1960s and 1970s.
Today, there are many different manufacturers that produce plexi-style amplifiers. These amps are often used by guitarists who are looking for a vintage, overdriven sound. Plexi-style amps can be found in a variety of price ranges, from affordable to high-end boutique models.