Preamp tube, harmonically rich tone, quantity: 1. The Groove Tubes GT-12AX7-M Select is a replacement preamp tube for guitar amplifiers. This is an exact reproduction of the popular Mullard ECC83, which was primarily used in music and hi-fi amplifiers of the 50s, 60s and 70s. The cathode is also shock proof, which increases stability and sound quality. The tone from this tube is very warm and musical.