Mullard CV4004 12AX7 Mullard CV4004 12AX7 tubes we have tested here at Tejas Tubes, are of amazing quality and have an exceptional tone. These tubes were manufactured in England by the Mullard company. They are renown for their tone in both guitar amps and hifi stereo. Try some out in your system and expereince the best!!! This is a suitable replacement for any 12AX7 tube type. Various brands available.