DescriptionFrom Mesa Boogie:Our SPAX7’s are a premium grade substitute for the standard 12AX7 – They are a “Special Requirement Preamp Tube” that possess an even higher standard of overall performance than our high-quality 12AX7 due to their incredibly low sensitivity to microphonic noise.  This is the ultimate preamp tube to use in all critical gain stage positions – The absolute best of the best!Substitute preamp tube in select, critical stages of MESA/Boogie Guitar & Bass Amps, Preamps, Power Amps, V1 & V-Twin Pedals.