MESA SPAX7 preamp tubes are a premium grade 12AX7 that possess an even higher standard of overall performance due to their incredibly low sensitivity to microphonic noise. This is the ultimate preamp tube to use in critical gain stage applications, particularly in first stage/input positions or other high gain stages in the preamp tube string. These tubes are selected during our rigorous 12AX7 testing process and when found to be substantially more stable and quiet in extreme high gain applications, they are fitted with a shrink-wrap shielding, further stabilizing the tube and reducing external interference and noise. MESA SPAX7s deliver the lowest noise floor for the highest gain output. While not required for optimum performance, SPAX7s are a great tube to have on hand as spares and for any applications where the absolute lowest noise floor and low-microphonic response is needed. MESA preamp tubes are among the highest quality available and selected for their exceptional full-range tonal quality, stability, low noise and low microphonics. They provide the maximum level of high gain output with low sensitivity to microphonic noise while producing a balanced response across the tonal spectrum that is extremely lively, open and expressive. All MESA tubes offer unsurpassed consistency and reliability for ANY tube amplifier, and certainly MESA amplifiers! Remember our dedication is to create the worlds finest tube amplifiers. And to achieve that goal requires a deeper commitment to tube quality because the reputation of our amplifiers depends on it.