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 our preamp tubes are tested in the most sensitive position of an amp running wide open to determine how noisy/quiet and stable they are. Those that pass are backed with the best warranty in the business - a minimum of six months. All MESA tubes offer unsurpassed consistency and reliability for ANY tube amplifier, and certainly MESA amplifiers!