The Groove Tubes SAG-AX7-MPI Matched Phase Inverter Tube Changing Kit consists of specially selected and matched 12AX7 vacuum tubes for critical audio or guitar/bass amplifier use. They"re intended primarily for matching the two sides of the triode for phase inverter or driver use. The Groove Tubes AX7 tube is a dual triode where the A and B sides are closely matched for use in critical high end audio applications or for use in the phase inverter or driver position of musical instrument amplifiers. The benefit of this tube in a guitar or bass amplifier is a matched output section. At lower frequencies such as dropped tunings, 7 string guitars, or bass guitars, the upper and lower portions of the output waveform will be much more equal. When this waveform is not equal, differences are cancelled out in the NFB loop of class A/B amplifiers and are noticed as a lack of sustain or apparent "dead spots" in areas of the fret board. At high levels at all frequencies, sustain will be improved. This is even more critical in Class A amplifiers such as the Vox AC-30 and Matchless amplifiers. Use this MPI in any application where a phase inverter / driver of the ECC83/12AX7/7025 tube is specified. The phase inverter / driver position of a guitar or bass amp is a current driving position, and not part of the tone generation stage. These tubes are selected for specific characteristics, so may be a 12AX7R, 12AX7C, 12AX7M or another 12AX7 type that has been selected for specific curve traces as the objective. One can not specify the tube type desired. If you want a specific type matched as an MPI you need to order the tube and then add the SAG labor charge on top of the tube. The stocked MPIs are generally made from 12AX7C, 12AX7R (WC and NOT WA) and the new 12AX7M. Any of these tubes may be supplied as they are matched for specs and vacuum tube curve traces and not for tube type. Use of a 12AT7 in place of a 12AX7 will yield more clean headroom in your output section. The two tube types are interchangable without any adjustment required, so feel free to experiment! Please contact GT SAG directly if you have any questions or specific application requirements. Country of origin: ChinaCountry of origin: China