Specially selected low-noise for early Manley mics, no “X” in the serial number code. These lo-noise 6072A are no longer available from Manley Labs. We recommend to order the conversion kit to convert your mic to use the 12AX7. That kit is shown under  Conversion / Update Kits here on this webstore, OR you may goto http://www.manleylabs.com and book service with Paul for him to convert the microphone for you.