Groove Tubes Gold Series GT-6201-M Preamp Tubes exhibit low noise, hum and microphonics and are specially selected for use in recording microphones.
A very quiet selected version of a 12AT7. These were used for critical applications such as microphones which use the 12AT7, studio gear, and vacuum tube preamplifiers that use a 12AT7 tube.
This is a Russia made NOS JAN spec tube (Joint Army/Navy Spec) made in the late 70's.
These GT-6201-M Preamp Tubes feature long lasting, really heavy duty dual triodes, usually used in the driver stage of an amp, aka: Phase inverter, and they take a beating driving the power tube changes.
Merkki Groove Tubes