New-Old-Stock (NOS) tube. The 6681 is a miniature high-mu twin triode especially suited for use in resistance-coupled voltage amplifier, phase inverters, multivibrators, and other applications in which high voltage gain is desired. Its electrical characteristics are essentially equivalent to those of the 12AX7. Intended specifically for use in mobile communications equipment, the 6681 may be operated without serious degradation under normal variations in supply voltage as encountered with automotive electrical systems. Also consistent with the requirements of this equipment, the tube is capable of withstanding appreciable on-off cycling.