A high μ (150) double triode, designed for use in high gain pre-amplifiers operating at low levels. It's charateristics are like an ECC83/12AX7 on steroids. It has about the same Rp (62.500 Ω) but 1.5× the transconductance (2.4 mA/V vs 1.6 mA/V). It is a replacement for the 13D7, but has higher gain. There are no direct substitutes. A very good ECC83 might work in some circuits, especially if feedback could be reduced and the heater was rewired. Otherwise the stages would have to be redesigned.
It was used in the Rogers Cadet and the VTL 'Minimal' preamp.
