The 7N7 combines two low-mu triodes in the lock-in construction. It is designed for use either as a phase inverter or as a voltage amplifier.  Widely used by audiophiles in audio amplifier / preamp applications.  Check out the following links for typical discussions and application:  https://vinylsavor.blogspot.com/2015/02/tube-of-month-7n7.html  

https://www.head-fi.org/threads/in-praise-of-the-7n7-tube-%E2%80%93.194962/ 

Location:  B5

R050124