The 7199 is a triode-pentode intended for use in hifi applications. The pentode section is for preamplifiers while the triode section is best used as a phase inverter.
Most notably used in Dynaco ST-70's and vintage Ampeg amplifiers.
These are getting incredibly hard to find in NOS condition. Yet, the good thing is these are rugged and long living tube. A small handful should be enough for a lifetime supply.