The 7199 is a vacuum tube, combining a pentode and triode. Typically, the pentode was used for the input stage, and the triode as a phase inverter. The tube was used in a number of American guitar amplifiers; the Gibson Guitar Corporation, for instance, used the 7199 in 1961's Falcon for the reverb circuit. Ampeg also used the 7199 extensively. The 7199 has been out of production for a considerable time, and as a result is becoming increasingly rare on the market. Another tube of the same type found in far more plentiful supply is the 6U8A, which is electrically identical, but with a different pinout configuration. The 6U8A can be substituted for a 7199 with a slight modification to the wiring of its tube socket.
References
7199 Wikipedia(Text) CC BY-SA