The 6SL7GT is a medium impedance double triode with high gain and superseded the 6SC7. It featured separate cathodes and was designed for use as AF voltage amplifiers, phase splitters and where required as a cascade amplifier of very high gain.
We also have the 12SL7 that differs only in respect to the heater rating.
The two triode sections are mounted side by side across the envelope.
The anode box side view.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 64 mm tall.
References: Data-sheet, 1043 & 1040. Type 6SL7GT was first introduced in 1942.