The 6SL7GT is a medium impedance double triode with high gain and superseded the 6SC7.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.
One triode face on. On the base cap are the words Made in USA and what looks like the GE logo.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 63 mm tall.
References: Data-sheet, 1043 & 1040. Type 6SL7GT was first introduced in 1942. See also1942 adverts.