The VT-229 is the US Government code for the commercial 6SL7GT and both Type designations exist on the base cap.
The valve is a medium impedance double triode with high gain and superseded the 6SC7.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.
The triode anodes are parallel to one another and set at an angle to maximise the length of the cooling flange that also clamps round the side support. The anodes are blackened for better heat dissipation.
The two triodes seen end on. The mica sheets are a close fit to the glass tubular envelope.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 67 mm tall.