This is a larger more powerful beam tetrode audio valve than the 6V6 receiver types. This was used for audio amplifiers and also for stabilised power supplies. Heathkit used a pair in parallel to produce a 400 volt 100mA supply.
The construction is of a traditional flat cathode with a zig-zag grid supported on notched rods surrounding the cathode. The beam plates are vertical [ shaped sections to the edges of the grid assembly. The anode is an open vertical box that encloses the rest of the electrodes.
The exhibit does not carry a makers name but the construction is identical to the Brimar 6L6G.
The heater wire is insulated and passes through the round ended rectangular cathode four times. The beam plates can be seen inside the anode.
The two halves of the anode are stitched together and the radiating flanges formed are in the centre of the working face where most heat is generated.
The saucer shaped getter holder is visible just above the pinch. The tape strap below the lower mica ties the two beam plates together.
The wide glass tube envelope is 34 mm in diameter and, excluding the IO base pins, is 91 mm tall.
References: Datasheet, 1040 & 1043. Type 6L6GT was first introduced in 1936.