The N18 (direct equivalent = 3Q4) is a miniature dry battery pentode that was designed for portable radio receiver use. The output of 270 mW maximum would only drive a small efficient loudspeaker, not hi-fi.
As this is a low power battery valve the anode is left bright as very little heat is generated.
Under the top gettering can be seen two thin wires coming out of the top mica at an angle. The wires connect to a tension spring and the mica ensures that the filament remains in the correct place relative to the grids.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet, 3002 & 1040. Type N18 was first introduced in 1950. See also1950 adverts.
