The TT12 is a transmitting beam tetrode for use in the low VHF region up to 90 MHz at full ratings and at reduced power up to 130 MHz. The drive power needed to drive to full power of 40 Watts out is 0.4 Watts. The anode dissipation is quoted as 20 Watts. Given the low control grid voltage, dissipation and output power, this valve was designed to operate as a Class C amplifier into a tuned anode load.
The base is a standard B9G but the envelope skirt is wider than normal at 44 mm diameter.
The Marconi badge is of a generous size and M-OV branded professional valves as Marconi.
The fixing pins at the base of the envelope are a continuation of the base pins. The oxide cathode can be seen as a flat strip. The anode is a complex shape to accommodate the beam plates. On the right can be seen the identification details and branding that is etched into the glass.
The wide glass tube envelope is 40 mm in diameter, and excluding the B9G base pins is 90 mm tall.