The MT9 valves were large transmitting triodes, specifically as a power oscillator but also described as a modulator, the L variant.
The MT9 series had a molybdenum anode capable of 750 Watts dissipation and an anode voltage of 10,000. At full load the anode would glow cherry red (approx 750°C).
In the 1925 2LO transmitter an MT9 was used as master oscillator.
The GEC make can be seen as can the screw terminals fitted for the connections.
Reproduction of a 1931 leaflet.
The balloon envelope is 162 mm in diameter, and excluding the leads is 180 mm tall.