The POVT111 (also marked as DA30) is essentially an uprated PX25 with a wider grid pitch and lower μ value. It was intended for use in push-pull pairs (Class AB1) to provide up to 45 W of undistorted audio for PA systems and transmitter voice modulation. In this respect it is nearly twice as powerful as the PX25.
Clearly the UK General Post Office used these valves in telephone repeater amplifiers.
The advantage of Class AB1 is that it allows very high peak output with large variable signal levels (eg. audio) but consumes relatively little power whenever the signal level is (briefly) low, as with speech). The penalty is that a large negative grid bias is required and the peak signal input must be correspondingly high. Up to 288 V pk - pk. However, as the grid circuit does not need to consume significant amounts of audio drive power, the preamplifier can be a low power type and the high peak grid voltage requirements can be met by using a push-pull drive transformer. Heavy but efficient and cool-running.
The rectangular anode is secured in all four corners by rods that pass to a clamp around the pinch stem. This makes for a rigid structure in valves like this with no top support for the electrodes.
The filament tension springs and hooks are fixed into an insulating glass rod.
Here the glass rod can be seen to be fixed to the anode side supports.
The balloon envelope is 65 mm in diameter and, excluding the B4 base pins, is 137 mm tall.