The 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.
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 envelope also carries the GPO Type designation of POVT111.
The four oxide coated filament loops with the springs held in the glass rod.
The filament tension springs and glass rod.
Here can be seen the folds in the ends of the springs and the small wire hooks that actually hold the filament loops. The box anode is supported on four rods that pass to a collar around the pinch stem.
The balloon envelope is 65 mm in diameter and, excluding the B4 base pins, is 141 mm tall.
References: Data-sheet & private communication. Type DA30 was first introduced in 1935. See also1935 adverts.
