The Mullard DAC1 is a battery diode triode. This would be used in 'straight' or TRF radio receivers as diode detector and first audio voltage amplifier. Alternately within a superhet receiver the triode would be the detector and the diode the AGC rectifier.
Unlike the Ever Ready version DAC1 this has a clear glass envelope.
The filament is a single strand running from the support on the pinch to the tension spring above the top mica. The triode is at the top with the diode anode below.
A closer view of the top mica components. The filament tension spring is welded to a short tape that is folded to pinch the filament in place,
The wide glass tube envelope is 30 mm in diameter, and excluding the Ct8 base pins is 91 mm tall.
Reference: 1043. Type DAC1 was first introduced in 1939.