The 705A is an EHT power rectifier with a PIV rating of 35,000 Volts. The average rectified current is 100 mA with a peak of 500 mA. The construction is all glass with the lead-out wires being fixed to the main pins that are held in a ceramic disc outside the envelope. The special base clamps to this ceramic disc.
The anode on this exhibit is a cylinder held in place by a single support that connects to the top pin. Western Electric made their version with three anode fins.
The side view of the anode fixing flange. The filament wires connect at the bottom to the two angled supports. The stout middle support is the centre tap and the filament wire connects to this support at the top.
The filament forms a double helix and both strands are wound round the top of the middle support. Both strands of the filament on this exhibit are broken and pieces of lie in the envelope.
The wide glass tube envelope is 57 mm in diameter, and excluding the B4A base pins is 115 mm tall.
References: Data-sheet & 1040. Type 705A was first introduced in 1943.