The ACL4 is an indirectly heated audio output triode. It has a side terminal to provide the extra contact. The move to indirect AC heating showed the limitations of the B4 base. After using side screw terminals for a while the B5 base was introduced.
Thrower places the ACL4 as a B5 based valve made by Three Six Two whereas in this exhibit the Osram identification can be seen etched on the side of the envelope and the cap is B4 with side terminal - probably earlier than 1933. The magnesium gettering is the mirror inside the envelope.
Turned round, the getter carrier can be seen facing the silvered side of the envelope. The anode is made of mesh and probably the mesh is woven from nickel wire.
The glass bead holds the heater top support in place. The insulation around the heater and the oxide coated cathode are both visible. The control grid is supported on two notched wires. The dome of the envelope is etched with the valve identification and in this case it is clearly visible.
A closer view of the rigid supports and the spiral control grid.
The coating on the cathode can be seen to be quite thick.
The balloon envelope is 51 mm in diameter, and excluding the B4 base pins is 101 mm tall.
Reference: 1043. Type ACL4 was first introduced in 1933.