The 6AQ5L (6928) is a modified 6AQ5 specially designed for long life operation in telephone systems. The 6AQ5 itself is a small audio output beam tetrode valve capable of 4.5 Watts in a single ended configuration.
The Ericsson 6AQ5L was made in 1955, the control grid was wound with gold plated wire, which has less tendency to grid emission. The changed form of the cathode provides a more constant grid-cathode distance. Moreover, since the diameter of the lateral wire was reduced from 0.08 to 0.05 mm, the cathode current will be more uniform over the whole surface. A higher transconductance is also obtained for the same minimum grid-cathode distance.
The Ericsson logo and made in Sweden.
The copper control grid supports below the lower mica and the U shaped heat-sink attached to the supports at the top.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins, is 60 mm long.
References: Data-sheet. Type 6AQ5L was first introduced in 1955.