From Electronic Engineering magazine, Jan 1955, 'Notes from North America'

A New Beam Switching Tube

A NEW type of beam switching tube is being developed by the Burroughs for use in computers.

The tube employs the beam forming properties of crossed electric and magnetic fields, the latter being provided by a small circular permanent magnet placed externally round the tube. Internally, the tube is composed of ten groups of electrodes mounted radially about a central cathode. The basic elements of each group are:

1. A spade to form and lock the beam.
2. A grid to switch the beam.
3. A target to give a useful output.

By forming the beam on any one spade an output signal is produced on the corresponding target plate. Since the beam may be automatically formed and locked on any spade, there are ten discrete outputs associated with the tube. An eleventh stable state occurs when all the elements of the tube are positive with respect to the cathode. This is the clear condition in which there is no output to any target plate. By lowering the voltage of any one spade from the clear condition, the following characteristic curve is obtained.

Of the three possible operating points, A, B, and C, only A and C are stable, while B is unstable. When the tube is in the clear condition, all the spades are in state C. By lowering the voltage of any one spade beyond point B with a suitable series load, it will assume its other stable state, which is at near-cathode potential. This affects the relationship between the electric field and the magnetic field so that the beam forms to that spade in the manner shown.

Over 90 per cent of the beam is utilized in producing an output with a pentode type characteristic. The remainder is used automatically to form and lock the beam in position. This is obtained without suppression, and a wide variety of target resistor load lines can intersect the constant current portion of the curve with negligible crosstalk. When a beam has been formed on a spade it can remain there indefinitely, or it can be advanced by lowering associated grid voltage. A voltage drop on the switching grid will disturb the electric field so that enough of the beam is deflected to leading spade to cause that spade to assume its a stable state (A). The beam will now be formed on this spade.

Because of their shape and position, the grid electrodes give a very uniform switching action. This action is the order of 0·1 microsecond, and draws negligible current. Since the switching grid will affect the beam only in region associated with the grid, the grid electrodes are conected in two groups, the odd numbered grids in one group and the even numbered in the other. In this way it possible to use a d.c. input and still secure single position stepping which eliminates the necessity for a critical width for this type of operation.