Hivac VS10G Trochotron

The device measures 88x45mm overall and weighs about 200g. It has 26 pins on the base (B26A) arranged as an inner ring of 9 and an outer ring of 17. The outer ring has a gap as a key.

The trochotron carries the serial number 1984 and also the number 6807.

The Trochotron is a form of counter which can operate at up to 2MHz. It is also referred to as the "beam switching tube" or "beam-X switch". It is a thermionic device surrounded by a magnet. The device has ten target electrodes arranged around the outside. An indirectly heated cathode is at the centre of the device. In between the cathode and each target is a J-shaped electrode called the "spade", and adjacent to these are flat electrodes called grids.

Alternate grids are connected together to form two groups known as the "odd" and "even" grids. Each spade and target has a separate connection. The photograph shows one set of grids connected together; the other grid connections are at the base of the structure.

With the correct voltages applied, the magnetic and electric fields are such that electrons spiral around the cathode in a trochoidal curve - thus the name Trochotron. With all the electrodes positive the beam is not attracted to any of them due to the uniform radial electric field created. When a spade voltage is lowered to ground, it distorts the field, allowing electrons to strike the associated target, and the beam locks in this position. The beam is moved around the targets by applying a negative pulse to the 'even' and then 'odd' grids, bumping the beam round each time.

See also ET51, 6700 and BX1000.

Here is a dismantled one, without the magnet!