The Technology of Anita Calculators Explained

The Technology Explained

This section has a brief description of the technology used in the early Anita calculators. Items covered:

Vacuum Tubes (Thermionic Valves).

Cold-cathode switching tubes.

Dekatron counting tubes.

Cold-cathode Numerical Display Tubes - "Nixie" Tubes.

1) Vacuum Tubes (Thermionic Valves)

Inside the glass envelope of a vacuum tube (thermionic valve), which is evacuated to a hard vacuum, are several electrodes.

In the the triode, which is the simplest type, is a cathode, an anode, and a grid. The cathode is heated, which causes a thin cloud of electrons to be emitted from its surface. The nearby anode is given a high positive voltage which<br>attracts these electrons and so a current flows. By varying the voltage on a grid which is between the anode and the cathode, and is in the path of the electrons, the flow of the electrons (ie. the current) can be varied.

The more complicated types such as the tetrode and pentode have extra electrodes to give different control characteristics.

Vacuum tubes were used in the same roles as transistors were later, though the circuit requirements of the two are considerably different.

The Anita uses a small number of vacuum tubes, several in the power supply and two in the logic circuits

Detail of the keyboard decoder board showing the GS10D "Dekatron" decade counter tube, with a small ECC81 vacuum tube (thermionic valve) to its right, which is part of a blocking oscillator<br>circuit providing driving pulses at a rate of 4000 per second.

One of the ECC81 double-triode vacuum tubes (thermionic valves) used in the Anita Mk8, with a 9-pin base.

Each of the dark-grey vertical structures contains one triode.

2) Cold-cathode Switching Tubes

In contrast to the vacuum tube which has a hard vacuum and a heated cathode, the cold-cathode switching tube has low-pressure gas inside and has an unheated cathode.

Although the early Anita calculators do use a small number of thermionic valves, the logic circuits incorporate large numbers of Cold Cathode Tubes. In contrast to the thermionic valves the Cold Cathode tubes contain a rarefied<br>inert gas (e.g. neon), and electrons are emitted in an avalanche from the unheated cathode when the voltage between the anode and cathode reaches a high enough value. The flowing current excites the gas inside and produces a glow -<br>this is the way that the small neon mains indicator lamps work. A relatively small voltage on a control grid between the anode and cathode has a great effect on whether a current of electrons flows or not and can switch it on and<br>off.

Cold-cathode trigger tubes operate in a similar way to the modern semiconductor thyristor.

The Anita Mk 8 uses 170 cold-cathode tubes, which were cheap and fairly robust, both electrically and mechanically. An article of 1965 on cold cathode tubes from Mullard Ltd. (a British manufacturer of<br>these devices who also produced a prototype calculator using them) gives an insight into why they were used in these calculators at the time:<br>"... They are an accountant's dream; a typical modern tube has a life<br>expectancy several thousand times better than the conventional thermionic tube, although they employ voltages of the same order. They are much cheaper than either semiconductor devices or vacuum tubes; they do not require costly<br>materials with a high degree of purity in their manufacture, nor do need transformers or cooling systems to operate. The tubes require no warm-up period and they can take severe overload."

One of the 12 counter/display boards of the Anita Mk 8 with the Nixie-type display tube mounted on the edge. The board holds ten XC-31 cold-cathode switching tubes and 5 black selenium rectifier diode blocks in a ring-counter<br>circuit.

Each switching tube is connected to a number in the display tube, and only one switching tube is turned on at any time. When the board receives a pulse, the next switching tube in the ring counter is turned on and the next<br>number in the display tube is illuminated. If the digit "9" is illuminated and another pulse arrives then the count wraps round the ring so "0" is illuminated and a carry pulse is directed to the adjacent<br>board dealing with the next decade.

The ring-counter circuit can count down as well as up.

Each display board stands vertically and plugs into the main logic board.

A small section of one of the display/counter boards, showing three Hivac XC31 cold-cathode switching tubes and a selenium rectifier.

A view of the logic boards of an Anita Mk 8 during operation. The horizontal main board, underneath, carries three vertical daughter boards. The tubes on the daughter boards are cold-cathode switching tubes and the black blocks<br>are selenium rectifier diodes.

When a cold-cathode tube is switched to the ON state it glows orange like a neon lamp, as can be seen in 5 of the tubes on the daughter boards. When the machine calculates the tubes flicker and a<br>different...