Demonstration Analog Computer with Vacuum Tubes

This is an English translation of the original German document “Demonstrations-Analogrechner mit Röhren” by F. Vogel (21.08.2010).

1. Overview

This small vacuum-tube analog computer serves to demonstrate electronic analog computing technology as it was used in the period from approximately 1945 to approximately 1960, before being very rapidly displaced by transistor- and semiconductor-based computers. The device allows the solution of simple, typical examples from the field of analog computing. The number of computing components present in the device is matched to these computing examples.

The analog computer is equipped with 10 tubes (4× 6U8, 3× EAA91, 3× OA2) and operates with supply voltages of +300 V and −150 V. The computing voltage (machine unit) is ±50 volts. Resistors that influence computing accuracy are selected to ±0.5% tolerance. The integrator capacitors have a tolerance of ±5%. The computer is built into a metal enclosure with dimensions 280 × 205 × 135 mm (L × W × H). The following computing components and auxiliary devices are available:

2 integrators / summers
1 pure summer
1 open amplifier / inverter
3 coefficient potentiometers
1 diode function generator for the function y = −x²
2 free diodes
2 reference voltage sources for +50.0 V and −50.0 V
1 display meter with address selector switch and measurement-range switch
1 stabilized power supply unit for +300 V / 20 mA and −150 V / 20 mA

The individual components are connected with patch cables to program the computer. These cables are available in two different lengths as well as branching cables.

2. Controls and Programming Panel

(Panel elements listed in the original: display meter, range switch, integrator control, address selector switch, programming panel, coefficient potentiometers, DC switch, main switch, null-balance control.)

3. Computer Components

3.1 Computing Amplifiers

The computer has four identical computing amplifiers (see circuit diagram 1). The circuit design of these amplifiers was adopted from the EC-1 analog computer, which was developed by the Heath Company as a training computer and came to market around 1960.

It is of course not possible to build a high-quality DC amplifier with only a single pentode-triode tube. However, through a very skillful circuit design, Heath managed to optimize the properties with regard to gain factor, linearity, and low output impedance to such a degree that the amplifiers fully satisfy the requirements of a training or demonstration computer.

The pentode of the input stage operates with a very high anode resistance of 10 MΩ and a screen-grid voltage of only approximately 10 volts. The second tube stage is coupled to the first stage without a voltage divider. In order to raise the total gain to approximately 1000, a small positive feedback is generated via resistor R15 (2.2 MΩ). To prevent the positive feedback from causing a tendency to oscillate, a low-pass filter (R14, C11) was inserted into the circuit. This does strongly reduce the cutoff frequency of the amplifiers, but this is of no significance for the intended application. The second tube stage operates as a cathode follower, thereby providing a very low amplifier output impedance. As a consequence of the direct stage coupling, the cathode voltage of the triode is very high above the zero potential. It is therefore shifted downward by approximately 120 V by the series connection of two glow-discharge tubes. From the determined gain diagram it can be seen that the computing amplifiers exhibit very good linearity in the normal operating range of ±50 volts.

The four computing amplifiers A1 through A4 have different, fixed computing functions:

Amplifiers A1 and A2 can optionally be used as integrators or as summers. They have two variable inputs x1 and x2 with a gain factor of 1, and an additional input IC for specifying an initial condition. A control switch allows three operating modes to be selected:

In switch position SUM, both amplifiers operate as summers: y = −(x1 + x2).
In switch position RUN, amplifiers A1 and A2 operate as integrators: y = −(1/T) ∫(x1 + x2) dt − IC. The integration time constant T is 1 second.
In switch position RES, the initial conditions of the integrators are defined: y = −IC.

Amplifier A3 is a summer with three variable inputs. Inputs x1 and x2 have a gain factor of 1; input x3 has a gain factor of 2: y = −(x1 + x2 + 2·x3).

Amplifier A4 is an open amplifier with two inputs x1 and x2 and a further input SP, through which the summing point of the amplifier is accessible: y = −A·(x1 + x2), A ≈ 1000.

If input x2 is connected to the output, amplifier A4 operates as a normal inverter: y = −x1. When used as an inverter, input SP must not be connected.

3.2 Diode Function Generator

As a typical example of the application of a diode function generator, the function y = −x² was realized. The curve of the function is approximated by five straight-line segments in the variable range 0 ≤ x ≤ +1 (i.e., 0 ≤ Ue ≤ +50 V). Circuit diagram 2 shows the construction of the diode function generator. The breakpoints of the straight-line segments are set by divider resistors R82 through R89. The slopes of the straight-line segments are adjusted with the controls P1 through P5. The maximum deviation of the approximated curve from the mathematical ideal is less than 1%. With two additional free diodes and an inverter, the diode function generator can be extended to the variable range −1 ≤ x ≤ +1 (i.e., −50 V ≤ Ue ≤ +50 V). Further details are described in computing example 6.3.1.

3.3 Coefficient Potentiometers

The three coefficient potentiometers are single-turn cermet potentiometers with a nominal resistance of 100 kΩ and a power rating of 0.5 W.

3.4 Reference Voltage Sources

The machine unit of the analog computer is ±50 V. The device therefore has two reference voltages of +50 V and −50 V, generated by the series connection of two selected Zener diodes each. The accuracy of these voltages is ±0.5 V.

3.5 Display Meter

The built-in moving-coil instrument has a scale with the zero point at center position and a scale division of ±100. A rotary switch allows four measurement ranges to be selected: ±100 V, ±50 V, ±10 V, and ±5 V. With an address selector switch, the display meter can be connected to the outputs of the four computing amplifiers or to the input jack “IN” on the programming panel.

3.6 Power Supply Unit

The power supply unit (see circuit diagram 2) supplies the computing amplifiers with two stabilized DC voltages of +300 V and −150 V. Since the total current requirement of the four computing amplifiers is only approximately +15 mA and −20 mA respectively, the power supply was built using three OA2 stabilizer tubes without additional series-regulator tubes. The power supply unit also provides the two reference voltages +50 V and −50 V.

4. Mechanical Construction

The analog computer is built into a metal enclosure with dimensions 280 × 205 × 133 mm (L × W × H). All controls and the programming panel are located on the front panel.

The electronics of the analog computer are housed on two circuit boards:

The amplifier board contains the four computing amplifiers, the diode function generator, and two free diodes.

The power supply board is populated on both sides. All power resistors are mounted on the underside of the board and are cooled through the ventilation grille in the base plate. This prevents the electrolytic capacitors on the top side of the board from overheating.

(Internal view photographs and component layout diagrams follow in the original.)

5. Circuit Diagrams and Component Layout Plans

Circuit diagram 1: Computing amplifiers
Circuit diagram 2: Power supply, diode function generator, free diodes, coefficient potentiometers, reference voltages, display meter
Amplifier board: component layout
Power supply board: component layout (top side)
Component layout (bottom side)

[Translation covers the first 8 pages; the original is 8 pages total — full document translated above.]