In the 1950s, before integrated circuits or even transistors, mainframe computers were built from thousands of power-hungry vacuum tubes filling massive cabinets. To simplify construction and maintenance of these computers, IBM invented a pluggable module with eight tubes;1 failed modules could be quickly pulled out of the computer and replaced. I came across one of these tube modules and wondered if it would still work decades later. Could I power it up and demonstrate it in a circuit, or would the components have failed with time?
Part I of my post discussed tube modules and described the IBM 705 that used this module. To recap, the IBM 705 was a large business computer introduced in 1954. It weighed 16 tons, used 70 kilowatts of power and cost $15 million (in 2018 dollars). A few dozen 705 systems were built, mostly used by large companies and the US government. For example, Texaco2 used the 705 for accounting applications such as payroll, marketing, and distribution. Even though the 705 was intended as a business computer, Texaco also used it for technical applications such as refinery simulation and pipe stress analysis. Below you can see the large CPU of an IBM 705 computer. Each of the four panels in the front held up to 80 tube modules.
The debouncer
By tracing out the circuitry of the tube module and studying old IBM documents, I determined that the module consisted of five key debouncing circuits. When you press a key or button, the metal contacts inside the switch tend to bounce against each other a few times before closing, so you end up with multiple open/closed signals, rather than a nice, clean signal. To use a key signal in a computer, it needs to be "debounced", with the multiple rapid transitions replaced by a single, clean transition. (Perhaps you have used a cheap keyboard that occasionally gives you double letters; this happens when the keyboard bounces more than the debounce circuit can handle.) In modern systems, debouncing is usually done in software, but back in the 1950s tubes were used for debouncing.
The IBM 705 was controlled from a complex console with control keys and neon status lights (below). The console was used for manual control of the computer, monitoring status, detecting and correcting errors, and debugging. (While memory could be modified from the console keyboard, programs were normally read from punch cards.) Tube-based debouncing circuits were used on many of the console keys to ensure proper operation, so that's probably the role of the module I examined.3
Vacuum tubes
Since most readers probably haven't used vacuum tubes, I'll give a bit of background. This module was built from a common type of vacuum tube known as a triode. In a triode, electrons flow from the cathode to the plate (or anode), under the control of the grid. The heater, similar to a light bulb filament, heats the cathode to around 1000°F, causing electrons to "boil" off the cathode. The anode has a large positive voltage (+140V in this module), which attracts the negatively-charged electrons. The grid is placed between the cathode and the anode to control the electron flow. If the grid is negative, it repels the electrons, blocking the flow to the plate. Thus, the triode can act as a switch, with the grid turning on and off the flow of electrons. The module I examined used dual triode tubes, combining two triodes into one tube for compactness.
Two vacuum tube circuits form the building blocks of this tube module: the inverting amplifier and the cathode follower. The schematic below shows a vacuum tube inverting amplifier (slightly simplified). If the input to the grid is negative, the flow of electrons through the tube is blocked, and the output is pulled up to +140 volts by the resistor. If the input to the grid is positive, electrons flow through the tube, pulling the plate output close to ground. Thus, the circuit both amplifies the input (since a small input change causes a large output change) and inverts the input.
The second circuit used in the module is the cathode follower, which is essentially a buffer; its low-impedance output let it drive other circuits. While the schematic (below) looks similar to the inverter, it has the opposite effect since the output is from the cathode, not the plate. If a positive input voltage is fed into the grid, the tube will conduct. The voltage drop across the cathode resistor will cause the cathode voltage (and thus the output voltage) to rise. On the other hand, if the input voltage is negative, electron flow will be reduced, shrinking the voltage drop across the resistor and reducing the cathode voltage. Either way, the cathode voltage (and output) will adjust to be approximately equal to the input voltage. The trick is that it's not a negative grid per se that blocks electron flow, but a negative grid with respect to the cathode.4 Thus the cathode follower essentially copies its input voltage to the output, but providing higher current.
Powering the filaments
The first step in making the module operational was to power up the vacuum tube filaments. Filaments usually operate at 6.3V AC for historical reasons (a 6V lead-acid battery contained three 2.1V cells, yielding 6.3V). Each tube's filament uses almost 3 watts, so a large filament transformer is necessary. The filament energy consumption is part of the reason tube-based computers used so much power.
We connected the transformer and inserted tubes one at a time to power up their filaments. Everything went smoothly—the tubes all lit up with a nice orange glow and none were burnt out. Each tube has two filaments (since they are dual triodes), so we could see two orange spots in each.
Powering the circuitry
Powering the tube module is inconvenient because of the multiple large voltages required. This tube module uses +140V, -60V and -130V, with input signals of probably 48V.10 The power supplies we had available only went up to +/- 120V, but I did some simulations with LTspice that showed the module should work with the lower voltages. We used a stack of power supplies to power the module, mostly vintage HP supplies from Marc's collection.5
The connector for the tube module probably hasn't been manufactured in 50 years, so I needed to find a way to connect wires to the module. I could have soldered wires directly to the module, but I didn't want to modify the module since it's a historical artifact. Instead I found that .110 quick-disconnect terminals fit (more or less) on the module's pins. To manage all the connections to the tube module, I built a small junction box to connect banana plugs from the power supplies to the tube module. This box also had a button to trigger the input.6
The trigger circuit
The schematic78 below shows one of the debounce circuits, which IBM called a "contact-operated trigger". (A trigger is the old term for a flip-flop, or a similar circuit that can be in two states.) The input goes through the resistor-capacitor low-pass filter on the left, smoothing out the input so the circuit won't respond to bounces. This goes to the grid (2) of a triode inverter amplifier as discussed earlier. The output from the first inverter (plate, 1) is connected to a second inverter (grid, 7) via the 91K resistor. The output from the second inverter (plate, 6) is shifted to the desired +30/-10 voltage range by a voltage divider (the 390K and 430K resistors). Shifting the voltage down is the reason a -130V supply is needed. 6 (The two inverters form a Schmitt trigger9 due to the connected cathodes and 3K resistor.) The output from this circuit is connected to a cathode follower (described earlier but not shown in the schematic below), which buffers the signal for use by other modules.
The diagram below illustrates how the debouncer functions. The red line shows the input, say from a button press. Notice that the switch opens and closes twice before closing for good. If the computer used this input as a control, it might perform the operation three times. The blue line shows the debounced signal, which turns on once cleanly. To perform the debouncing, the debouncer uses a resistor-capacitor filter to smooth out (i.e. integrate) the input signal into a slowly-changing signal; this is the green line. When the green signal gets high enough; the output turns on. Note that the output stays on even though the green signal drops in the last bounce. This is due to the Schmitt trigger; it turns off at a much lower level than where it turns on.
Using the tube module
The tube module contains five debounce circuits, each made up of a trigger and a cathode follower. The diagram above shows how these circuits map onto the eight tubes. Each cathode follower (CF) uses one triode (half a tube) so there are two cathode followers per tube.11 Since there's one triode left over, the last debouncer (5) has a double cathode follower for a higher current output. The photo below shows that someone marked the top of the module with red and greed dots indicating the two tube types and functions, probably to simplify maintenance.
We tested one of the five debounce circuits in the module. Although the tube module contains five debouncers, some resistors were knocked off the module over the years, so debounce circuit 4 was the only one we could use without repairs. Below, you can see the power and signal wires hooked up to the tube module, along with an oscilloscope probe to view the output. At the left, we have wired a neon bulb to the debouncer's status output. In the computer, these status outputs were connected to neon bulbs on the console or on maintenance panels.
The oscilloscope trace below shows the debounce circuit in operation. The input (yellow) is a pulse with contact bounce. You can see a bunch of large spikes due to bounce, as well as couple bounces of longer duration. There is also some bouncing at the end of the pulse. In contrast, the output (green) is a clean signal with a sharp transition. The bounce and noise in input signal could cause erroneous operation if used in a computer, for instance causing multiple operations. On the other hand, the output signal provides a single clean pulse to the computer, ensuring proper operation. Note that the output signal turns on and off about 1.3 ms after the input signal; this delay is due to slow charging and discharging of the R-C filter.
Once we had the tube module operational, we hooked it up to a vintage HP pulse counter. Without the debouncer, pressing a button would result in multiple counts; each bounce incremented the count. But with the tube debouncer in the circuit, each button press resulted in a single count, showing the module was functioning. Marc's video below shows the pulse counter in operation. Because the pulse counter could only handle 5 volt inputs, we used a resistor divider to reduce the voltage from the tube module. The resistor divider was implemented with large resistor decade boxes; they are on top of the stack of power supplies in the earlier photo.
Conclusion
Despite its age, the tube module still worked. (At least the one of the five debouncers we tested.) I was a little concerned about putting high voltages through such old electronics, but there were no sparks or smoke. Fortunately Marc had enough power supplies to power the module, and even though we fell a few volts short on the -130V and +140V the module still functioned. The module was bulky and consumed a lot of power—I could feel the warmth from it—so it's easy to understand why transistors rapidly made vacuum tube computers obsolete. Even so, its amazing to think that the principles of modern computers were developed using vacuum tubes so long ago.
Thanks to Carl Claunch for providing the module. Thanks to Paul Pierce, bitsavers and the Computer History Museum for making IBM 700 documentation available.
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Notes and references
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Although each module contained eight tubes, this does not correspond to a byte. The eight tubes generally don't map onto eight of anything since circuits can use more or less than one tube. Also, the byte wasn't a thing back then; IBM's vacuum tube computers used 36-bit words (scientific models), or 6-bit characters (business models). ↩
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For a detailed list of companies using the IBM 705 and their applications, see the 1961 BRL report. This report has extensive information on early computers and how they were used. ↩
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By studying the IBM 705 schematics, I found that the debouncing circuit was used with many keys on the IBM 705 console, such as reset, clear memory, initial reset, machine stop, display, test start and test stop. The debouncer was also used for relay-controlled console signals such as machine stop, manual stop, manual store, memory test, half/multiple step and display step. I didn't see this specific tube module in the schematics, so the module could have been part of an IBM 705 peripheral or the earlier IBM 702 computer. ↩
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The cathode follower might seem like it should oscillate: when the cathode is low, current will flow, pulling the cathode high, which will block current flow, making the cathode low again. Yes, it could oscillate if the wires are too long, for instance. But it's designed so it will reach a stable intermediate point where everything balances out. ↩
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To supply the module with power, we used two vintage HP3068A power supplies (60V each) for the -60V and -120V. An HP6645A DC power supply provided 120V. A modern Protek 3003B power supply provided 30V for the input. We counted pulses using an HP 5334B universal counter. ↩
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There's a second output from the inverter's plate. This high-voltage output is used to drive a neon indicator bulb, showing the status of the circuit. The 1MΩ resistor limits current through the neon bulb. ↩
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The schematic says the input is "from key, relay or CB." A "CB" is a circuit breaker, but not in the modern current-protection sense. In old IBM machines, a circuit breaker was a microswitch triggered by a rotating cam, providing a timing signal. That is, it breaks the circuit as it opens and closes. Circuit breakers were common in electromechanical systems such as tabulating machines and card readers. ↩
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The schematic is from 700 Series Component Circuits, an IBM document that describes the wide variety of circuits used in IBM tube modules. If you want to understand tube modules in detail, this is the document to read. The contact-operated trigger is described on page C-35 and the schematic is on page C-36. The cathode follower is discussed on page A-43. ↩
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The tube module uses Schmitt triggers, which were invented in 1937. A Schmitt trigger provides hysteresis; when it turns on, it stays on until the input drops significantly. The Schmitt trigger is implemented by connecting the cathodes together, along with a series resistor. When one triode turns on, the cathode voltage will rise due to the resistor, similar to the cathode follower circuit. But since both triodes have the same cathode voltage, the rising cathode voltage will tend to shut the other triode off. Since it's harder for the other triode to turn on, the Schmitt trigger will stay in its current state until there is a large voltage swing on the input. ↩
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Vacuum tube systems used many different inconveniently-large voltages. The table below (from 700 Series Component Circuits) lists the voltages used by the IBM 704 (scientific) and IBM 705 (business) computers. I was surprised that the scientific computers and business computers used totally different voltages, but historically they were entirely different systems. IBM's 701 scientific computer started as the "Defense Calculator" project, while the 702 business computer came from IBM's TPM II (Tape Processing Machine) project. Thus, the two branches of the 700 series ended up with completely different architectures. (Among other differences, the 701 used binary while the 702 used binary-coded decimal characters.) They also ended up with different hardware components.
IBM's vacuum tube computers use a large variety of voltages. Based on 700 Series Component Circuits, page E27.The power supply manual explains some of the voltages and their uses. A +500V supply was used by the IBM 701 to power its electrostatic memory system, which stored bits on the surface of Williams tubes. Later machines used core memory instead, with the voltages listed above. The +15V and -30V were used as diode clamps to keep output voltages in the proper range. The +220V supply was typically used by AND gates, while OR gates used -250V. Tubes typically used the +150V supply for plates and -100V for cathodes. In the business computers (702/705), most logic used +270, +140V, -12V, -60V, -130V and -270V. ↩
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The triggers are implemented with type 6211 tubes, while the cathode followers are implemented with the more powerful 5965 tubes. From the photo, it may appear that the tubes don't match the locations since there are four tubes with metallized tops and four tubes with metallized sides. However, the tube markings indicate that all tubes were in the right locations. The location of the shiny getter is independent of the tube type. ↩
