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Before Transistors

FIGURE 1 This is an infinite heat switch—an electromechanical device capable of handling 3000W with a resolution that would normally be associated with a PWM circuit.
Written by Brian Millier

How Did They Do It Back Then?

As part of Circuit Cellar’s celebration of its 400th issue, Brian looks back at some ingenious electromechanical devices that performed necessary functions, using existing technology. Many were so clever that they are still in use today—even while microcontrollers are used in just about everything.


Both at work and at home, we expect that most of the devices we use daily will contain some form of a microcontroller (MCU), or at least electronic circuitry. Sometimes this trend goes overboard, and we hear talk of toasters that are Internet-connected.

Many of these consumer and commercial/industrial products were introduced 50-75 years ago—prior to the invention of transistors, never mind complex digital integrated circuits. How did the engineers and designers of the era design products that, while not as sophisticated as those available now, nevertheless did the job adequately? When I began my career at General Electric Consumer Products Division, I quickly discovered just how cheaply electromechanical components could be made, and that these components could still do the job in a reliable way.

In this column, I’m going to describe several components and circuits—found in industrial, scientific, and consumer products—that I’ve worked with and found to be extremely ingenious. Let’s start close to home.

A HIGH-POWERED KITCHEN

Electric stoves have been around for 100 years. Regardless of their vintage, they all contained a number of “surface units” for heating foods in pots and skillets. These heaters consume about 3000W, on average, and ideally should be finely adjustable in terms of heating power delivered. Early stoves contained surface units in which the spiral Calrod heating element was made up of three discrete heaters of different wattages. A complex mechanical switch would “dial in” eight different combinations of these three elements, providing a rough control of heating power. This wasn’t ideal, and in time, the infinite heat switch was invented (Figure 1).

Using modern components and techniques, controlling an AC-powered 3000W heater today would likely be done with a PWM generator feeding a solid-state relay (which is basically an opto-coupled TRIAC mounted in a heatsink enclosure). In the 1970s, when infinite heat switches were introduced for stoves, SCRs were quite new and still expensive, and TRIACS were not available.

Referring to Figure 2, SW1 is a set of high-current contacts, one of which is connected to a length of bimetallic strip. A bimetallic strip is made up of two dissimilar metals that each have different thermal coefficients of expansion. When heated, a bimetallic strip will bend in proportion to how much heat is applied. In this device, the bimetallic strip has a small heater coil wound around it. The dial of an infinite heat switch is connected to a contoured cam. When it is moved from the off position, it closes both SW2, the on-off switch, and SW1 containing the bimetallic strip. At this point, the 240VAC will pass through both switches and power up both the surface unit and the small heater wound around the bimetallic strip. When this small heater heats up it will bend the bimetallic strip so that it opens the contacts of SW1. Both this small heater and the surface unit will then stop heating, and in a few seconds, the bimetallic strip will cool off and SW1 will again close. While this diagram doesn’t accurately reflect the shape of the cam, suffice it to say that as you rotate the dial knob for more heat, the cam will adjust the position of the bimetallic strip in such a way that it must heat up more to open SW1’s contacts. The whole on-off cycle will take 10-40 seconds, depending upon the dial setting, and the PWM duty cycle will vary from about 10% to 100% in fine increments.

FIGURE 1
This is an infinite heat switch—an electromechanical device capable of handling 3000W with a resolution that would normally be associated with a PWM circuit.
FIGURE 1
This is an infinite heat switch—an electromechanical device capable of handling 3000W with a resolution that would normally be associated with a PWM circuit.
FIGURE 2
This is a block diagram of an infinite heat switch. In the text, I describe how it operates.
FIGURE 2
This is a block diagram of an infinite heat switch. In the text, I describe how it operates.

When I first encountered them in the mid-1970s, infinite heat switches were made by Robertshaw, and they still are. Back then, I’m sure they cost no more than a few dollars to manufacture—a tiny fraction of what it would cost to perform the same function using PWM plus a solid-state relay. From experience, I know that they can easily last 20+ years in normal service. They are still in common use in stoves today.

THEY PUT VACUUM TUBES IN CARS?

Even if you are too young to have ever used them, most electronics people know what vacuum tubes are. Compared to transistors, they get hot, draw much more power, and are a lot more susceptible to shocks and vibration. They’re not something you would expect to see in a car. However, car radios were a popular option from the 1950s onward. While transistors had been invented in 1947, they were neither robust nor high enough in performance to operate in car radios for many years. So, for at least a decade or so, vacuum tubes were used in car radios.

Vacuum tubes often use 12V for their filaments, which matches the 12V car battery. However, to operate efficiently, they need at least 100VDC for their plate electrode. Using a “B” battery (around 100V), which was employed in old home radios, was not practical in a car. So, the 12VDC battery voltage had to be converted into AC, stepped up using a transformer, and then rectified back to a high enough DC potential to run the vacuum tubes. It would be easier to do this in today’s cars since alternators are now used to charge the battery. Internally they produce AC voltage which is rectified before it leaves the alternator. That AC voltage could feed a transformer directly, but automotive generators of that era produced DC voltage only.

Figure 3 shows a picture of the device that made vacuum tube car radios possible. It was called a vibrator. This one was made by Cornell Dubilier, which was well-known for its capacitors. I’m guessing Cornell Dubilier got into the vibrator business because it already used those cylindrical cans to house its power supply electrolytic capacitors.

Figure 4 is a schematic diagram of the circuit using such a vibrator in a car radio. Basically, a vibrator is like a two-pole relay, designed to handle being switched on and off rapidly, and for a long duration. The 12V battery voltage is applied to two of the vibrator’s four terminals. The current passes through a set of N.C. contacts to the vibrator coil, which energizes it. Once energized, it opens that N.C. contact, and the relay coil is deactivated. This part of the vibrator acts much like the old electromechanical buzzers used in the past, except that here we don’t want to hear the buzzing sound—therefore the can surrounding this vibrator is sound-insulated. I can’t recall the frequency that these vibrators operated at, but it was somewhere between 50 and 100Hz. The second set of contacts is SPDT and basically switches opposite ends of the radio’s power transformer to chassis ground. With 12VDC from the battery supplied to the center tap of the primary, we are effectively supplying a square wave AC voltage to the transformer’s primary winding. The voltage from the transformer’s high-voltage secondary is full-wave rectified (by a vacuum tube rectifier, if my memory serves me correctly) and filtered.

FIGURE 3
This is a vibrator unit that was used in early car radios to provide enough voltage to operate the vacuum tubes that were used in car radios at the time.
FIGURE 3
This is a vibrator unit that was used in early car radios to provide enough voltage to operate the vacuum tubes that were used in car radios at the time.
FIGURE 4
This is a schematic diagram of the high-voltage power supply used in older car radios containing vacuum tubes.
FIGURE 4
This is a schematic diagram of the high-voltage power supply used in older car radios containing vacuum tubes.

It speaks well of the engineers at the time that they could design an AM radio that would pick up distant RF signals clearly while in the presence of electromagnetic interference (EMI) from the spark plugs, distributor, generator commutator, and the contacts in the vibrator itself. Vibrators were not expensive in those days and lasted for many years.

SLOWLY DRIFTING AWAY

After learning about the automotive vibrator in the last section, can you think of another place where such an electromechanical device could be used? Let’s consider industrial process controllers, specifically ones in which temperature is controlled—possibly high temperatures in large ovens. The only temperature sensors capable of withstanding high temperatures—rugged and can work with long signal leads—are thermocouples.

Thermocouples produce only low millivolt-level signals even over a high-temperature span. These tiny signals must be amplified greatly before they can be used in some form of PID controller. However, amplifying a slowly changing DC signal by a large amount requires a high-gain amplifier, with a frequency response down to DC. Modern op-amps with zero-drift features are common today, but they weren’t 30-70 years ago when such controllers were required. It was difficult to design a high-gain DC amplifier that did not suffer from some amount of drift over temperature/time (especially using vacuum tubes). This drift could severely affect the accuracy of the controller, and many processes are critical regarding process temperature.

If you instead consider a high-gain AC amplifier, you can see that a multi-stage amplifier (needed for high gains) can have its stages coupled via capacitors. Any DC drift in a particular stage will not pass through this capacitor to subsequent stages. Therefore, a good solution is to convert the low-voltage thermocouple signal into an AC voltage, amplify it with a high-gain, drift-free AC amplifier, and then convert it back to DC again at the output.

Today, such switching is generally done using MOSFETs. They have fairly low RDS values, and don’t generate any DC offsets of their own (which would be meaningful given the low voltages produced by thermocouples). They can suffer from a phenomenon known as charge injection, but this isn’t much of a consideration at the low switching frequencies needed for this type of application.

However, MOSFETs weren’t around back in that era. Instead, if you consider the automotive vibrator, it has all the attributes needed to do the DC-AC conversion. In particular, it has basically 0Ω contact resistance when the contacts are closed, and it doesn’t generate any offset voltages of its own. And there’s another bonus: If you add a second SPDT set of contacts to the vibrator design, you can use that set to synchronously rectify the AC signal at the output of your AC amplifier. Again, no offset voltage errors are introduced, as these are only mechanical contacts.

In this case, such devices were called choppers. Figure 5A and Figure 5B are examples of such choppers. In a car, there was only a DC voltage available, so the vibrator needed its own set of contacts to switch the coil on and off rapidly. My memory is a bit hazy on this now, but I believe that the chopper’s coil was fed an AC voltage and the switching was performed at 60Hz (at least here in North America).

I had some spare choppers in my lab, which were used in chart recorders (another device that needed to be able to amplify tiny DC signals in a stable, drift-free manner). They were somewhat smaller than the ones shown in Figure 5. They didn’t turn my lab into a museum when I retired, and in time, all of these older parts were disposed of, so I have no photos.

A LIGHTBULB MOMENT

I recently read that the USA is banning incandescent light bulbs. Today, of course, the media tries to make a controversy about everything, so there is some confusion about whether this is a ban or just new, stringent energy efficiency regulations. One way or the other, almost no one uses incandescent lights for lighting any longer, since LEDs are cheap and vastly more energy efficient.

Figure 6 is a photo of a miniature light bulb like that which played a significant role in the first product ever produced by Hewlett-Packard—now a large multinational conglomerate in the computer/electronics industry. No, they didn’t start out producing light bulbs. Instead, Bill Hewlett and David Packard’s first product was an audio signal generator which was initially manufactured in David Packard’s garage in Palo Alto. You can Google “HP 200A” for a photo of the original HP 200A generator, but those photos are not clear enough to meet Circuit Cellar’s publishing standards.

High-quality audio signal generators were, and still are, essential in the audio industry. In particular, low waveform distortion and a wide frequency range are required. Getting both qualities simultaneously makes the design more difficult, but the Wien bridge oscillator configuration is one of the better choices. Figure 7 is a schematic diagram of the Wien bridge oscillator in HP’s original design. The frequency-determining components are R1,C1 and R2,C2, where R1=R2 and C1=C2. Capacitors C1,C2 are two sections of an air-variable capacitor and R1,R2 are switched by the frequency range switch. The combination of vacuum tubes V1 and V2 provides AC voltage gain. For a Wien bridge oscillator, the gain of the amplifier must be >3 for the circuit to sustain oscillation. However, if the gain is too large, the oscillator will saturate. But, even before such saturation, the sine wave amplitude would not remain constant if the gain changed.

FIGURE 6
This is a miniature incandescent lamp like the one that was an integral part of H.P.’s first product: the HP 200A audio signal generator.
FIGURE 6
This is a miniature incandescent lamp like the one that was an integral part of H.P.’s first product: the HP 200A audio signal generator.
FIGURE 7
This is a schematic diagram of the Wien bridge oscillator circuit used in the HP 200A audio signal generator.
FIGURE 7
This is a schematic diagram of the Wien bridge oscillator circuit used in the HP 200A audio signal generator.

Incandescent bulbs have a large positive temperature coefficient. That is, the hotter they get, the higher their resistance becomes. For lighting purposes, this is a double-edged sword. On the plus side, as the AC mains voltage varies (from time of day and load conditions), an incandescent bulb will compensate, to a fair degree, in terms of brightness. The downside is that the bulb’s filament is always cold when you first turn it on, and its lower cold resistance leads to a momentary current surge at turn-on. That’s why incandescent lamps often burn out right after you turn them on.

From Figure 7, you see that the bulb is placed in the cathode circuit of V1. The AC audio signal is coupled to the lamp via R24 and pot R25. When the signal amplitude increases, it feeds more power to the lamp, increasing its resistance. The higher the resistance present in the cathode return path to ground, the lower the gain of V1. This negative feedback, combined with the particular characteristics of the R19 lamp, act to produce a constant amplitude audio signal—even when switching ranges.

While there are other, more complex ways of building a Wein bridge audio oscillator with a constant output amplitude, none of these alternative circuits are even close to the simplicity and cost of the HP 200A’s light bulb scheme. The HP 200A was granted US Patent #2268872 in 1942.

Figure 8 is a photo of the somewhat newer HP 200CD which uses somewhat different circuitry from the original HP 200A design, but still uses vacuum tubes and the incandescent light bulb. There was one in my lab when I arrived, which I used, and which was still working when I left 30 years later.

FIGURE 8
This is an HP 200CD audio signal generator. It was a bit later model of the HP 200A. I used one in my lab.
FIGURE 8
This is an HP 200CD audio signal generator. It was a bit later model of the HP 200A. I used one in my lab.
THE GLOVES CAME OFF

I have another clever use of light bulbs. In the Department of Chemistry at Dalhousie University where I worked, there were numerous glove boxes, like that shown in Figure 9. In case you’re wondering, the gloves don’t extend outward like that when you are using them! The enclosure is hermetically sealed and often filled with a gas other than air—for chemical and/or safety reasons. The gas used may be expensive, so for that and safety reasons, it’s useful to be able to know if there are leaks in either the enclosure or the gloves themselves. If you cut a hole in the glass envelope of an incandescent lamp, you can operate it with something other than the vacuum under which it’s accustomed to operating. If this modified lamp were operated in our normal atmosphere, containing oxygen, it would quickly burn out due to oxidation of the filament. You’ll have noticed this if you’ve broken a light bulb that’s powered up. However, many of the gases used with glove boxes do not oxidize the lamp’s filament, so you could power up this modified bulb in that atmosphere and it wouldn’t burn out.

Figure 10 shows a simple schematic of such a leak alarm. The 120V mains power is applied to the incandescent bulb (a 120VAC 7W night light bulb works well) through resistor R5. The AC voltage across R5 is adjusted downward by pot R3 to provide 2.0VDC after rectification/filtering by D1 and C1. Under normal conditions, the lamp is lit up and there is enough voltage across R5 to turn the optocoupler on. Under these conditions, the 2N3904 does not conduct and the alarm buzzer doesn’t sound. If the glove box leaks and lets in oxygen from the outside air, the lamp quickly burns out and the optocoupler shuts off. This raises the voltage on the base of the 2N3904 and the SP1 buzzer sounds. Also, a positive 5V can trigger an optional timer module to start accumulating elapsed time. This allows the glove box operator to know for how long the proper gas atmosphere has been lost, and to act accordingly.

Like HP’s audio oscillator, this design, using a light bulb, is much simpler than other ways of accomplishing the same function. I built quite a number of these devices for some of the many glove boxes that were used in our chemistry department.

FIGURE 9
This is a glove box apparatus that is used in chemistry labs to handle chemicals either that are dangerous to humans or that must be handled in an atmosphere made up of gas(es) different from Earth’s normal atmosphere.
FIGURE 9
This is a glove box apparatus that is used in chemistry labs to handle chemicals either that are dangerous to humans or that must be handled in an atmosphere made up of gas(es) different from Earth’s normal atmosphere.
FIGURE 10
This is a schematic of a simple glove box leak detector based on a simple miniature incandescent light bulb. Like the HP 200A, it’s a novel use for a simple light bulb.
FIGURE 10
This is a schematic of a simple glove box leak detector based on a simple miniature incandescent light bulb. Like the HP 200A, it’s a novel use for a simple light bulb.
MECHANICAL ACTUATORS

Aside from robotics, there are many other areas in which mechanical actuators are needed to perform some physical function. Depending upon the task, either stepper motors or servomotors might be the best solution. Linear actuators are another common solution, but if they are electrically driven (not pneumatic or hydraulic), they would generally be driven by either a stepper motor or a servomotor.

In almost all cases, you need some positioning information fed back to the controller. This sensor would often take the form of a digital rotary encoder, or maybe just a potentiometer if the motion was limited to <360 degrees of rotary motion. Alternately, a linear potentiometer could be used if the motion was linear.

What if we add the criteria that there must be some haptic feedback as well? In simple terms, think of haptics as the controller needing to know how much resistance the actuator is encountering when it’s moving toward its targeted position. Possibly add to this the need for limit switches so the actuator doesn’t destroy something when it tries to move beyond some mechanical limit. Suddenly, the design of the controller/actuator becomes quite a bit more complicated. Modern controllers with powerful MCUs and intelligent sensors can handle this without too much difficulty. However, before transistors and IC chips, controlling an actuator electrically was difficult enough that pneumatics/hydraulics were often used instead, particularly in industry.

There was, however, one ingenious device called the selsyn, invented back in 1925, that handled all of the following:

  • Physical motion (actuator)
  • Position feedback
  • Haptic feedback

Figure 11 is a photo of a small selsyn. It’s configured like a small three-phase motor, with an additional coil (terminals R1,R2) mounted on the device’s rotor and connected externally via slip-rings. Its mechanical rotary output comes from the threaded shaft on the right. The stator is made up of three Y-connected coils (S1,S2,S3) spaced 120 degrees apart. While a selsyn is a motor, it can equally act as a generator. In fact, selsyns are always used in pairs (one transmitter and one or more receivers). Figure 12 is a schematic diagram of a basic selsyn receiver-transmitter pair. The transmitter’s S1,S2,S3 coils are connected to like-named coils on the receiver. When an AC excitation voltage is applied to each unit’s rotor coil, it will induce a voltage in the three stator coils. The phase of each of those signals will be displaced by 120 degrees and will vary as the transmitter shaft is rotated. These three voltages are applied to the receiver, which will cause the receiver’s rotor to move to match the position of the transmitter.

FIGURE 11
This is a selsyn unit—they were later re-labeled “synchro.” Two such units, wired together, can transmit the rotary position of one unit to be displayed on the other unit.
FIGURE 11
This is a selsyn unit—they were later re-labeled “synchro.” Two such units, wired together, can transmit the rotary position of one unit to be displayed on the other unit.
FIGURE 12
This is a schematic diagram showing how two synchros are wired together.
FIGURE 12
This is a schematic diagram showing how two synchros are wired together.

Selsyns are commonly used to transmit the rotary position of some remote mechanical device to a receiver, which is configured with a dial, allowing it to be read like a meter. This is referred to as a torque system. However, you can also use them as an actuator: a person can rotate the transmitter selsyn shaft, and the receiver selsyn will move to match that rotational position. If the receiver encounters some resistance in achieving that position, that will be felt by the person rotating the transmitter (haptic feedback). The amount of torque developed by the receiver selsyn is small—somewhat limited by the torque that the operator can exert on the transmitter. This is called a control system. Optionally, the transmitter’s output signals can be amplified and fed to a servomotor if a larger torque is needed. During the Second World War, the term selsyn was replaced by the term synchro.

As a teenager, I was fortunate enough to get a truckload of electronic military surplus equipment removed from the DEW line in northern Canada. I was delighted to find a lot of military-grade 6LC, 12AX7, 12AU7 vacuum tubes, power transformers, and audio power transformers. These all made their way into guitar and hi-fi amplifiers that I built. I came across a lot of selsyns as well, but didn’t know what they were used for until much later in life. There was no Google back then.

MUSICAL INSTRUMENT EFFECTS PROCESSING

I mentioned my interest in guitar amplifiers in the last section. From the 1960s onwards, electric guitars and organs played a large role in rock and popular music. Using only vacuum tubes and early transistors, many sound “effects” were invented back then that are still popular and routinely used with electric guitars and organs. In general, these fall into four categories:

  • Tremolo (amplitude modulation)
  • Vibrato (frequency modulation)
  • Phasor/flanger (phase modulation/comb filtering)
  • Reverb/delay (basically introducing echo into the signal, over time)

Let’s examine the vibrato effect—specifically as it was implemented in the famous Hammond tonewheel organs such as the B3. Briefly, Hammond tonewheel organs generate the frequencies needed for each note on the keyboard (plus a lot of selectable harmonics) using metal wheels, which are machined with a sine-wave pattern along their circumference. These wheels are rotated at a fixed speed by a synchronous motor, and a pickup coil is placed near the wheel’s circumference, which generates a sine wave at a specific frequency. Depending upon the model, there are 91 or more of these wheels/pickup coils. That amount is needed to produce the fundamental tones for all of the notes within the keyboard’s range, plus many user-configurable harmonics. Figure 13 is a photo of the tonewheel generator assembly with the wheels and coils clearly visible. This sound generation method is known as additive synthesis and is still one of the best synthesis methods available, even using today’s complex digital integrated circuits. In Circuit Cellar #328, I designed a Teensy MCU-based Tonewheel organ synthesizer. There is much more background on these organs contained in that article (“Simulating a Hammond Tonewheel Organ—Part 1: Mimicking a Mechanical Marvel,” Circuit Cellar 328, November 2017) [1].

FIGURE 13
This is a photo of the tonewheel generator as was used in the famous Hammond B3 organ. The tonewheels, which have “teeth” something like gears have, are clearly visible, as are the associated pickup coils.
FIGURE 13
This is a photo of the tonewheel generator as was used in the famous Hammond B3 organ. The tonewheels, which have “teeth” something like gears have, are clearly visible, as are the associated pickup coils.

A big advantage of the tonewheel organ was that every note was properly in tune, without any adjustments needed, due to the fact that the tonewheels were rotated by a synchronous motor driven by the very accurate 60Hz power mains. The downside of this is that there was no easy way to introduce vibrato—a slow modulation of the note’s frequency.

Today, with fast MCUs and large amounts of RAM and such, we could take the digital signal representation of a fixed-frequency sine wave, and introduce vibrato as follows:

  • Feed the digital sound samples into a large circular RAM memory buffer.
  • Maintain input and output buffer pointers into that buffer.
  • Slowly manipulate the position of the output pointer with respect to the input pointer in such a way as to introduce a phase/frequency variation.

This is a bit of a programming effort but quite doable with today’s MCU and memory devices. But how would you do this back in the 1930s when the Hammond tonewheel organ was designed? Hammond’s solution was quite ingenious. Figure 14 is the electrical schematic for what was called the Vibrato scanner. The electrical signals representing the notes being played enter at terminal 2, to the left. You can see that there is a whole series of LC networks that are series-connected. The time constant of each of these LC networks is large enough to introduce a significant phase shift to the incoming signal. This phase shift steadily increases as you move from left to right in the LC network. At each “tap” of the cascaded LC network, there is a signal that goes to what is labeled the scanner. For now, consider that to be a 16-position switch. The scanner switch is rotated by the same synchronous motor that turns the tonewheel generator. As the scanner rotates, it will select various taps of the LC network, and the varying phase shift applied to the input signal will be enough to produce a pleasant vibrato effect. There is also a switch that can select which switch taps are fed by the LC network—this allows for various amounts (depths) of vibrato effect.

FIGURE 14
This is a schematic diagram of the vibrato circuit in a Hammond tonewheel organ. The heart of this circuit is the cascaded LC network and its associated scanner switch.
FIGURE 14
This is a schematic diagram of the vibrato circuit in a Hammond tonewheel organ. The heart of this circuit is the cascaded LC network and its associated scanner switch.

Were the scanner to actually be a mechanical switch, it would produce frequency variations in discrete steps. Also, the switch would have to be make-before-break or the signal output would interrupted with small intervals of silence.

Using a mechanical switch wouldn’t work in practice. The digital vibrato solution, that I presented above, would have thousands of elements in the buffer array, making each sample close together in time. Thus, you could achieve a smooth vibrato response. Here we only have 16 “switch” positions on the scanner and that isn’t nearly enough resolution.

Figure 15 is a picture of the inside of the Scanner. You can see that at each scanner position, there is stacked a series of copper plates. While not visible, the rotator part of the scanner is also a series of stacked copper plates, and is spaced between the fixed plates. This forms an air capacitor which results in basically a variable capacitor between the rotator and each of the 16 fixed scanner capacitors (three of which are removed in this photo). As the scanner rotates, it will smoothly mix in various proportions from any two adjacent fixed scanner capacitors. This will provide a smooth vibrato signal.

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FIGURE 15
This is a photo of the vibrato circuit’s scanner. This is basically a “switch” using capacitive coupling. See the text for more details on why this was so ingenious.
FIGURE 15
This is a photo of the vibrato circuit’s scanner. This is basically a “switch” using capacitive coupling. See the text for more details on why this was so ingenious.

The capacitance of a small air variable capacitor, such as those 16 found here, is quite small. I don’t think that specification is available, but I would estimate it to be in the tens of picofarads. Given such a small coupling capacitance, the impedance of the amplifier following it must be high or there would be poor low-frequency response. Since they used vacuum tubes for the amplifiers in these organs, the high-impedance criterion wasn’t hard to meet. However, the shielding of both the scanner assembly and the shielded cable leading to the following amplifier had to be good, or there would have been excessive hum in the organ’s output signal.

The sound of Hammond tonewheel organs was so exceptional that they produced about 2 million of them between 1935 and 1975. Even though the newest of them would now be 50 years old, many thousands of them are still in use. They were originally designed for churches, but I suspect that the remaining ones are used mostly by rock/pop music bands. If you ever had a chance to see the complex mechanical components inside of one of these organs, as I have, you would be astonished to know that they were sold for about $1200 dollars when first introduced in 1935.

CONCLUSION

This article was intended as a special one-off for the 400th edition of Circuit Cellar. However, while looking back over the last 50 years in which I’ve been active in electronics, I collected many other ideas/devices like the ones described here—more than would fit into one column. If there is reader interest, I may sprinkle these other ingenious design ideas into future Circuit Cellar editions. I hope you enjoyed the trip back in time. 

REFERENCES
[1] Brian Millier, “Simulating a Hammond Tonewheel Organ—Part 1: Mimicking a Mechanical Marvel.” Circuit Cellar 328, November 2017

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • NOVEMBER 2023 #400 – Get a PDF of the issue

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Brian Millier runs Computer Interface Consultants. He was an instrumentation engineer in the Department of Chemistry at Dalhousie University (Halifax, NS, Canada) for 29 years.

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Before Transistors

by Brian Millier time to read: 21 min