Search today’s Library Catalog for “Vacuum Tubes” and you’d be greeted to a collection of guitar maintenance books! Nowadays, vacuum tubes are relegated to a niche market due to guitar players’ preference for the distortion provided by an overdriven tube stage with third harmonics over that provided by transistor amplifiers.
Once, however, there was a time that tubes were bleeding-edge technology, enabling the invention of radar, radio, television, and computers.
The Beginning of an Era: Edison and Thermionic Emission
The physical effect behind vacuum tubes was first noticed by Edison before the term ‘electron’ even existed.
Early light bulbs consisted of a single wire (called a filament) that glowed when electricity was sent through it. In 1882, when experimenting on how to improve his light bulb filaments, Edison saw that inserting an additional wire into the light bulb allowed current to be controlled through the wire by switching the light bulb on and off. The amount of positive voltage applied to the wire controlled the current.
Figure 1. A carbon-filament light bulb from Edison’s lab in Menlo Park. Image used courtesy of Terren in Virginia. [CC BY 2.0]
U.S. Patent number 307,031 for an Electrical Indicator was awarded to Edison in November of 1883. The discovery was referred to as the Edison Effect, which today is known as thermionic emission because a thermionic material (one which emits electrons when heated) was used in the filament. The heated filament caused electrons to be emitted and they were collected by the wire.
Early tubes mimicked Edison’s configuration, which used a glass housing, a filament to provide electrons, and an anode or plate operating at a positive electrical charge to collect them.
Fleming and the First Diode
In 1904, J. A. Fleming produced the first diode. Named for the number of wires, diodes (two electrodes) were used as a switch. This type of tube was also referred to as a thermionic valve since the results mimicked mechanical valves in use at the time.
The diode controlled one-way flow of current and was used in amplitude modulated receivers, but had no amplification and couldn’t amplify signals detected. This directly heated type of emitter gave way to tubes with a separate cathode to provide the electrons, referred to as indirectly heated. The materials of the filament/cathode determined the operating voltages needed to emit electrons.
Vacuum Tubes in Power Applications
Tube technology and manufacturing evolved, increasing the number of wires/collectors/controllers to supply triodes, tetrodes, pentodes, each configuration overcoming weaknesses in the previous offerings and providing improvements for signal handling and amplification.
Tubes found use in signal applications, which used a relatively lower voltage, and in power applications, where the voltages were larger. A coating of barium or strontium oxide was used when the anode was operated at less than 400 volts. At higher voltages, these emitters were damaged, so tungsten or thoriated tungsten was used. For power applications, these materials could withstand temperatures as high as 2400 deg Celsius.
Specialized tubes like cathode ray tubes were used in television receivers, radar, and computer systems and fire control displays. Phototubes converted light energy to electrical energy.
With the intricacies of making tubes, not every manufacturer made every tube. The companies of the day portioned the market: Sylvania made one type, RCA another. Some of the tubes are shown in Figure 2.
Figure 2. An assortment of tubes
When designing with tubes, engineers manually consulted operating characteristics charts, like that shown in Figure 3—a visible Ohm’s Law.
Figure 3. An operating characteristics chart
These charts show the plate current/plate voltage variations of a device over a range of voltages. Tubes heat up during operation and their characteristics change, so there is a range of conditions to consider. Various device characteristics were compared to find the right combination of plate voltage/current, filament power and resistance needed.
Today, online tools are available to help with that should you need them.
For more information on early tube technologies, please check out the AAC textbook.
The Residential Experience: Designing for the Right to Repair
Televisions of the 1950s and 1960s required high voltages to drive the picture-tube. A black and white television had about 17,000 V stored in the capacitance of the picture-tube. Color televisions used 33,000 V. With those voltages, it seems strange looking back that tubes were considered replaceable by the owners. After all, the commercial devices in use had trained service fleets supporting them. Yet, during these years, replacements were available at almost every type of outlet: gas stations, grocery stores, appliance stores.
Issues with tubes were easy to troubleshoot. If the device didn’t work correctly, the problem tube was not ‘glowing’ while the remaining tubes were!
You then removed the tube, walked it down to the neighborhood gas station, checked it at their tester, picked up a replacement to put back in the device. If you prefer not to bother, there were many locally-owned small businesses to repair radios and TVs; they had service trucks that came to your home to fix your TV.
TV sets in particular put a lot of stress on vacuum tubes and failure rates were high; service trucks were a common sight in the 1950s. These businesses have essentially vanished now.
Both personal and commercial testers were available to test questionable tubes.
Figure 4 shows a commercial tube tester an appliance store might have and a portable tester a TV repair shop might use.
Figure 4. A commercial tube tester (left) and a portable tester used in tube repair shops
Enabling easy maintenance speaks to the designs of residential devices at the time. The line cord was mounted in the back cover of televisions, allowing it to be easily disconnected. The replaceable tubes were separated from high voltage areas. You heard of 10-year olds fixing their family televisions. That attitude was part of the era, a time after World War II, coming back from the austerity the war imposed.
Manufacturers were expected to provide replacements for their equipment. People serviced and replaced components on their cars; they expected to be provided a means to service and replace components of their electrical devices. Compare this to current “right to repair” movements centered around device manufacturers and the ensuing regulatory battles that have popped up across the globe.
One facet of this conversation is that economies were different, as were manufacturing environments. Today, it’s more economical to replace equipment than to find someone who can work on it and replace a bad component that might be manufactured into a circuit or nano board. And the new version most likely offers more features.
Ending an Era: The Switch to Semiconductor Components
While tubes provided the amplifiers used in radio, radar, television, and computers, they had the drawbacks of being easily damaged by vibrations and shock. They needed a lot of power for heating. And they certainly weren’t that portable. Devices powered by tubes were not always consistent—tuning in a radio station required “a touch” because broadcasted signals could fade in and out. Equipment also ran hot, changing signal characteristics.
Semiconductor components had none of these issues. By 1959, semiconductor components were proving to be dependable and efficient. As they became widely available, tube-based systems were replaced by solid-state equipment.
An Engineering Learning Curve
In the 1960s, IBM, with the largest field support force at the time, retrained their entire division on the new devices which were replacing the tube based systems. Sometimes the changing technology led to confusion.
Dennis Myer, P. Eng and AAC contributor, relates some recollections of this time:
“I started in the electronics field just as tube technology was being replaced by solid-state. Although none of my formal education included tubes, one of my first assignments as a designer included the replacement of a tube-based analog computer with a solid-state minicomputer. It was my pleasure to get a detailed introduction to the design and operation of the tube system from one of the seasoned designers at the time.
One of the more colorful anecdotes passed on to me was regarding a younger engineer who had just learned the major weakness of tube reliability was thermal management. That engineer then promptly went home to drill a vent hole into their tube-based TV only to learn how unfortunately close the circuit board was to the bottom of the set. The result was a main board riddled with holes and, eventually, a new TV set.”
The demand for vacuum tubes fell off quickly. A technology that took decades to bring into prominence went obsolete within a year. Semiconductors, followed by integrated circuits, transformed the electronic markets.
Today, specialized tubes are still manufactured, mostly outside of the United States, and tubes continue to have a place in specific applications. Even as we enjoy our awesome electronic devices, it shouldn’t just be our guitarists who respect the tube.