THE HISTORY OF VACUUM TUBES
The main problem with the wireless receivers during the “Wireless Telegraph Era”, was the DETECTOR. All the detecting options that were used (Electromagnetic, Coherer, Galena Crystal, Electrolytic and others), were unstable and not sensitive.
In 1881, while looking for a way to prevent the carbon coating in his carbon filament light bulb, Thomas Edison added a metal plate inside the sealed bulb. He noticed that when the metal plate was connected to a positive voltage, current flows from the filament to the metal plate through the VACUUM inside the bulb. It was called Edison Effect and a patent was awarded to Edison in 1884.
There was no practical use for this Edison patent, until John Ambrose Fleming (who worked in the past with Edison) while working for Marconi in 1904, found out that the Edison Effect can be used for detecting wireless signals. He also found that the bulb with the metal plate, can RECTIFY ALTERNATING ELECTRIC CURRENT (AC) to DIRECT CURRENT (DC). Fleming was awarded the patents in England and the USA for this invention.
Thermionic Diodes based on Fleming’s patent have been produced and replaced the crystal detectors in the Wireless Receivers, with better and stable reception. In 1906, Lee de Forest tried to improve the diode tube performance by affecting the conductivity. Adding an external wire wrapped around the bulb and later on inside the bulb, resulted variations in the flowing current. De Forest have connected the wireless signal from the antenna tuning circuit to the wrapped wire. The headphones were connected to the metal plate circuit. The result was a more sensitive detector which Lee de Forest called AUDION.
Early experimental Diode Early Experimental AUDION AUDION Advertisement Early Triode
The sealed glass bulbs that de Forest experimented with, were partly filled with gas. He believed that the variable conducting effect with the wrapped wire, is due to the gas inside the bulb. At the same time that Lee de Forest experimented with the Audion, Robert von Lieben in Austria developed a vacuum bulb with three elements, but it was filled with Mercury vapor and acted like a THYRATRON .
Langnuir from Gottingen University in Germany worked on the research of the thermionic emission in vacuum bulbs. In 1909, he moved to GE USA to research the capabilities of Tungsten filaments in Audions. In 1913, he published the results (partly based on the research of Richardson of the UK regarding thermionic emission). His conclusion was that no gas is required in Audions, but high degree of vacuum.
The research of Arnold of AT&T on the efficiency and life of filaments in Audion bulbs included adding a layer of oxide over the filament coil. This improved both the electrons emission and the life of the Audion bulb.
The first use of the Audion as an amplifying device was in 1911 by Lowenstein in the USA. His purpose was to develop a Telephone Repeater for the Trans Atlantic Telephone Line. In 1913, Lee de Forest designed a three Audions amplifier with a total amplification of 120. Lowenstein and de Forest proposed to Bell Telephone a design of amplified repeater for the Trans Atlantic Telephone. Bell Telephone decided to develop and produce the amplified repeater, based on the Arnold amplifying bulb.
From then, the three element vacuum tube with the signal amplification capability (later on it was called TRIODE), was used in many future applications. The improved vacuum tube wireless receivers, were used mainly in the Wireless Telegraph services for land and ship to shore. The increased sensitivity of the receivers extended the range of Wireless Telegraph communication, compared to the limited range with the detector receivers..
The first use of the three element vacuum tube (Triode) for generating oscillating waves was made by Meissner in Germany. It was called Meissner Oscillator Circuit. Later on, Hartley and Colpitts in the USA designed different oscillating circuits that are named after them.
Meissner Oscillator Hartley Oscillator
From there on, it was possible to design a wireless transmitter that could generate a pure sine wave, comparing to the wideband splatter of the Spark-Gap transmitters. In the new vacuum tube wireless transmitter, the pure wave triode oscillator frequency is amplified by Power Triode Amplifier stages.
The triode tubes became the building blocks of radio receivers in the “Broadcast Radio Era” that started in 1920, with the first radio broadcast stations. It was not possible to transmit voice or music by Wireless Spark Transmitters. The pure CARRIER signal of the vacuum tube transmitter, could be Amplitude Modulated (AM) by Triodes Audio Amplifier
TETRODE and PENTODE
After many years of practice and experience with triode amplifier circuits, radio engineers noticed that the internal capacitance between the Plate and the Control Grid caused problems in high frequencies. To overcome these problems, another Grid or Screen was placed between the Control Grid and the Plate. The new vacuum tube was called TETRODE or Screen Grid Tube. The PENTODE is another development of power amplifier tubes. It contains an additional Suppressor Grid between the Screen Grid and the Plate. It is connected to the Filament (Cathode), in order to repel the secondary electrons that are emitted from the Plate, back to the Plate.
Until the end of the 1920’s, all the vacuum tubes used Directly Heating Filament, which means that the electrons are emitted from the filament coil, or the Oxide layer over the Filament. This fact required supplying to the Filament electric power from a BATTERY. The reason is that any AC voltage or ripple in the filament circuit would be amplified by the vacuum tube. For this reason, all the vacuum tube radios operated with batteries.
An improved heater filament was developed with a metal cylinder coated with oxide that covers the heating filament. The oxide coated cylinder became the Cathode and was isolated from filament circuit. The new Indirectly Heated Vacuum Tubes were able to be supplied with AC power source for the filament, without any AC signal amplification. From the end of the 1920’s, all the radios for home use were designed for operation from the AC mains power.
VACUUM TUBES BASES AND STYLES
The first experimental vacuum tubes used flying leads for electrode connection. Some used the Edison Light Bulb base for the filament. The first commercial types used 4 pins base like the UV-4 and later on the UX-4 base for triodes like: UV201, UV201A, UX201A and 2A3.
Early AUDION Detector
De Forest AUDION
with UV-4 Base
5 PIN BASE
6 PIN BASE
With the development of theTetrode, Pentode and mixer/converter tubes new tube bases with more pins were designed. The most popular was the OCTAL 8 pin base. It was used with the G and GT glass style tubes and with the military metal types from the 1940's. The LOKTAL base is another 8 pin base with locking capability.The miniature all glass tubes of the 1950's were without base and include 7 pin and 9 pin styles.
OCTAL 8 PIN TUBES
9 PINS 7 PINS
A further miniaturization of the vacuum tubes brought the NUVISTOR metal tube and the SUBMINIATURE glass tube. Both were used in portable equipment for military and commercial applications. The subminiature glass tubes have been used mainly in Hearing Aid devices.
Medium and high power transmitting and industrial tubes required thicker electrodes with other base configurations than the receiving tubes.
The production of commercial TRANSISTORS in the mid 1950's, put an end to the Vacuum Tubes Era.
© 2023 by Success Consulting. Proudly created with Wix.com TELECOM MILESTONES Along the History of TELECOM and Electronics Technology