A History on valve amplifiers

Introduction

The vacuum tube is also called ' thermionic valve' or valve.

The invention of the vacuum tube or thermionic valve brought the dawn of the age of electronics. Its invention enabled the wireless technology of the day to move forward. Many new and exciting applications were found for these devices, first as telephone repeater amplifiers and then many other applications that were not always linked to wireless and as a result the new area of electronics was born.

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Beginnings

The first vacuum tube was not made until the beginning of the 20th Century, but the foundations for its discovery were laid many years before. Professor Guthrie made one of the first discoveries in 1873. He was investigating effects associated with charged objects and he showed that a red-hot iron sphere that was negatively charged would become discharged. He also found that the same did not happen if the sphere was positively charged.

The American inventor named Thomas Edison took the next major step in 1883. Edison was developing electric light systems and one of the major problems that he was facing was their short life. Although the filament life was a problem, the main limiting factor was that the bulbs quickly became blackened. Initially it was thought that this was caused by atoms of carbon from the element hitting the glass. As it was known that the particles leaving the element were negatively charged, experiments were carried out to prevent them hitting the glass. One method that Edison tried involved placing a second element into the envelope. He reasoned that if he placed a positive charge on the second electrode, particles could be attracted away from hitting the glass of the bulb. Edison experimented with the polarity of the charge on the second electrode and he noticed that when the second element was made positive with respect to the filament then a current flowed in the circuit. When the potentials were reversed he noticed that this did not happen. Edison was fascinated by the effect but uncharacteristically he did not find a use for it. Even so it became known as the Edison Effect.

Over the years Edison demonstrated the effect to many other leading scientific personalities including Preece, a well known British electrical engineer and more importantly to Ambrose Fleming, the professor of electrical engineering at University College London. Although no developments were made for a number of years the seed had been sown for later discoveries.

More developments

Like Edison, Fleming was also fascinated by the effect and performed some experiments around the idea. For example in 1889 he had some bulbs made up for him by the Ediswan Company in the UK. Using these bulbs he reproduced the Edison Effect, although again this was performed using a steady state charge. It was not until a few years later that he observed that if an alternating current with a frequency between 80 and 100 Hz was passed through the bulb, then only one half of the cycle was passed. In other words it was rectified to produce a direct current.

At this time there was a lack of understanding about the operation of the device and this prevented further progress from being made. However the situation improved when Sir Joseph Thomson discovered that atoms were made from even smaller particles, one of which was a negatively charged particle, an electron. Accordingly it was quickly realised that it was electrons that were being emitted from the heated filament in the bulb, and it also provided the reason why they were attracted to an electrode with a positive charge.

Fleming's oscillation valve

In addition to his work at University College London, Fleming also acted as a consultant to the Marconi, who at this time was rapidly increasing the distances over which wireless signals could be used for communication. For example, in 1901 he made the first transatlantic transmission, and then sought to improve the performance that could be achieved. Fleming rightly saw that the major limitation in the sensitivity of the receiving equipment was caused back the lack of sensitivity of the detector. At the time coherers and magnetic detectors were used, and both of these instruments were very inefficient.

Fleming decided that he needed to seek ways of improving this situation, and in November 1904 whilst he was walking along Gower Street in the West End of London, he had what he called "sudden very happy thought". He wondered if the Edison Effect could be used to rectify what he called the "feeble to and fro motions of electricity from an aerial wire". Fleming instructed his assistant to set up an experiment and to their great exhilaration they were quickly able to prove that the idea worked.

Fleming called his new invention an "oscillation valve" because it acted in a similar way to a valve in a pump that allows gas or water to move in only one direction. He patented the idea that was clearly a major step forwards in wireless technology. Even though the vacuum tube was still in its infancy it was still a major improvement over the coherer or magnetic detectors that were available at the time.

Despite its clear advantage over other detectors, Fleming's oscillation valve or vacuum tube was not widely used. Valves or tubes were difficult and expensive to make and their heaters consumed large amounts of power and this had to be supplied by expensive batteries. Additionally some cheaper devices were discovered in 1906. Devices that were forerunners of the Cat's Whisker detectors that were used in crystal sets until the mid-1920s were discovered. In fact two different patents were filed, one by Ferdinand Braun for a crystal detector using hydrated crystals of manganese oxide and the other by H. Dunwoody for a crystal detector using carborundum. These devices had many limitations but they were very much cheaper than Fleming's oscillation valve and as a result they were quickly adopted.

The Audion

Even though crystal detectors were very successful, several people continued to investigate whether they could develop thermionic or vacuum tube technology whilst avoiding any infringement of Fleming's patent. It was de Forest, an American who had been working on a variety of areas associated with wireless who made the next and crucial vacuum tube development. He had been researching Fleming's diode valve and having investigated the idea he took out some patents for improvements in 1905 and 1906 where he introduced a third electrode. However in 1907 he took out a patent for a three-electrode device where the additional electrode which was placed between the anode and cathode had a fine grid structure. He called this device his Audion which he used as a leaky grid detector, not realising its full potential.

It was not until 1911 that the vacuum tube was used as an amplifier. After this discovery people were quick to try to exploit it. De Forest built an amplifier using three Audions and demonstrated it to the telephone company A.T & T. Although the performance was poor they saw its potential and soon started to build repeaters using vacuum tubes which they had improved. Naturally as soon as the tube was used as an amplifier, people were quickly able to use it as an oscillator. Indeed, one of the problems soon encountered was difficulties in preventing oscillations in view of the high values of grid anode capacitance.

Further Improvements

In these early days of vacuum tubes, their operation was not fully understood and there were a number of misconceptions. One idea that was held it was thought that some gas molecules were needed in the envelope for it to operate correctly. These tubes were known as "soft" tubes. It was not until 1915 when an American scientist named Langmuir proved that gases were not required in the envelope. Soon after this discovery new highly evacuated tubes known as "hard" tubes were produced and these exhibited far better levels of performance.

In addition to basic improvements the full evacuation of the envelopes brought a number of other improvements. Filaments could now be coated to improve their electron emission. Previously any coatings would have been contaminated. Filament temperatures could also be reduced and this improved reliability as well as reducing the heater current consumption.

Large numbers of the new tubes were manufactured. A French engineer named Ferrie designed one designated the TM valve for the French military authorities. Over 100 000 of these were produced. In Britain a similar valve called the Type R was produced and this was manufactured in equally large quantities.

More in the envelope

With the very high levels of anode grid capacitance exhibited by these early vacuum tubes, it was very difficult to prevent the circuits that used them from bursting into oscillation when they were used at frequencies above a few hundred kilohertz. Several attempts were made to overcome the problem. In 1916 H.J. Round produced a low capacitance valve known as the Type V24. For this tube, Round used the novel idea of keeping the anode lead away from the grid by passing it out of the top of the glass envelope and not through the base at the bottom. Whilst this gave a significant improvement, it was not the complete answer.

The solution to the problem was derived in 1926 by adding a further grid. Known as the tetrode this vacuum tube used a second grid known as a screen grid between the first or control grid and the anode. Its introduction reduced the anode to control grid capacitance to almost zero and solved the problem of instability.

Then in 1929 the tetrode itself was improved by the introduction of another type of vacuum tube termed the pentode. This tube had a total of five electrodes, the additional one being a third grid called the suppressor grid. This overcame the problem encountered with the tetrode of a discontinuity in its characteristic caused by electrons bouncing of the anode.

Apart from making improvements in the operation of valves by adding additional grids, further improvements were made in the heater arrangements. One of the main problems with early tubes was that the circuit configurations were limited because the heater and cathode were one and the same. It was discovered that the cathode could be indirectly heated and this meant that the heaters could be electrically isolated from the cathode. This had the advantage that the heaters did not need to be run from a battery supply supplying DC. Instead an AC supply derived from the mains could be used. This was a major improvement because it meant that size of radios could be considerably reduced as could their running costs.

Increase In use

The introduction of the tetrode and pentode brought revolutionary improvements in performance. As a result the use of vacuum tubes rose dramatically. Not only were thermionic valves used in radios which by this time were very popular, but they also found many other uses. By the late 1930s many thousands of different types of vacuum tube were being manufactured, and there was a large number of different manufacturers which were appearing both in the USA and in Europe.

Many of the vacuum tubes introduced in this period have long since disappeared from common use. However there are a few which were very successful remaining in new designs for a long time. One such valve was the 6L6 used in many guitar amplifiers until quite recently. In many ways it was quite revolutionary because it was the first beam tetrode. It used a new technique to overcome the discontinuity in the characteristic of the tetrode caused by electrons bouncing of the anode. Rather than using a suppressor grid it used a new arrangement connected to the screen grid. This tube became so popular that it was later modified for RF applications by giving it a top cap for the anode. This vacuum tube was called the 807 and was widely used in transmitters in the Second World War and afterwards.

Prior to the war all tubes had used special metal or plastic bases attached to the glass envelope to hold the pins. After the war miniaturization and improvements in manufacturing techniques enabled the pins to be mounted into the glass envelope. By doing this much smaller tubes were made and costs were reduced.

Vacuum tubes today

The reign of the thermionic valve did not last forever. Developments in semiconductors that resulted in the invention of the transistor in 1948 meant that smaller, reliable, and less power hungry devices could be made. The success of the transistor was far from instantaneous. It took until the 1960s before domestic radios used them widely, and even then many vacuum tube radios remained (and still remain) in service for many years afterwards. Today thermionic technology is still used but in limited areas. High power transmitters still use tubes, and cathode ray tubes are still used in their millions. But to many people, transistors do not have the same life as a tube. They don't have the warm friendly glow of a valve. However to those who worked with tubes, there will never be anything that can equal them.