Diodes and Triodes

The English physicist John Ambrose Fleming worked as an engineering consultant for many technology firms of his day, including Edison Telephone; in 1904, as a result of experiments conducted on Edison Effect bulbs imported from the USA and while working as scientific adviser to the Marconi company, he developed a device he called an "oscillation valve" (because it passes current in only one direction) or kenotron, which can also be used as part of a radio wave detector. Later known as the Fleming valve and then the diode, it allowed electrical current to flow in only one direction, enabling the rectification of alternating current. Its operation is described in greater detail in the previous section.
In 1907 Lee De Forest placed a bent wire serving as a screen, later known as the "grid" electrode, between the filament and plate electrode. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. DeForest called his invention the "Audion". In 1907, DeForest filed[2] for a three-electrode version of the Audion for use in radio communications. The device is now known as the triode. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard. The Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. The first true vacuum triodes were the Pliotrons developed by Irving Langmuir at the General Electric research laboratory (Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode. Pliotrons were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true vacuum tubes. Historically, vacuum levels in production vacuum tubes typically ranged between 10 µPa to 10 nPa.
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the overdrive that tube amplifiers exhibit at high volume levels (known as the tube sound). To remedy the low volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or standing current. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called grid bias.
Batteries were designed to provide the various voltages required by tubes in early radio sets. In North American terminology, the "A" batteries provided the filament voltage. Although North American terminology calls this the A battery, most of the English-speaking world knows it by a descriptive label: the LT (low tension) supply or battery. These were often rechargeable—usually of the lead-acid type ranging from 2 to 12 volts (1-6 cells) with single, double and triple cells being most common. Because these batteries produced 2 V, 4 V or 6 V, tube heaters were designed to operate at those voltages—a scheme which continues to be followed today. In portable radios, flashlight (torch) batteries were sometimes used.
The "B" batteries (in North American English) provided the plate voltage. These were generally of dry cell construction, containing many small 1.5 volt cells in series. They typically came in ratings of 22.5, 45, 67.5, 90 or 135 volts and were made of series-connected zinc-carbon batteries. To this day, plate voltage is referred to as B+, but only in America. The rest of the English-speaking world calls this the HT (high tension) supply or battery.
Some sets used "C" batteries (North American English) to provide grid bias, although many circuits used grid leak resistors, voltage dividers or cathode bias to provide proper tube bias. Most of the English-speaking world calls this simply the 'grid bias battery'.