Minor condensed Tube History:
As most know, there are several types of tubes available
for consumer use. They are the diode, the triode, the tetrode, the beam
power tube, and the pentode. I placed them in this particular order for
the number and use of electrodes in each.
In the Flemming "valve" (what the Europeans call tubes), there was merely the cathode and anode. Sound familiar to you solid state junkies? It ought to. It is also known as a DIODE. The anode, or plate, was connected the the B+ terminal of a high voltage battery (22 volts or more. Although some tubes were made to be used with 12 volts for automotive electronics). This caused a sufficient positive charge on the plate to attract the free floating electrons. The cathode is connected to the negative side of the battery. Now current can flow, due to the same typical rules for current flow. However, in the diode, if one were to reverse the battery connections, no current could flow. This proved an intriguing fact that allowed the Flemming valve to be a valuable asset to radio at the turn of the century. It now became the "detector", dividing the R.F. portion of an A.M. radio signal in half and allowing only the fluctuations of amplitude to pass, hence the audio portion of the Amplitude Modulation of R.F. interestingly, about eighty years before the thermionic diode, the principles of semiconductor technology had been first discovered. But that is a story for the solid state page.
It turns out that Edison had played a bit with the diode originally. In order to minimize the problem of a dark film forming inside his precious invention the light bulb, he inserted a plate to attract stray particles. He used a D.C charge across the two elements, the filament and the plate. He found that when connected in one polarity, with the positive to the filament and the negative to the plate that nothing would happen. No current flowed. But connecting the battery the other way, current flowed. This came to be known as "the Edison effect". He could have been the first one to invent the valve (tube), but his business sense told him that there was no money to be made from this, and abandoned the work he did in order to promote the light bulb, D.C. current generation, the phono, and whatever other ingenious gadgets that he invented or stole. Had he done so, instead of fighting with Tesla regarding using A.C or D.C. for powering america, he would have realized that he could have his cake and eat it too! He would have been the god-king of electrical power and technology, providing both A.C. and a way to rectify it into D.C., AND made the electronic equipment to be used!
The TRIODE, as the name implies, has three major electrodes. This was invented by Lee DeForest. Filament connections are not considered an electrode although many tubes use the filament as a cathode, also known as direct heated cathode. The three electrodes are the cathode, the plate, and the control grid. The cathode is the source of current. It is heated to the point where the chemical coating is excited to the point of ionization. That is, electrons are "boiled off". The molecules in the coating become so energized with heat energy that the outer electrons jump off, making an electron "cloud" around the cathode. This is known as Thermionic Emission. Therm for heat, and ionic for the lack of electrons in the molecules of the coating on the cathode, making them positive ions.
The triode is relatively a very linear device. When biased at the proper point, the triode will produce an audio signal that is low in distortion and high in fidelity. A little inverted feedback (formally known as negative feedback) carefully and skillfully placed will minimize any distortions to a large degree without affecting the sound quality except for the better. We will discuss this later.
It has been found that the triode has a limitation; frequency response. It was only good for up to a couple of megahertz. This was due to what is known as Miller effect. It is a capacitance between the electrodes. Even though measured capacitance could be as low as 1.5 picofarads (1.5X10^-12)(It can be lower in tetrodes and pentodes) owing to the high impedance of the grid to plate, the capacitance will affect frequencies down to even audio. From the formula f=1/(2PiXcC), where Pi is 3.14, Xc is the capacitive reactance (or impedance of the grid to plate in this case), and C is the interelectrode capacitance, if we plug in the values of capacitance and impedance, we arrive at the frequency where a -3 decibel (drop in amplitude by 1/2) occurs. For instance, if the impedance between the grid and the plate was 12 megohms and the capacitance was 2 picofarads, then the frequency of roll off would be at about 40 kilohertz. Which means that the pole is actually starting at a lower frequency withing the audio range. So we can see that using the rule of thumb that roll off is at 6db per octave, when we get to a couple of megahertz the triode would be useless.
A different type of device was needed in order to increase the frequency response of the tube in order to accommodate the R.F. needs of the public and government. Hence came the TETRODE. This is a triode with an extra "accelerator" or screen grid installed that allowed a faster electron flow, because the screen grid was biased more positive than the plate. It being a grid and not a complete plate, while it may attract some electron flow, most electrons simply zoomed past towards the plate. Along with that is the fact that now there are two capacitances in series. From the formula Ct=C1-C2 one can derive the total capacitance of two capacitors in series. Instead of adding like resistors do, the capacitances actually subtract. The capacitive reactances actually add, which in essence would make the above example have an incresed frequency. If the added capacitance was also 2 picofarad, the total capacitance would be 1 picofarad, and the frequency roll off would be 80 kilohertz. Frequency response increases dramatically, as well as gain. (I wonder what the "Heisenberg" pattern of the electron flow is in this tube. Does it look like a Young double slit pattern at the plate? Is it random? Does this affect the sound? A topic for a physics page.)
The BEAM POWER TETRODE, or BEAM POWER PENTODE, as it is also known, is a tetrode with "concentrator" plates installed and connected at cathode potential. What this does is concnetrate the flow of electrons to the anode plate through the openings of these plates by repelling them due to their being negatively "charged". The plates are actually connected to the cathode. The electrons that would have normally gone that direction now goes in the direction of the openings, joining the electrons that are going that way normally. Also, the grids are aligned so as to aid in making beams of the electrons. This has the effect of giving more power and higher gain. How the concentration accomplishes the power thing I have yet to find out. If anyone can email me and let me know how concentrating electron flow to narrow areas of the plates allows it to put out more power, as opposed to a standard pentode or tetrode, I would appreciate it and add it to this page with honorable mention, of course.
However, the accelerated electrons caused a problem with tetrodes and beam power tubes. Noise. It turns out that the accelerated electrons hit the plate too hard and bounce off, or knock away electrons native to the plate, known as secondary emmision, only to be re-attracted to the plate, or back to the accelerator grid. This made more noise, and power and gain loss. Another genius thought if there was another grid placed after the accelerator close to the plate, but biased at ground potential or cathode potential, then the secondary emission would be forced by the negative charge on this grid back toward the plate. Hence the PENTODE was born. So now there are two, three, four and five electrodes in tubes. There are more in certain types of tubes, but since this is an audio page, we won't worry about those.