- Written by Administrator
- Published on 04 December 2007
A tube is much like a light bulb, as it is constructed of glass, and contains a heated filament in a vacuum. The heated filament gives off light, which we can see, just like a light bulb. However, it is not the light that is useful in tubes, but the electrons which hover around the heated filament. Tubes contain other elements which are the basis for their function. Besides the filament (called the cathode), a metal strip (called the anode or plate) is situated near the periphery of the tube. When a positive voltage (in the hundreds of volts) is connected to the plate, the electrons at the cathode, by their negative charge, are attracted to the positive charge of the plate (remember opposites attract). Flow of electrons from one point to another is called "current" (measured in amperes, or "amps"), and movement of electrons from the cathode to the anode falls into this category.
Types of Tubes
A tube with just a cathode and anode is called a diode, and since the current can only flow from the cathode to the anode, it is a tool (called a rectifier) for changing alternating current, like that which comes from the socket in your wall, and which changes direction of flow 60 times per second (60Hz), to direct current, which flows only in one direction. Diodes are the fundamental tube in power supplies, which convert alternating current (AC) to direct current (DC) for use by the amplifier. This gets rid of the 60Hz "hum" that would otherwise be passed on to the speakers, and then allows the amplifier to convert the DC current, produced by the amplifier's power supply, back to AC current that alternates at the frequencies of the music. Since rectifiers still have pulses from the AC current, even though it is only in one direction, power supply capacitors are used to store the DC and feed it to the amplifier stages as smooth (filtered) current, with no fluctuations. However, the diode can only rectify. It cannot amplify. We need to add a third unit to the tube for that, called a "grid". It is a thin metal mesh, placed between the cathode and the anode. This type of tube is called a triode. If we apply a negative voltage (and it does not take very much negative voltage to do this) to the grid, the electrons are repelled and not allowed to flow from the cathode to the anode plate. So, if the audio signal (small voltage) is applied to the grid, and its small voltage fluctuates with the sound, there will be a fluctuating amount of current flowing between the cathode and anode, in time with the music, and since the anode plate voltage is very high in comparison to the grid voltage, voila, we have amplification. Besides triodes, there are tetrodes which have a screen grid as well as the original (control) grid, and pentodes which have the control grid, screen grid, and suppressor grid.
Single Ended Amp
The function of all the grids is to modify the current flowing between the cathode and anode plate, in one way or another, and the multigrid tubes can produce higher powered amplification, but, unfortunately, more distortion. Triode based tube amplifiers were the first, the simplest, and, in the opinions of many, still the best amplifiers. Their sound is incredibly pure, in part, because they are simple. The finest in this category is called a single ended Pure Class A triode amplifier (see below for description of classes of operation). Usually one output tube is used, and the plate is connected to one wire on the primary winding of an output transformer (the secondary winding of this transformer supplies the speakers), while the other wire of the output transformer primary winding is connected to the power supply. Only a few watts of audio power (7-30 watts) can be obtained from such amplifiers, but the sound is marvelous. However, they don't have much application in home theater, because the sounds in movies often require high power to reproduce them realistically. On the other hand, this type of amplifier would be wonderful to have as part of a separate audio system for listening to music at modest volume levels. You should audition a single ended triode tube amplifier in any case, just so you know what they sound like. Models begin at about $1,000.
Push Pull Amp
To get more power out of a tube amplifier, tetrode and pentode tubes are used, and in a different configuration than single ended, a configuration called "push pull". In this type of amplifier, the output tubes (at least two are required) are utilized by connecting the anode plates to the two wires of the output transformer primary winding. One tube "pushes" the signal, and the other "pulls". Often, several tubes (in multiples of two) are connected in parallel to get more power. The push pull tube amplifier is capable of producing 1,000 watts RMS per channel, plenty for home theater, but this much power will cost in the range of $20,000 for a stereo pair because they are hand made in small quantities.
Push pull tube amplifiers with outputs of 125 watts per channel are available at more reasonable prices, and, if you decide that the tube sound is for you, such amplifiers could be worked into a home theater system. For example, a tube amplifier could be used to drive the front left and right stereo speakers, while a less expensive three channel solid state amplifier (see below) could be utilized for the center channel speaker and the two rear surround speakers. In this system, when playing music, the "bypass" mode could be selected on the surround sound receiver, which directs all the sound to the front left and right stereo speakers, just as in a standard two channel stereo system, and the center channel as well as the rear surround channel would be turned off (muted). Thus, you would have the tube sound when playing CDs in normal stereo, and a combination of tube and solid state for home theater. If this idea appeals to you, work carefully with your dealer to match the best combination of components, and listen to the complete setup before making the final purchase.
Solid state (transistor - semiconductor) amplifiers are manufactured in the millions, being assembled by electronic robots in many cases. The result is that you can purchase amplifiers with outputs in the range of hundreds of watts per channel for hundreds of dollars instead of thousands of dollars in the case of tube amplifiers. Of course, you can also spend that $10,000 tube amplifier price mentioned above for a solid state amplifier, but you don't have to unless you really want the absolute top of the line equipment. Superb sound can be purchased with budgets that most of us can handle.
Types of Transistors
Transistors are like tubes in a way. They act as valves for the flow of electrical current, and can be used to amplify by using one of the connections on the transistor to control the flow, like the grid is used on the tube as described above. There are other connections on transistors which are the equivalent to the cathode and anode of tubes. Three popular types of transistors used in amplifiers are called "Bipolar", "J-FET" (Junctional Field-Effect Transistor), and "MOSFET" (Metal Oxide Semiconductor Field-Effect Transistor). FET type transistors perform more like tubes (musical quality) than bipolar transistors, but bipolar transistors are more capable of high current output than FETs, so they both have their specific uses. B,C, and E are Base, Collector, and Emitter, respectively. G, D, and S are Gate, Drain, and Source, respectively. Transistors have several layers sandwiched together. Impurities are added to the silicon which makes it conduct under specific circumstances. This is why they are called "semiconductors". If the impurity, e.g., gallium, causes the layer to be positively charged, the layer is called "P", while impurities that result in the layer being negatively charged, e.g., antimony, make the layer "N". The sandwich defines whether the transistor is PNP or NPN. In the above illustration, P types are shown. In the case of N types, the arrow would be in the opposite direction for schematic diagrams. Darlington circuits are often used, which consists of a transistor pair, with the collectors connected together, and the emitter of one transistor connected to the base of the other. This produces an amplifier stage with current gain that is the product of the individual gains of the two transistors.