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God I'd love a vacuum tube component. It'll make me go wild :D Okay so this time I'm going to try to make a comparison between vacuum tubes and transistors. I'm not going to involve performance much, but I'm going to focus on how they operate instead.
So the first major difference is how they start conducting. Transistors need a base current and voltage before they start conducting. Tubes rely on a different phenomenon. It's called thermionic emmision. They have a filament, which heats up a cathode. Some tubes may have direct cathode heating although that's become obsolete in later tube models. When the cathode heats up it starts emitting electrons. These negatively charged electrons head out to the nearest attractor which is the plate. This is the first and most simple variation of tubes. It acts like a diode. But there is nothing to constrain the current flow except the internal resistance of the tube. This is where the grids come in place. A typical tube may have one or many more grids which control the flow of electrons in some manner. The first grid is typically negatively charged in respect to the cathode. This way when negatively charged electrons start heading off for the plate the grid somewhat repels them, returning them to.the cathode, thus restricting the current flow. There are many ways to bias a tube in such way, but two stand out. Fixed bias and autobias. Fixed bias means directly putting a negative voltage on the grid through a battery or a separate power source. Autobiasing the grid is done by placing a resistor from the cathode to ground, which developes a voltage drop, thus raising the cathode potential above 0V. The grid is then given a ground reference through a bleeding resistor, and once again becomes more negative in respect to the cathode. Fixed bias is often used in the power stages of the circuit, while autobias is more convenient in the preamp stages. Now, for the other grids. Typically triodes have a big interplate capacitance and considerably low gain, with a big miller effect. That makes them unsuitable for high power applications and VHF applications. This is where the tetrode was invented. By adding a second grid, at almost the plate potential, you reduce the parasitic capacitance effects, increase the gain and make the tube suitable for VHF applications. But that created a so called tetrode kink, which is a negative resistance in the characteristics of the tube. This is due to secondary emmision from the plate. See the second grid accelerates the electrons so much that they cannot be fully absorbed by the plate, so some of them (especially at lower voltages) get bounced off the plate and return to the second grid. It then developes current flow and dissipates power, leading to efficiency loss. The kink also meant low stability, and tendency for oscillations. That's where pentodes come into place. Adding a third grid, at ground potential repels most of the secondary emmision back to the plate, thus minimizing the kink and the efficiency loss problem. The second major difference is the impedance differences between tubes and transistors. Transistors are often used in low impedance circuits, driving speakers directly while tubes can only work in a high impedance environment (with some exceptions). The high voltage, low current, high impedance output, must be transformed into low voltage, high current, low impedance in able to match the speaker impedance. This is done by an impedance matching transformer, like showed in the circuit. Now the primary of the transformer has an impedance of 6.4kOhm and the secondary is only 4ohm. The preamp stages need extra filtering to reduce hum, so that's where the 4.7k and the 47uF caps come into place. That's basically all i could think off, for now. If you have any further questions post them in the comments and me and my fellow colleges will try to sort them out.
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