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The triode vacuum tube is a 3 element tube (the filament is not counted as an element but there is a 'heater' filament in all vacuum
tubes). In some triode tubes, there is no cathode element, instead the filament is coated and emits electrons directly. In this case
the filament is the cathode. There are other vacuum tube types using more grid elements and provide higher gain than the triode. However, the triode tends to produce cleaner audio. |
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The plate of the vacuum tube requires a positive voltage, in this example a 300 volt DC power supply is used.
The cathode is connected to ground, usually through a resistor value in the range of 200 ohms to 3,000 ohms.
Connecting the cathode directly to ground will also work but may cause the tube to operate out of it's linear
range and result in distortion unless a bias supply is used on the grid. Bypassing the cathode resistor with a
capacitor will increase gain but also increases distortion some. There is a certain amount of natural negative
feedback at the cathode without the bypass capacitor. Negative feedback reduces distortion by canceling
non-linear components. The output signal of the vacuum tube at the plate is inverted 180 degrees with respect to the input grid signal. The output signal at the plate is also inverted 180 degrees with respect to the signal at the cathode. The grid and cathode signals are identical. The coupling capacitors ( .1 mfd @ 400V ) are dry type, non-polarized capacitors. The cathode bypass capacitor ( 25mfd @ 50V ) is an electrolytic polarized type and the positive lead must be connected to the cathode. The resistors ( 150K, 470K and 2.2K ) are 5% 1/2 watt carbon composition resistors. Carbon composition resistors are more rugged and tend to hold up better. Tubes require a socket to plug into. All wiring is done at the tube socket terminals. The circuit above shows DC voltage used on the filament. In the past, the usual method of filament voltage has been using AC voltage. DC is better for reduced hum. |
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Bias is required on the grid ( control grid ) to put ( or bias ) the tube into the middle of its linear operating curve.
The control grid controls the flow of electrons. When the control grid is more negative, no electrons flow. The more
positive the control grid is the more electrons flow. If the control grid is not biased into the linear operating
range then the tube either becomes a switch, on or off, or will amplify non-linear resulting in high distortion. Shown above are two bias arrangements for a vacuum tube. The tube on the left is "self" biased and the tube on the right requires a negative bias supply. |
| The circuit above has been modified. The cathode bypass capacitor has been removed and R1, C1 added. R1 and C1 "feedback" signal from the plate to the cathode. C1 blocks DC voltage from the plate to the cathode but allows signal through. The amount of feedback is determined by R1. This is called negative feedback and is used to cancel out distortion. Negative feedback can also be used to alter the frequency response of an amplifier such as boost or reduce low or high frequencies. Negative feedback also reduces the gain of a stage. |
TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE
TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE
TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE
TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE - TRIODE VACUUM TUBE