As early as 1761, it was known that heating a wire, or filament, could create a glow, called “incandescence.” By the middle of the 19th century, there were several scientists working on the problem of creating a sustainable electric lightbulb. One of the discoveries made was that a filament burns up in the presence of oxygen, and methods of evacuating (or e-vacuum-ating!) a glass bulb were being developed. It is well-known that Thomas Edison is credited with developing the first commercially viable electric light bulb. During his research, he wondered why certain filaments broke, or why they didn’t burn evenly. His experiments led him to put an electrode inside the light bulb so as to measure results. He discovered that not only did the filament emit photons (light!) but that electrons also traveled across the vacuum from the filament to the electrode (electric current)! Later, it was discovered by the British physicist John Abrose Fleming that these diodes (two-electrode devices) could be used to detect radio waves. It was also discovered that these devices convert AC (alternating current, such as is used for municipal power) to DC (direct current, which is used in all electronic circuits). The EZ81 tube in your People’s Amplifier acts this way. The technical term for this is rectifying. Some amps have a solid-state rectifier, usually based on a diode bridge. When the amplifier requires an increase in current, a tube rectifier takes a little time to recover. This imparts a sponginess in response to attack that we perceive as being musically expressive, often referred to as “sag.” Solid state rectifiers do not have this characteristic.
In 1907, radio pioneer Lee De Forest developed a three electrode vacuum tube known as a triode. De Forest was trying to develop a more sensitive radio receiver. This third electrode (called a grid or control grid) was used to control the flow of electrons from the filament (or cathode) to the plate (or anode). This is why tubes are sometimes referred to as valves. Voltage on the grid affects how much current flows through the tube, kind of like how a faucet controls how much water flows through a spigot. It was also discovered that the device behaved as an amplifier, increasing voltage and power at the plate according to varying voltage on the grid. The 12ax7 tubes in your People’s Amplifiers are triodes. Actually, they are both dual triodes, which means there are two tubes in one enclosure. The pre-amp tube amplifies the relatively weak signal from your guitar’s magnetic pickups to a line-level signal appropriate to pass through the rest of the circuit. Many amplifier designs will put the first gain control in between the two triodes in the preamp stage. The People’s Amp runs the two preamp triodes coupled together, and the volume and tone controls select how much and what frequencies are passed along to the next stage: the phase inverter.
The phase inverter in your People’s Amplifier is also a dual-triode 12ax7. It doesn’t amplify the signal, but through some electronics wizardry, it inverts the waveform of the signal, splits it in half and sends each half of the signal to one power tube, the two el84 tubes. This bouncing back and forth from tube to tube in the power section is referred to as “push-pull” or Class AB operation
Those el84 tubes are pentodes, which means that they have a five electrodes. In addition to the triode’s filament (cathode), plate (anode), and grid, pentodes have a screen which makes the operation of the tube more stable, and a suppressor which keeps too many electrons from heating up the screen. The operation of a pentode is exactly the same as the operation of a triode, but it is capable of dealing with much higher voltages, and thus produces much more amplification.
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Love my saggy ol’ People’s Amp!
[…] tube, which has had it’s heaters fired up by that other 6.3 volts. You might remember from From Glowing Wires to Amplifiers that one of the earliest discovered functions of a vacuum tube was to operate as a diode. Diodes […]