How do vacuum tubes amplify

We will have a look at what each of these parts does in the assembly. The added parts in this image are metal grills used to control the flow of electrons and reduce capacitance, thus improving the performance of the tube immensely.

A cathode is a positively charged pole. It is an element with a slight positive charge located at the centre of the tube, and which emits a stream of electrons negative charge in the right conditions. These electrons pass through the vacuum to a second plate with an even stronger positive charge.

This difference in charge attracts a high flow of electrons because unlike charges tend to attract. With the right control, this flow can be used to manipulate a signal.

The second condition necessary to ensure the flow of electrons is heating. The cathode needs to be heated so that electrons can flow freely. The filament will be located next to the cathode in the centre to facilitate this; alternatively, the element itself will be the cathode and coated with special material to provide the flow of electrons.

A plate with a high positive charge surrounds everything else inside the vacuum tube. It works by attracting the negatively charged electrons towards itself inside the vacuum tube, which makes it the anode. It collects the electrons emitted by the cathode at the centre and also picks up the signal from your guitar pickup. This is where things get really nifty.

A fourth component called a grid comes between the plate and the cathode. The grid is a piece of metal connected to the input from your guitar, which charges it and gives it a small positive or negative charge depending on the incoming signal. This is why the cathode is not really negatively charged but instead has a slightly positive charge.

Without any current flowing in it, the grid will be negative by comparison, which helps to repel the electrons in the anode and keep them in place until a voltage is applied to attract them.

Obviously, this process is far more complex than this, but you can now understand the basic operation of a vacuum tube. There are other components in a tube amp that work together with the vacuum tubes to become the kind of amps loved by the best musicians everywhere. There are a few more principles of electricity that you have to understand that play a part in the operation of a tube amp as a whole. First, the relationship between current, voltage, and resistance is defined as:.

In other words, the voltage is the product of the current and resistance measured between the ends of a conductor or circuit. If you think of current as the amount of water flowing through a pipe, voltage is the pressure pushing the water and resistance is caused by any restriction to this flow.

Secondly, mains electricity is supplied as alternative current AC that changes direction times every second. This electricity comes in the form of a sine wave, but audio equipment such as tube amps operate on a steady DC current. Thus, a tube amp will have a device to convert mains electricity from AC to DC, then a transformer steps it up and down as required to supply it at different voltages to the other components in the system.

While the tubes in the circuit operate at over V, the elements in the valves require 6. Thus, the first step is to step up or step down the electricity supply as required and send it to the different circuits. The rectifier valve converts or rectifies the AC signal from the supply to a DC signal that the tubes can use. In older amps, this was also done using a tube valve rectifier, but silicone diodes have since replaced them in more modern equipment.

The different electrical waveforms look like this :. As you can see, an alternating current represented by the green line has a sinusoidal up and down waveform, while we need the more stable direct current.

Because the waveform keeps changing direction, the rectifier works by correcting this reversal to produce a unidirectional waveform.

The rectified DC waveform is not completely straight. Instead, it tends to look more like the pulsating waveform above. These capacitors continue to store a lethal dose of charge even when the tube amp is switched off. Due to the high voltages involved, this charge can be lethal if you touch the wrong place and they dump this charge inside your body.

The high DC voltage produced by the capacitors then goes to the plates in the tubes, which require a high positive potential to attract electrons from the cathode. If further voltage or current regulation is required down the line, it is accomplished by the use of resistors. Like a constriction in a pipe, resistors can be used to control and reduce the amount of voltage and current in a system.

They dissipate electrical energy depending on their resistive value, whose units are ohms. If you ever build your own tube amp or would like your technician to help you experiment a bit, you can always suggest changing the resistors in the output stages of the circuit.

Many valve amplifiers have two sets of tubes: preamp tubes and power tubes. The preamp tubes are the first set of tubes that receive the guitar pickup signal. Their work is to pre-amplify the guitar signal to a level that can be applied on the larger power tubes, and they also drive reverb or tremolo effects. The 12AX7 type of preamp tubes is one of the most common.

The 12 in the specification indicates the amount of heater voltage they take as opposed to 6. The most common 12AX7 tubes have a gain factor of , which makes them one of the most powerful options available. The most important job of preamp tubes is to give character to the input signal. They add rich harmonics, create distortion, and add sustain to the sound at high volumes; this is known as overdriving the preamp.

I mean, how does the different components help in amplifying the source wave? A vacuum tube is a small tube from which all the air has been removed. The first vacuum tubes had just two elements, a heater or filament and a plate, and were called diodes. They rectified AC to DC. The filament is heated by a small AC voltage, for example 6. In , De Forest invented the triode. He added a cathode which surrounded the filament. The cathode is heated by the filament and was a better source of electrons.

He also added a control grid in the form of a spiral of wires or an open mesh so the electrons can pass through in between the cathode and plate. If the voltage applied to the control grid is lowered below that of the cathode, the amount of current from the cathode to the plate is reduced. Note that this effect -- varying the output via a voltage -- is similar to how MOSFETs are controlled by varying the voltage on the gate.

This is in contrast to Bipolar Junction Transistors BJTs , which use a varying current through the base to control the output. To make an amplifier, a large positive voltage several hundred volts is applied to the plate through some sort of load. Then a signal is fed into the control grid. A relatively small amount of change in the grid voltage causes a much bigger change in the voltage across the load. In the picture below , is probably about as simple a one-tube amplifier that is possible, but it illustrates all of the basic concepts.

The speaker acts as the load. Triodes worked fine for audio frequencies, but some problems arose when applied to radio frequencies RF.

In order to improve the triode, additional grids were added. First a screen grid was added in between the control grid and the plate, then a suppressor grid, until finally a five grid tube was invented called the pentagrid. It became very popular and was used in most of the AM radios manufactured from the 's on until transistorized radios began to replace them.

There are also vacuum tubes with two triodes in them. The filament is shared between both halves, but nothing else. They would be used in two stage amplifiers like this one. Each half of the dual triode is drawn as if it was separate, but the dotted lines indicate that the two sides belong in one envelope. The 12AX7 is one of the most popular dual triodes ever produced and and an estimated two million per year are still being made in Russia and China vacuum tubes are no longer manufactured in the US.

In almost all cases they are used to amplify a signal, or at least in audio equipment. There are many types of tubes and the simplest is a diode. You have likely heard of modern diodes which are used in circuits to control the direction electricity flows. The name diode comes from di meaning 2 and that is has only 2 electrodes inside. There triodes, tetrode, pentodes, and so on. In basic terms they are glass tube with a cathode and anode that pass electrons from one side to the other.

However vacuum tubes are a little more complex than that. Below is a diagram outlining the parts of a common vacuum tube. Not all tubes are constructed exactly the same however most tubes in audio applications will be similar. The path of electricity through the tube will help you understand what it is doing to the current. The Signal that is being amplified and the grid voltage when applicable.

The grid voltage is variable from negative to positive. This voltage will control the speed at which electrons move from the cathode to the plate, when positive, they move faster, and negative grid voltage has the opposite effect. The Heating element is constant voltage which is specific to the tube being used in the circuit. The heater is what gives the free electrons the energy to be released from the cathode and flow then to the plate.

This is why you want to let your tubes warm up a bit before actual use.