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Understanding Tube Amps

This is presented to help you understand how vacuum tubes work, and how they are used in amplifiers.
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Chapter Index

The Tube preamp stage

Tone Stacks

* Phase inverters

* Power Supplies for Tube Amps

Output stage - does class matter?

How magnetism works, or rather how magnetism acts

Electronic Theory and Amplifier Repair

Know What Your Problems Really Are

Getting Ready to Work

{This is a limited-content document that will be used in my upcoming Tube Amp Class and Book.}

Vacuum Tubes in Amps

by Mike Fratus, www.Fratus-Amplification.com

Vacuum tubes are still used in a large number of products.


Recent interest in tubes has exposed the fact that colleges and schools have entirely abandoned teaching about the vacuum tube, even though the operation of it is fairly easy to understand, and much easier than the physics of semiconductors. In fact, once you understand the activity inside tubes, the activity inside solid-state devices is much easier to understand.

Before We Begin

To do anything in electronics work, you need to understand something about electrons, the things that make electronics possible. You have to know what to expect them to do. And yes, they do things.

The Underlying Theory

I remember that when I was little, I wondered how stuff worked and had wild ideas about how things worked without any knowledge of even what was inside things. It took many years to get around to electronic things, and delving into how they worked. So, I am attempting to pass on what I had to learn to understand electronics the way I do, without all the false starts and side trips, and with a minimum of intensive mathematics.

Atoms And Their Parts

Atoms are the building blocks that make up everything we can see and a lot of things we can't. But even the atoms are made up of smaller building blocks. Every atom has at least one proton and one electron. The element Hydrogen has only these two particles. These two fundamental particles are the charged particles of the atom. The proton has a positive charge, and the electron has a negative charge. Even protons and electrons are made of smaller parts, but we won't talk about that. It is interesting, though, when you find out that things are made of even smaller things, and so on. Atoms are like the bricks or boards that make up a wall or a fence.

Protons tend to stay put, and electrons are the atomic particles that can move from one atom to another, and form bonds with other atoms. When conditions are right, they can and will move. You can't see them, but they are there all the same. We can actually measure their movement.

Atoms are known as "The Elements" in modern schools. (Not the Bruce Willis movie, but that was a good movie. The Fifth Element is Boron, by the way.) There are lots of elements, and the last one I know of is #118. You have heard their names before, I'm sure. Most science classrooms have the chart called "The Periodic Table" posted somewhere. The table starts with Hydrogen, Helium, Lithium, and so on, in the order of how many protons they have. Hydrogen has one proton, Helium has two protons, Lithium has three protons. They also have more-or-less the same number of electrons as protons, as well. The third particle, the Neutron, has neutral properties that we won't consider important for our subject here. We’re talking about electrons mostly.

The Surprising Universe.

What we see as we look around is not really the way things are. That is, it’s not a complete representation of reality. What we are really seeing when we look at or touch any object is actually the fields around and forces between atoms. Atoms themselves are vanishingly small. The distance between atoms are enormous compared to the atoms themselves, but that just shows how important these forces and fields of energy are to our universe. If it wasn't for these fields, objects would just pass through each other. The forces holding atoms together and onto other atoms is quite powerful.

Electrons And Their Job.

Protons and neutrons are stuck in the middle region of the atom, called the Nucleus. Electrons are mobile. They spend most all of their time orbiting the nucleus. Electrons also are free to come and go, which is their job - orbiting some nucleus somewhere. They can jump to another atom, and start orbiting it, or keep going, hopping from atom to atom, as they are forced to or are attracted elsewhere. That is their job. They represent part the energy content of the atom, and define bonds between atoms that they visit regularly. The grouping of atoms makes the materials that we experience in the world.

Conductors And Non-Conductors.

Electrons travel in a most peculiar way, by jumping the orbit of its own atom and orbits of adjoining atoms. Each atom has a "normal" number of electrons that orbit it. If another atom will accept electrons or provide them, it is considered a Conductor. If it will neither accept or provide electrons, it is considered a non-Conductor, or an Insulator. We use these properties to control where electrons go, and where they don't. There are special exceptions to this - see SEMICONDUCTORS. But normally, something is either a conductor or and non-conductor.

Two Ways Electrons Travel in normal conditions.

    1. Electrons can go hopping from atom to atom according to whether there is a place for it to move to. That is called a "hole." A hole is where an electron COULD be if there were any available. They can also be inclined to move when an extra electron arrives from another atom. So one of the electrons has to move on - it does not matter which one - but there are only so many electrons allowed orbiting an atom, usually the same number as the number of protons.
    2. Electrons can also become "mobile" when they acquire too much energy to stay in orbit around an atom. Instead, they act like a swarm of gnats, looking for a place to land. Light and Heat can also supply enough energy to free electrons, as can other types of radiation.

Those are the two major modes of movement. The first way happens inside wires and other conductive substances. The second way happens in a vacuum around a conductor.

What Is A Battery?

A battery is an electron pump based on chemical activity. The electron flow ceases when the chemicals quit reacting.

What is a power supply?

A Power supply is also an electron pump, but usually run from your AC line current. Electron flow ceases when you turn off the power switch.

Why do we need power supplies?

The negative terminal supplies electrons by having excess electrons. The positive terminal wants or draws in electrons. Using these two terminal is what we will call a "closed circuit." The electrons come out one terminal, travel through the circuit, and go into the other terminal. That was a very short sentence, but tends to turn common thinking on its head because it is the opposite of conventional thinking.

Ahhh. Controversy. We Like That.

There are two prevailing, but completely opposite ways of considering how electricity flows.

Electron flow diagram

Electron Flow says that electrons are provided by the Negative terminal of a battery, and return to the Positive terminal.

Current flow diagram

Conventional Current Flow says that current comes out the Positive or Plus terminal and travels to the Negative or Minus terminal.

I learned Conventional Current Flow at first. Sure, the current comes from here and goes to ground. That works for casual thinking, but when you have to understand how tubes work, you will see that you will only understand tubes if you use the Electron Flow viewpoint of electricity.

So, from now on in this discussion, electrons come from the negative terminal of the power source and move towards the positive terminal.

Let’s take a moment to define two terms: Voltage and Current. The way I learned it seems to work: Voltage is how badly electrons want to get somewhere; Current is how many want to get there at the same time. (That is why the battery cables on your car are so thick... many electrons need to travel at the same time to turn the starter motor. That is a lot of current.)

Vacuum Tubes

What is a tube?

A vacuum tube is an evacuated tube (all the air is sucked out) with two or more conductive elements in it. One terminal, called the Cathode, is connected to the battery's Negative terminal and provides the electrons which travel through the vacuum (like an astronaut in space) and land on the Anode, which is connected to the Positive terminal of the battery, and is more commonly called the Plate.

During their travel through the circuit, the electrons use the 1st mode of travel as discussed above, until they reach the tube. Then they travel by way of the 2nd mode, through a vacuum. Then they travel again in mode 1 until they reach the battery's other terminal.

The above described tube would be a "cold cathode" tube. You probably have one like that as your laptop backlight, a tiny fluourescent tube.

The kind we usually use in amplifiers has a heated cathode. In it, the cathode is a metal tube with a heated wire inside of it, which makes the electrons more mobile or "anxious to get somewhere." A heated-cathode tube can operate at lower voltages and carry more current than a cold-cathode tube of the same size.

What will tubes do?

Depending on the construction of the tube, they can:

1. Act as a one-way valve by only allowing electrons to travel in one direction.

2. Act as an amplifier of very small signals (voltages) and small currents.

3. Act as an amplifier of large signals and heavy currents.

How do tubes work?

1. The Heater warms up the Cathode, and electrons form a cloud around it, ready to go somewhere. (They are too energetic to settle back down.)

2. The cathode is connected to the negative supply voltage, which provides it with a ready source of electrons.

3. There is a Plate, or Anode, that is connected to a positive voltage, and the electrons REALLY want to get there.

4. In between the cathode and plate can be nothing or several gateways called Grids.

A. If there is no grid, then the tube is a Rectifier or Diode ("Two-electrode").

B. If there is one grid, it is a Triode ("Three-electrode").

C. Then from there on it goes Tetrodes, Pentodes, and so on.

Now, here is the important part that explains how tubes work.

5. The grids control the flow of electrons through the tube. The reason this is true has to do with electric fields and potentials and stuff like that. Some grids act like a throttle, some act like fences, some act like mirrors, depending on how they are charged, which is called Bias.

What tubes are the ones to learn about?

The Triode are the PREAMP tubes in your amp. The Tetrodes and Pentodes are the OUTPUT tubes in your amp. If you only learn those, you can understand your amp.

Normal operation of tubes

(Tube amps designers use the term "Stage" for the building blocks that make up an amplifier.)

There are preamp stages, tone control stages, and output stages.

The way the stages are designed and the order that they occur has a major influence on the way an amp sounds. That is why you can't just put in different tubes and make a Fender sound like a Marshall.

You would have to change the Tone Stack, the output tubes, and the output transformer. Well, that would be a start. The more you change of the design, the further you get away from Fender design.

The way each stage is designed is why you can't just put in a different tube and make one model sound like another, or why a crappy amplifier model is only going to sound like a slightly better crappy amplifier after you put a $150 NOS tube in it. It really isn't lack of appreciation after all.

The most common preamp stages are a design called self-biased/grounded-grid. The grid voltage reads zero volts with no signal, and the cathode voltage is about 1.5Volts with a 12AX7 preamp tube. The plate voltage ranges from 80 Volts or so, up to 280 Volts or more.

Now, in that amp design, the tube is regulating the current flowing through it in a delicate balancing act. When a signal voltage is impressed upon the grid of the preamp tube, for example your guitar signal, it disrupts that balance and this disruption is amplified by the tube.

[The reason is, the grid is close to the cathode and surrounds it, so any small change in the grid voltage results in a large change in the current flow. This appears as a varying plate voltage that is the amplified signal.]

Normal output stages can be self-biased/grounded grid designs, or fixed bias designs. A few amps have both types, or can switch from one to another.

The output tubes act in a similar way, except they are larger and the signal sent to the grid is already quite large. The same thing applies, though. A small variation of voltage on the grid equals a large variation of voltage (and current) at the plate. But in this case we want lots of electrons to flow - lots of current. Then the Output transformer converts this large-voltage/large-current variation to a small-voltage, REALLY large-current signal to drive the speaker.

So, that is how tubes, preamps, output stages and tube amplifiers in general work.

Brought to you by Mike Fratus at Fratus-Amplification.com copyright 01/29/2017... Unique Visitors: