Wilder Amplification

A forum member requested that I write up an article regarding amplifier classes of operation, i.e. Class A, Class AB, Class B, etc etc. As I was thinking about how I would write the article, I came to the conclusion that in order to gain a full understanding of amplifier classes of operation,there first needs to be an understanding of how a valve works. So this article is the first of a series of articles regarding power amp operation.

I have divided this article into two parts to avoid overwhelming you with too much info to digest all at once. Don't try to read and fully understand it all in one setting...there's a lot of material to remember. Read some...take a break...come back and read some more, re-read if you have to. Don't try to digest it all at once as you will find it confusing if you try to and will quickly lose interest.

What is a valve?

More commonly known as a vacuum tube in the US, a valve is basically an electrical "valve" that uses an externally applied voltage to control the electrical current that flows through it. It can be thought of as a voltage controlled pot. Instead of having a mechanical assembly and allowing you to control its resistance by rotating it like a pot does, it uses an applied voltage to control its effective "plate resistance", which controls electron current flow through it.

The first type of valve was called the "diode" valve. Diode is a Latin term, "Di" meaning 2 and "Ode" meaning "elements". Inside of a diode tube you have two active elements excluding the filament, or "heater".

The diode tube has a "cathode" and an "anode". The cathode can be either the heater itself (directly heated cathode type...heater is referred to as the "filament" in these types of diode tubes, i.e. 5U4, 5V4) or it can be a metal sleeve that is wrapped around the heater (indirectly heated type, in which the filament is referred to as the "heater" since its only purpose in these tubes is strictly to "heat" the cathode). Examples of indirectly heated cathode type diodes are the 5AR4/GZ34. Also, most power and preamp tubes are the indirectly heated cathode type.

The cathode is typically connected to the negative side of the power supply, which has an excess of electrons as compared to the positive side. When the cathode is heated, there is a coating on the cathode that will emit electrons from the cathode itself. This is known as "thermal emission of electrons. This creates a "space charge", which is basically a "cloud" of electrons that are being boiled off of the cathode. This cloud of electrons surrounds the cathode in the free space around it.

Electrons are negatively charged. Opposite charges attract while like charges repel, just like magnets. By having an excess of electrons on the negative side of the power supply, this is what gives the negative side of the supply its negative charge.

Just the same, the positive side of the supply has a positive charge. This is due to there being a shortage of negative electrons on the positive side. Since the negative side has more electrons than the positive side does, this creates a charge difference, which we see as a "voltage". This voltage is a force that wants to pull the electrons from the negative side of the supply to the positive side in an effort to neutralize the charges. Again opposite charges attract.

Back to the diode valve. So now our cathode is heated and is connected to the negative side of the supply. This means that our heated cathode has a cloud of electrons that are being boiled off of the heated cathode and waiting to go somewhere.

Let's place a plate (known as the "anode" plate, or more commonly referred to as the "plate") inside the valve and place it in close proximity to the cathode, but not physically touching the cathode. If we connect the positive side of the power supply to this plate, thereby positively charging the plate, the negative electrons that are being boiled off of the heated cathode will flow through the space between the cathode and plate right to the plate...and back to the positive side of the supply.

However, if you try to reverse it so that the electrons are on the plate and the cathode is on the positive side, the cathode will have no electrons to boil off of itself because they're on the plate. Since the plate is not heated, it cannot emit electrons so all current flow stops. Thus, the diode valve acts like an electrical "one way valve", hence the name.

OK, so let's go back to our original circuit. We now have the cathode connected to the negative side of the power supply (which in amplifiers happens to be ground) and the plate connected to the positive side and current is flowing. Well, since a diode valve has a fixed value of plate resistance, the only way we really have to control current flow through the valve is by changing the voltage applied across the cathode and the plate. But what if we wanted to control the plate's voltage by contolling the current flow through the valve? We would have to have a way of controlling the valve's effective plate resistance to have a way to increase/decrease current flow through it.

This is where the valve now becomes a TRIODE (3 element) valve. A 3rd element is placed between the cathode and the anode plate. This element is known as the "grid". This name comes from the fact that the first grids were basically a metal "screen", much like window screen material, that is placed in between the two original elements. Later, the grid became a spiral of very thin wire wrapped around two posts set a distance apart from each otehr and most of the electrons pass right through the space between the spiraled wire. Current flow from the cathode to the plate passes right through the holes in the grid (VERY little of it actually flows onto the grid).

By applying a voltage force to this grid, we can now increase or decrease current flow through the tube. If a positive voltage is applied to the grid, this causes more current to flow from the cathode to the plate because the positive charge on the grid pulls more negative electrons from the cathode. Again, most of these electrons pass right through the grid and end up at the plate, so current flow to the plate (i.e. plate current flow) INCREASES. Applying a positive voltage to the grid basically decreases the valve's effective plate resistance by adding a second positive voltage to the grid to assist in pulling more electrons from the cathode.

Now let's apply a negative voltage to the grid. Remember like charges repel? Now the negative electrons from the cathode are being repelled by the negative voltage on the grid. This creates a "resistive field" between the cathode and the grid that the electrons must pass through, which effectively increases the resistance between the plate and the cathode, known as the valve's "plate resistance". As a result of the plate resistance increase, current flow to the plate DECREASES.

A small voltage change on this grid creates a large current flow change through the valve from cathode to plate, which in turn creates a voltage change on the plate that is larger in amplitude than the voltage change on the grid. Example, a 1 volt change on the grid might create a 10 volt change in plate voltage. We now have amplification.

Now we get into PENTODE valves. A PENTODE (5 element) valve is a valve with...yep you guessed it...5 active elements. We now have 2 more grids...the "screen grid" and the "suppressor grid". The original grid is now referred to as the "control grid". We'll get into why in a bit.

The screen grid is a 2nd grid placed between the control grid and the plate. It typically has a positive voltage applied to it that is just a few volts lower than the plate. Inside the valve you have electrodes (triode = cathode, grid and plate) that are placed in close proximity to each other. This creates a capacitance that exists between the electrodes, much like how a capacitor has two metal elements in close proximity seperated by a dielectric. In the case of a valve, the free space in the valve makes a dielectric.

The purpose of the screen grid is to reduce this capacitance that exists between each element, known as "interelectrode capacitance". This increases the efficiency of the valve.

So now we have an amplifying tube. However, as current flow to the plate increases, so does the temperature of the plate. Eventually the plate may reach a temperature hot enough for the plate to emit electrons. This will cause some of the electrons that flow to the plate to be emitted from the plate. This is known as "secondary emission of electrons". These electrons would normally get sucked up by the positive charge on the screen grid, causing some power loss. This is where the "suppressor grid" comes in.

The suppressor grid is connected to the negative side of the power supply. Its job is to "push" these stray electrons emitted from the plate back onto the plate. The negative charge on it from being connected to the negative side of the supply creates a repelling force that repels electrons (electrons being negative...like charges repel) back onto the plate. This reduces power loss within the valve by not allowing the stray electrons emitted through secondary emission to be sucked up by the screen grid, hence they are forced to flow to the plate, and through the output transformer.

Stay tuned for Part 2 to this article.

__________________
Jon Wilder
Wilder Amplification

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