Valves Or Tubes To You – An Introduction

Just like vinyl, valve or tube amplifiers rapidly went out of fashion as solid state semiconductors, CD’s and streaming became the new, lower cost norm. In reality all that happened was that vinyl and tube amplifiers took a back seat for a while while users investigated these new far lower cost technologies. Now, as time has moved on, many are once again hearing and appreciating the benefits of these older technologies and as in the case of vinyl are rapidly moving over to it and dropping plastic CD’s. The convenience and choice of digital streaming however still consumes a huge share of the music industry.

So it would appear that together with vinyl the older valve technologies are gaining in popularity, fame and fortune again. I therefore thought that a highly simplified introduction into valve technology and how they work maybe of interest to some readers out there. Do not worry this is not a highly technical, deep dive into valve electronics and design, just a brief overview of what they are, how they work and the different types and uses. So here goes.

First – What is an electrical current?

For those of you who are still confused as to what positive and negative means read on. An electrical current is nothing more than a whole load of electrons all moving generally in the same direction at the same time. Materials are made of atoms that contain a nucleus of protons (+ve charge) with generally an equal number of electrons (-ve charge) orbiting the proton/neutron nuclei cluster.

For now we shall assume that all materials in their natural state are electrically neutral generally having an equal number of electrons and protons. (Equal and opposite charges cancel out.) More importantly the protons are BOUND to the nucleus of an atom and cannot normally be removed. Materials known as conductors, most metals like; copper, gold, silver etc. have atoms that will easily release, or give-up, their outer most electrons due to the weak attractive force between them and the atoms protons. (The atoms of insulators do not have “free” electrons).

Copper Atom; 29 electrons, 29 protons and 35 neutrons

The orbiting electrons maybe released by giving them additional energy from; light, heat, chemically or applying magnetic or electrical fields, a bit like a soccer player kicking a ball from under another’s foot. Once free and in order to get the electrons to all move in one direction we need to apply an attractive or repulsive force to them. This force is generated by creating an electron imbalance. Just like magnetic attraction and repulsion, electrons are repelled by other electrons and attracted to positively charged areas. As the positively charged protons cannot move, a positive charge in nothing more than a number of atoms that have a shortage of electrons as they have been moved somewhere else.

Positively charged copper atom

The simplest and most common electrical force to get these electrons to move is a chemical battery or a power supply. Batteries use chemicals to separate the electrons from materials and keep them separated, causing a build up of electrons at the negative terminal and an equal but opposite build up of a positive charge at the opposite terminal. This difference in charge level is referred to as a potential difference and is measured in volts.

Connecting a conductor between the negative and positive terminals of the battery causes the weakly held outer electron, the “free” electrons in the conductor, to flow into the positive terminal causing electrons to flow out of the negative terminal to replenish those removed from the conductor that joined with the positively charged atoms inside the battery. The battery chemicals continue to free the electrons continuing the process. This flow of electrons is what is termed an electrical current and is the basis of all your electronics.

NOTE: Many people think that electrical circuit current flows from positive to negative……well it does not. It is the other way around!

So now you know where electrons come from, what a current is and how electrons behave. Lets apply that to valves or as they are often referred to, tubes.

Valve/Tube Technologies and Types

The first valve was invented in 1904 by British engineer John Ambrose Fleming who patented the thermionic valve. This first valve was called a diode as it only has 2 metal electrodes an anode or plate (positive) and a cathode (negative).

Basic Diode Valve Symbol

All valves operate by thermionic emission. So we now need to understand what this is before we take a peak inside the different types of valves to see how they work and what they are used for.

Types of Electron Emission

We now know that a conductor contains many free or loose electrons. These electrons may also leave its surface if given enough energy to overcome their attraction to their nuclei. Such energy may be provided by heat, electron bombardment, or light. There are three types of electron emission:

Thermionic Emission

In simple terms thermionic emission is the liberation of electrons from a metal surface or electrode by virtue of its temperature. So at the “center” of every valve you will see a red glowing element, the heater, that raises the temperature of the cathode metal of nickel or molybdenum to about 750 °C or 1,400 °F. This cathode is generally coated in various rare earth materials like barium or strontium carbonate in order to increase the number of electrons released. In modern valves the cathode always surrounds and is heated by, a tungsten filament that requires a DC or AC voltage of typically 6.3volts at currents up to a couple of amps.

The electrodes of valves are always contained inside an evacuated sealed glass bulb or ‘envelope’ so that there are few atoms of air/oxygen to cause the heater element to burn out or impede the movement of the electrons. Once the tube is evacuated and sealed any remaining air is removed by the ‘getter’, a small metallic device that combines with any remaining gases and absorbs them.

Simplified Diode Valve Top View

Diode Valve Electronic Symbol

The heated cathode releases electrons creating a cloud of electrons or “space charge” around it. If we attach a DC power supply/battery, as shown below, these negatively charged electrons that have been freed by the heat energy are attracted to the positive anode, thus causing an electron current to flow across the valve and in the external circuit as shown.

NOTE: A miniature valve, the Nuvistor, is sometimes used in high-end audio (and RF) voltage amplifiers due to their low noise figures. Due to its size and design Nuvistors run at lower temperatures.

The Nuvistor

Diode valves, like all valves, will ONLY conduct electrons in ONE direction, as shown in the graph above, from the cathode to the anode. That makes the diode an ideal device for converting AC electricity into DC electricity, a rectifier, one of the valves earliest and still current uses. Some of these valve rectifiers are dual diodes enabling full-wave AC rectification by one valve.

Dual Diode Valve Electronic Symbol

Typical rectifier valves include: EZ81, 5Y3, GZ34/5AR4 and 5U4.

GZ34 – Dual Diode Rectifier

Basic Valve Construction

Diode

Triode

Nuvistor – Triode

Controlling The Flow Of Electrons – The Triode

In order to make a valve do more than just act as a rectifier we need a way to control the flow of electrons that move from the cathode to the anode. Clearly altering the value of the battery voltage in the above diagram will cause the current to increase for higher voltages and decrease for lower voltages. However, this is of little use as this anode or plate supply voltage normally needs to remain at a fixed value for correct operation.

Electron flow control is achieved by inserting a wire grid, known as a control grid, between the cathode and anode as shown below. Such a valve is referred to as a triode (3 electrodes) and is extremely common and very popular.

Simplified Triode Valve Top View

Triode Valve Electronic Symbol

If a mesh or grid is placed around and close to the cathode, it will allow electrons to pass through its gaps to reach the anode. However, if a negative voltage is applied to the grid with respect to the cathode it will repel the electrons reducing the number that reach the anode, reducing the anodes current flow and the voltage drop across the anodes load. If the control grid voltage is made sufficiently negative it will cut-off all electron flow across the valve.

Very small changes in the grids voltage can produce very large changes to the valves anode current (electron) flow. The triode therefore acts as a voltage change to current change converter and with a suitable anode resistive load an effective voltage amplification of up to 100 or greater may be achieved.

Common audio triodes would include; low level voltage amplifiers like the dual triode ECC81/82/83 and the single high power triodes like the 300B and 2A3.

ECC83 – Low voltage dual triode

300B – Power Triode

The Tetrode & Beam Tetrode

The triodes amplification and performance at high frequencies (MHz) can become reduced due to the internal capacitance between the anode and control grid (Cag). This capacitance can also cause instability or oscillations as the phase of this capacitive feedback signal is positive and increases the input signal.

By inserting a second grid, the screen grid, between the control grid and the anode this capacitive coupling can be significantly reduced, creating the tetrode that has 4 electrodes.

Tetrode Valve Electronic Symbol

The screen grid is held at the same AC voltage as the control grid, effectively stopping the capacitive feedback, restoring the tetrodes gain and stopping instability. However, this screen grid now requires a high DC voltage so as not to repel the electrons back to the cathode and accelerating them to the anode.

Unfortunately under certain circumstances the addition of the screen grid accelerates the electrons to such an extent that when they hit the anode they cause secondary emission, and electrons are ejected from the anodes surface. This effect causes an unhelpful “kink” in the valves electrical characteristic.

This non-linear kink limits the valves use for audio and in particular large signal amplitudes due to the signal distortion that it creates. However, by modifying the physical structures of the cathode, two grids and anode and then adding beam forming plates, a beam tetrode is created that removes this kink. These beam forming plates concentrate the electron flow into strong narrow beams significantly reducing secondary emission. These beam forming plates are electrically internally connected to the cathode. Beam tetrodes are very popular being used in many audio (and RF) power amplifier designs.

Simplified Beam Tetrode Valve Top View

Beam Tetrode Valve Symbol

Alternative Symbol

In the 1950s, the ultra-linear audio amplifier circuit was developed for beam tetrodes, like the Leak TL50 plus. This amplifier circuit links the screen grids to taps on the output transformer, and provides reduced intermodulation distortion.

1xEF86 (pentode), 1xECC83 (dual triode), 2xKT88 (beam tetrode), 1xGZ34 (dual diode rectifier)

Leak TL50 Plus Schematic showing the two KT88 beam tetrodes

Examples of audio power beam tetrodes would be the KT66, KT88, KT90, 6CA7, 6L6, 6550, EL84 and for RF power the 4CX250B.

KT88 – High Power Beam Tetrode

The Pentode

An alternative method of reducing the tetrode “kink” is by adding yet another grid between the screen grid and the anode, the suppressor grid, creating the pentode that has 5 electrodes. The supressor grid is often externally connected directly to the cathode. The action of this suppressor grid is to repel secondary emission electrons back to the anode. Although very linear for small signal amplification, the pentode still shows a form of “kink” at low anode voltages, still doesn’t handle large amplitude signals too well plus the suppressor grid absorbs a small amount of power.

Pentode Electronic Symbol

Pentodes are very popular for use in both audio low voltage and audio power amplifiers. Examples would include; low power voltage amplifiers: EF80, EF86 and 12AX7, high power tubes: EL84, EL34, PL81, 6CL6, 6F6, 6G6, and 6K6GT.

EF86 – Pentode low voltage amplifier

EL34 – Power Pentode

NOTE: Due to the operational similarity of the Power Pentode and Power Beam Tetrode, manufactures have made equivalents of each so that one maybe be directly replaced by the other with no circuit modifications.

Other Valve Types

There are three other basic types of valve but they are only used in RF equipment, they are:

• Hexode – 6 electrodes – 4 grids
• Heptode – 7 electrodes – 5 grids
• Octode – 8 electrodes – 6 grids

Their main use is in RF circuits as oscillators and mixers.

There are numerous other valve types that combine the above basic valves into one glass envelope plus many other specialized valves for specific circuit designs, RF, displays, power supplies and photovoltaic use. All in all there are thousands of models of valves, however, most are now replaced with modern semiconductors.

Summary

There we have it, the diode rectifier and four basic audio types of valve all using the same thermionic principle but each building on the previous in order to overcome a technical issue.

Coming soon. A brief introduction to semiconductors and bipolar and field effect transistors. After all, they must compose 99.99% of everything electronic these days.

Want to know more about valves visit The National Valve Museum web site.

Want to know more about amplifiers and their class types see this post: Amplifier Classes – A Quick Primer.