This page is dedicated to all those Audiophiles who want to build the very best amplifier that money - or lack of it - can buy.

This page is dedicated to the memory of that great progression of valiant people who have given a lifetime of commitment to the pursuit of high fidelity.

It recognises their contribution by recalling the great advances made - as well as revealing to the discerning reader many of the outrageous claims made by builders of commercial amplifiers and sound reproduction equipment.

The author makes no claim whatsoever as  to the validity or accuracy of any statement or information contained in these pages and no liability will be accepted for any error or omission of fact.

No warranty is made as to the workability or performance of designs or information described herein.

WARNING - VACUUM TUBES OPERATE AT HIGH VOLTAGES THAT CAN BE FATAL OR CAUSE PERMANENT PHYSICAL MALFUNCTION OR DISABLEMENT. DO NOT ATTEMPT TO CONSTRUCT  OR WORK ON AN AUDIO AMPLIFIER UNLESS YOU HAVE BEEN TRAINED OR ARE OTHERWISE QUALIFIED TO DO SO - WE WANT YOU TO LIVE TO ENJOY THE FRUITS OF YOUR LABOUR.
 

1.    VACUUM TUBES, ELECTRON TUBES, THERMIONIC VALVES

The VACUUM TUBE is an electro-mechanical device that enables an electrical current to be controlled be external electrical means.

The Vacuum Tube is commonly known around the world by the names "Vacuum Tube, Electron Tube, and Thermionic Valve."

The RCA Receiving Tube Manual RC-19 (1959) describes it so:

"The Electron Tube is a marvellous device.

It makes possible the performing of operations, amazing in conception, with a precision and a certainty that are astounding.

It is an exceedingly sensitive and accurate instrument - the product of co-ordinated efforts of engineers and craftsmen.

Its construction requires materials from every corner of the earth.

Its use is world-wide.

Its future possibilities, even in the light of present day accomplishments, are but dimly forseen; for each development opens new fields of design and application.

The importance of the Electron Tube lies in its ability to control almost instantly the flight of the millions of electrons supplied by the cathode. It accomplishes this control with a minimum of energy.

Because it is almost instantaneous in its action, the electron tube can operate efficiently and accurately at electrical frequencies much higher tha those attainable with rotating machines.

An Electron Tube consists of a cathode, which supplies electrons, and one or more additional electrodes, which control and collect these electrons, mounted in an evacuated envelope.

The envelope may be made of glass, metal, ceramic, or a combination of these materials."
 
 

The Vacuum tube is called a "Valve" is some countries because it performs the traditional function of the valve - ie a valve controls the flow of fluid in a device, usually a pipe or tube, by limiting the volume of flow of fluid to a pre-determined proportion, or percentage, of the maximum possible rate.

A "valve" is not a "regulator". The function of a "regulator" is to limit the rate, pressure or volume of flow to within a range controlled within pre-determined limits.

As we will see in this paper, electrical current behaves just like a fluid and this analogy will be referred to several times.
 
 

2.    HOW VACUUM TUBES WORK

The Vacuum Tube is an extremely simple device.

In its simplest form, the 'diode", the vacuum tube comprises a cathode and an anode mounted in a vacuum chamber.

A high direct current voltage is applied between the anode and cathode. The cathode is heated to stimulate it to emit electrons. The electrons are then free in space in the vacuum tube, and are attracted to the anode, setting up a stream of electrons, just like water in a pipe.

In a diode the volume of electrons flowing is controlled by the load in the circuit. Diodes are therefore not "valves".

The maximum current that can flow in the circuit ("prospective current") is determined by the capacity of the mains, battery or generator supply limited by the sum of the internal impedances (reactance) of the vacuum tube, circuit components (eg transformers) and the load.

Diodes have limited application and are therefore mainly used to rectify (convert) alternating current to direct current, in which application they offer outstanding performance and reliability if used correctly.

Vacuum tubes are not very efficient, giving up about half the input energy as heat, transferred to the atmosphere and mounting components by convection and conduction respectively. Electrons generated by the hot cathode but not used by the load are also released as heat, described as "plate dissipation".

In directly heated tubes the cathode is heated by itself and is called a "filament" because the electrons are emitted directly from it, just as in a light bulb. Filament warm-up time is quick so current flows soon after the tube is switched on.

In indirectly heated tubes - ie where the cathode is physically separate to the heater, the cathode is called a "cathode", to describe its true function. Cathode warm-up time is slow, so current flows some time after switch on - usually controlled by tube design to about 11 seconds from cold to full warm up.
 
 

3.    ELECTRON TUBE TYPES

Electron Tubes are designated by the number of electrodes used in their design and construction.

In directly heated tubes the "filament" is classed and counted as an electrode.
In indirectly heated tubes, the "heater" is not classed or counted as an electrode.
In most cases except the "monode" the Electron Tube contains an ANODE ( the "plate") and a CATHODE (the directly heated "filament" or indirectly heated "cathode"). The remaining electrodes are called "grids" and are used to control the flow of electrons between cathode and plate.
Certain types of gas filled tubes may vary slightly from the above configuration.

a)    MONODE  - Single Filament Electrode eg LIGHT GLOBE or LAMP. (Not usually classed as an "Electron Tube" even though they are typically a vacuum tube.)

b)    DIODE - Two electrodes

c)    TRIODE - Three electrodes

d)    TETRODE - Four electrodes

e)    PENTODE - Five electrodes

f)    HEXODE - Six Electrodes

g)   HEPTODE - Seven electrodes

h)   OCTODE - Eight electrodes

i)    BEAM POWER TUBE - Further refinements produced the Beam Power Tube, which may be a tetrode or pentode whose performance is enhanced by mechanical manipulation of the electron beam, to produce substantially greater efficiency, power output and reduced distortion.

Examples of  the Beam Power Tube are demonstrated in the 6L6 family of designs, which include the 6L6GT, 807, 1614, 1625, 5881, 7027A, 7581A/6L6GC and KT66.

Other Beam Power Tubes include 6AQ5, 6CZ5, 6DZ7 (2 x 6BQ5), 6V6GT, 6005, 6550/KT88, 6973, 7581A/KT66, 8417, KT66 and KT88.

These tubes evolved from the 42 and 6F6 family which were pentodes. Comparison of their performance shows reduced heater power requirements and substantially improved performance.

Please note the 6BQ5/6DZ7/EL84/7189, 6CA7/EL34, and 7591/7868/6GM5 families are not Beam Power Tubes but are Pentodes.

Beam Power Tubes may generally be identified by a box-like structure on top of the plate assembly, which extends down inside the tube between the plate and the control grids to direct electron flow.
 
 

4.    TUBE FUNCTION

Electron Tubes may be used in a very wide range of applications.

In audio amplifier and modulator applications the primary functions of Electron Tubes are:

a)    Rectifier
b)    Voltage Stabiliser
c)    Voltage Amplifier
d)    Phase Splitter or Phase Invertor (push-pull circuits only)
e)    Power Output

Tube signal output is controlled by direct current voltages applied to the electrodes of the tube.
 

To be continued........