Valve Guitar Amplifier – Tube Amp Basics Explained

Valve Guitar Amplifier – Circuit Design and Operation Explained

This page was created to explain the basic operation of a valve guitar amplifier. The sound and circuit design used in  guitar amplifiers which use valves (or tubes as they are known in the USA) in the pre-amplifier and output stages is completely different from what is achieved from solid state amplifiers. Solid state amplifiers use transistors, diodes and IC’s, a technology which became widely available to circuit designers in the 1970’s. Before then guitar amplifiers used valves (also known as vacuum tubes). The concept is the same, a guitar with a low output coil or pickup is plugged in to the amplifier and a greater amplified sound is then output from the amplifier speaker cabinet.


What are Valves or Tubes?

Inside a 12AX7 Valve
Inside a 12AX7 Valve

A valve (or tube) is a sealed glass tube with a vacuum and various wiring elements inside similar to a light bulb. The wiring is split into separate functions. The basic valves started with just a Cathode and Anode. When a +Voltage is placed on the Anode and a -Voltage is placed on the hot cathode, a current can flow between them, but not the other way around. This function is described as a ‘Rectifier’. In the earliest valves the Heater Filament and cathode were the same element.

From approx 1940 on wards the majority of valves (except rectifier valves) being manufactured had the heater filament and cathode are separate elements. The majority of valve filaments are 6.3V as early vehicle batteries were 6.3V. The temperature of a filament can be as high as 2,000 deg C. In small pre-amp valves the filament current consumes around 2W of power, in a power output tube this can be over 10W. In order to control the flow of current between the cathode and anode a Grid is added. A valve with only a single grid between anode and cathode is a Triode.

The triode is a a fine helix (spiral) wire placed close to the Cathode.  Without a voltage being applied to the grid the current between anode and cathode will be maximum. By applying a negative voltage to the Grid (-V) the current between Anode and Cathode is reduced. By applying a small audio signal to the grid it is possible to modulate the current between Anode and Cathode which will appear as a large amplified signal, as a perfect replica of the input.

From this basic design concept many variations of valves have evolved with various performance characteristics. This allows them to be used in many applications (not just guitar amplifiers). Gradually with the invention of solid state transistors valves were phased out as transistors allowed for smaller design, reduced manufacturing cost, increased reliability and circuits which consumed lower power.

An example would be car radios, these were originally designed using valve technology and were large and bulky items which consumed power from the car battery. By using transistors the radio design could be reduced in size and the power requirements from the car battery reduced.

This is also true in the field of guitar amplifiers with valve amplifiers all but disappearing from shelves in the 70’s onwards. However, the solid state version cannot replicate the great tone and dynamics of the valve hence why some iconic brands continued to use them.

Valves for audio amplifiers are categorised into three primary groups.

  • A Small (noval base) pre-amp twin triode 12AX7 and 12AU7 (12AT7 & 12AY7 similar but less used)
  • B Small (noval base) power tubes for 10 to 15 Watt amps. EL84 and 6BQ5
  • C Large (octal base) power tubes for 30 to 100 Watt amps. EL34, 6CA7, 6550, KT88, KT90

Each of these valves are explained elsewhere on this site.

Amplification – Creating Volume and Sound

The amplification of the guitar comes from the circuit which has been designed within the amplifier. There are various circuit types and also power outputs however the basic concept is the same. The low voltage output from the guitar is fed to the grid of the pre-amp tubes. This signal is then amplifier and fed to the grid of the power output tubes. The tubes are fed with high voltage from the transformers within the amplifier circuit. The power fed to the power output tubes becomes a replication of the the input signal and is fed to the speakers within the cabinet. By adding additional circuitry it is possible to adjust the input signal to add additional tone qualities – bass, middle or treble and also adjust the volume.

By adjusting the input signal strength it is possible to overdrive the valves creating distortion. This effect is what makes valve amplifiers unique. By adjusting how the valves are driven a great number of dynamic sound qualities can be created at the speakers. Distortion is a “clipping” of the input signal. Basically the top/bottom of the waveform is chopped off meaning the original clean signal is modified and the distorted signal is then amplified and output.

By adjusting the amplifier settings the guitarist can experience clean melodic tones to wild dirty sounds. It is the characteristics of early valve amplifiers which created some of the greatest bands and sounds of the 50’s, 60’s and 70’s.

Read our Guitar Amplifier Technology Pages

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