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Valve amps. The Lowdown....

Published: Tue December 23, 2008  News Feed


Historical Background of guitar amps

Rock music evolved from Blues, the music of the streets. Most musos’ of the 50s and 60s were poor and guitar amps made to a budget. Some but not all technical principles of amp designs were well thought out.

Fender and Marshall were the dominant and most copied brands. The powerful amps had 4 output valves in parallel push-pull and gave approx 60 - 100Watts.

The Myth
Most amp manufacturers were conservative from a previous generation with a background of country, jazz and religious music. They were horrified by the anti-religious, drug driven, sex crazed rock musos of the 60s’ diving their amps at full power into hard distortion for sustain. When these amps were first designed in the 50s, it was inconceivable they would be used in this way.

The Reaction
Many manufacturers reacted by making warranties void if amps were driven at full power, some threatened to cut off retailers who sold their amps to bands that played ‘music of the devil’.

The parody of this historical contradiction has been rewritten, to fit mythical beliefs that brilliant designers created these amps for what ‘rock musos’ wanted.


Myths and Legends

Valve technology is intrinsically the most elegant means by which a speaker is able to reproduce music. This is not because valves have magical qualities but because the technology by which they function is not achievable by other means.

In ten thousand years from now Valve amplifiers will possible be the only remaining technology from the 20th century they will be still be being used and loved with as much passion as when first invented. As much as there is no proof that Valve amplifiers can sound superior to solid state I for one if given the choice will only listen to music through Valve amplification.
Before solid-state technology, Valve amps were manually assembled by large teams of women in conditions that would not be accepted today. For domestic application the majority were not well made. Before manufacture, designs were scrutinised and modified to reduce production cost. Valve count kept to minimum, cheapest components used at voltage rating limits, safety standards almost non-existent.

The heart of a valve amplifier is the output transformer and accounts for most of the bulk and weight, it is also the most difficult and highest cost item to make. Size and therefore performance was reduced to minimum, especially when used for musical instruments. Only a few brands were made with technical excellence.

By the mid 1960s, valve amp technology started to leap forward, but too late. As sold-state arrived, manufacturers competed to be first to make the change. This new technology reduced size and costs. It enabled production to be mechanised and staff reduced.

The first solid-state amps used germanium transistors, which performed poorly, until silicon arrived.
The intense marketing of solid-state amps caused valve amps to be perceived as worthless, except for musicians.

Virtually mountains of them ended up in the landfills of our expanding cities.


The Basic Principles

A Valve is an extension of the light bulb. Theoretically inside the valve is a vacuum. The hot filament is called ‘Cathode’ (Let’s not forget a T.V. is a’ Cathode ray tube’ ).  Around the outside of the Cathode is a cylindrical metal tube called ‘Anode’.

When a +Voltage is placed on the Anode and a -Voltage placed on the Cathode, a large current can flow between them, but not the other way around. This is called a ‘Rectifier’ or diode.

Grid: A fine helix (spiral) wire called ‘Grid’ is placed between the Cathode and Anode. A small variable voltage (music signal) on the Grid varies the large current between the Anode and Cathode. The small varying input signal is now amplified to a large varying current.

The result is very linear. Why this happens is a mystery. The fact that it works and the universe exists is a miracle. It pays to be humble.

(emitter base collector) are complementary to valves (cathode grid anode).

Transistors are related to crystals. Their individual function is non linear and have to be arranged in compound groups to behave as a linear circuit.
Solid-state amps operate at low voltages (10 - 100V).
Valves amps operate at high voltages (200 - 600V).
Speakers operate at approx (0 - 40V).

The Output Transformer converts the high operating voltages of valves to the lower operating voltage of speakers. A transformer has 2 separate coils of wire (primary and secondary) wound around an iron core.

Electricity flowing through wire causes a magnetic field around the wire and visa versa, a changing magnetic field causes electricity to flow through wire.

The varying amplified current of the valve is connected through the first coil of wire (primary) and creates a varying magnetic field.
The varying magnetic field created by the primary coil, causes electricity to be generated in the second coil of wire, which is wound tightly around the first.

Electricity is transferred to the second coil only when the magnetic field is changing, not stationary. The iron core of the transformer keeps the magnetic field contained so little is lost. The transfer is very efficient.

The secondary coil is connected directly to the speaker.
The reduced secondary voltage is adjusted by the ratio of turns between the 2 coils. Eg 1,000 turns on the primary and 100 turns on the secondary would change the voltage 10:1.

Most output transformers have a turn’s ratio of approx 20:1.


Why do Valve Amplifiers sound different?

When technology changed from valve to solid-state, it was noticed that solid-state amplifiers lacked warmth and bass performance, and had to be twice as powerful as valve amplifiers, to sound as loud.
Current Drive:

Solid-state amplifiers behave in ‘Voltage Drive’. This acts as a short circuit (zero output impedance, or 100% damping factor) across the speakers, causing excessive damping, which reduces efficiency, limiting responsiveness.

Valve amplifiers behave in ‘Current Drive’. This represents an open circuit across the speaker without over damping, allowing maximum response and efficiency.

Sensitivity: Valve amplifiers (current drive) are sensitive to crossover resonances and speaker impedance variations. Quality speaker systems often used passive crossovers that were second-order, constant impedance, and critically aligned to avoid resonant effects. Some quality speakers had copper caped pole pieces, which helped damp impedance variations.

Note:- With solid-state amps in voltage drive, power decreases as the speaker impedance rises. With valve amps in current drive, power increases as the speaker impedance rises.

Therefore a flat speaker impedance is synonymous with a flat frequency response.
Modulating Offset:

The output of solid-state amplifiers is directly connected to 2 DC power supplies through the output transistors. The instantaneous non-symmetry within the music waveform (particularly from the bass notes) is averaged as a modulating DC offset. This modulating offset is small, but it varies the efficiency of the speaker, introducing inter-modulation distortion, amplitude modulating the music.

This problem is mostly eliminated by the trend to use an active sub-bass.

In valve amplifiers the output transformer isolates the speaker from the amplifier electronics. No modulating offset can be produced.
Open Loop Gain: Feedback

Solid-state amplifiers are inherently nonlinear.
They have a very large ‘open loop gain’ approx 20,000. The amplifier output is (feedback) to the comparator input to reduce the gain to approx 50. Therefore 99.9% of this feedback corrects all instability and non-non-linearity of the amplifier, as explained in amplifiers.

The speaker also acts as a large microphone. All non-linear movements and vibrations within the speaker cone, (caused by reflected nodes, chaotic resonances etc) of which there are many, are regenerated back into electricity by the voice coil.
This re-generated signal from the voice coil is inadvertanetly fed-back to the solid-state amps comparator input, and re-amplified back to the speaker as recycled distortion.
Re-amplified Distortion:

This re-amplified distortion is audible by comparison, when switching between a solid-state and valve amplifier in real time. By paying close attention, it is heard as a fine spurious inter-cluttering within the music. This is clearly noticeable with efficient speakers but not with inefficient speakers.
Valve amplifiers are inherently linear.

Their natural gain is small and therefore require minimal or no negative feedback.
Their designs do not require them to have comparator inputs as with solid-state amps. The small amount of negative feedback in valve amps is only required to provide damping to the speaker.
Valve Amplifier Design

From physics we know certain things must be a particular order and size to be efficient and this is acutely so with valve amps.

60Watts is the minimum power capacity for an amplifier to bring quality speakers to life with full fidelity.

The reason:

The dynamic range of music can exceed 60db (power ratio 1:1,000,000).
Transients within the music can exceed 20db (power ratio 1:100).
Therefore amps under 60Watts and ‘Single Ended Class A’ will not be discussed in this text.

Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS systems, audiophile stereo amplifiers, military applications (such as target acquisition and radar) and very high power radio and UHF television transmitters.

Want To Know More?

Read The Tube Amp Book

  • Tubeopedia - Wiki of electronic tubes and related topics
  • The Vacuum Tube FAQ - Henry Pasternack’s FAQ from rec.audio
  • The Audio Circuit - An almost complete list of manufacturers, DIY kits, materials and parts and ‘how they work’ sections on valve amplifiers
  • Conversion calculator - distortion factor to distortion attenuation and THD
  • AX84.com - Although oriented towards valve guitar amplifiers, AX84’s free schematics and theory document apply well to any tube/valve project
  • lenardaudio.com
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