A Guitar Amp Buyer's Guide

If its a small portable amp, large 4×12 or a simply a valve head. The choices are seemingly endless.

So we decided to put together a guide covering the many options available and try  highlight some of the features which make each unique.

Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

In real simple terms amplifiers come in several forms valve amps, transistor amps & modeling amps.

These in turn can be a head and cab, a combo ( or combination of head and cab) amp.

There is even acoustic amps and even mini amps to consider !

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.
History
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.

Transistors:
(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

Source:

• 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

Our Top 5  Guitar Head Amps

	Hayden  MoFo The MoFo by Hayden is an all-valve guitar amp with a whole lot of attitude, and best of all, handmade in England! Unlike other mini guitar amp heads, the MoFo is definitely not a one trick pony. From stunning bell like clean tones with real depth to plexi-esque classic rock crunch, right up to full on metal mayhem, the 30 watt MoFo head does it all. And yes…MoFo does stand for that word. But you’ll have to use one to find out why…it rocks!
	Vox Night Train Night Train is one sexy beast. Solid enough to inspire confidence, Night Train weighs less than seventeen pounds – and the “armoured lunchbox” dimensions provide extreme portability. Finished with a cool chrome mirror-finish, the signature VOX diamond design reveals that inspiring vacuum tube glow. And of course, the Night Train NT15H includes its own padded carry case to protect your investment.  And to make things even more appealing, you get a FREE Vox Wah pedal when you buy Night Train at Dolphin!
	Vox AC4 Head Here’s another greta Vox product, but an altogether different affair than the Night Train. The Vox AC4 offers pure vintage charm and, at 5 watts, is the quietest head amp of the bunch – as well as the cheapest, at under £200! The AC4 is ideal for recordings, where you crank up the amp to get the best, crunchiest valve tones. This up-to-date Class A version retains the distinctive EL84 power tube of the original and features a 12AX7 powered pre-amp, delivering a classic British tone
	Marshall Haze 15 Ever dreamed of owning a all-valve head amp by Marshall, but couldn’t afford one? Your dreams can now become true, thanks to Marshall’s new Haze series, which brings Marshall quality to a more affordable price range. The 15W head is the ultimate in Marshall portable valve tone. On its own you can take it anywhere, safe in the knowledge that you will never have to settle for someone else’s equipment.
	Orange Tiny Terror Small, loud and already legendary, the Orange Tiny Terror is without any doubt one of the most popular head amps we sell. Why? This very versatile workhorse won’t let you down in recording sessions or at gigs, and it’s quite affordable, too! The Tiny Terror is ideal for any guitarist seeking professional grade tube tone at a price range that won’t break the bank. With 15 watts of available Class A valve power, the Tiny Terror is a highly versatile workhorse suited to a myriad of guitar playing scenarios.

Our Top 5  Guitar Cabs Marshall MG 4X12B cabinet Due to the technical nature of the Marshall Amplification products advertised on this page it is highly recommended that you contact us by telephone or by coming into our store to discuss your particular requirements and to arrange a demonstration before buying any… VOX V212BN The VOX AC30 Custom Classic amp is the culmination of over 45 years of guitar amp design and manufacturing. VOX has taken the best AC30 designs and added a number of useful, very cool features to create the most tonally flexible and affordable AC30 to date! Here’s the perfect speaker cabinet for your AC30 head: the V212BN 2 x 12″… Hiwatt 4×12 Fane loaded Cab Solid Marine ply construction and loaded with ledgendary Fane speakers. 300 watt power handling 16 ohm impedance. Laney LT212 Lionheart Series Cabinet. 2×12 If you have the Laney L20H, you won’t want to compromise that great Class A tone by using any old cabinet. Instead, why not get the matching Laney LT212 cabinet. Featuring 2x 12″ Celestion Heritage G12H Drivers and closed back construction, this cab gives thick, deep tones with growling bottom end power. This stylish and great sounding cab also… Orange PPC212 There are many guitarists who are quite happy to plug their head into any cabinet they can lay their hands on. It’s a pity as you only need use your ears to realize the difference the right cabinet can make to your amplifier’s sound. Too many manufacturers compromise their cabinets by using inexpensive speakers. This foolhardy decision can be disastrous!… Our Top 5  Guitar Combos VOX VT30 Sophisticated modeling technology combined with the VOX Valve Reactor power amp circuit that uses a 12AX7 vacuum tube results in sound quality that overwhelms the competition. With sound quality that overpowers its rivals, the VOX Valvetronix VT30… VOX Pathfinder 10 The Pathfinder series of VOX amplifiers represents truly outstanding value for money yet captures the very essence and personality of the legend that is VOX. The look and feel of the Pathfinder is inescapably VOX, possessing all of the style of its bigger family members. Small yet perfectly formed, these amps are ideal for practice and recording. Those… Line 6 Spider IV 30 The 30-watt Spider IV combo features 12 amp models and seven Smart FX (three effects simultaneously). Each Spider IV preset, amp model and effect exceeds the renowned Line 6 standard of sound quality. Every aspect of the tone, from the guitar input to the three-quarter closed-back cabinet and Celestion® speakers, delivers exceptional definition, clarity and feel. Fender 65 Twin Reverb w/ 2×12 Jenson Speakers This amp has seen action in every imaginable musical venue and continues its musical legacy today! Whether you play rock, jazz, country, or whatever, this is what a clean electric guitar sounds like! Or add an upside-down Stratocaster guitar and a fuzz box … Fender’s 85-watt (at 4 ohms) Vintage Reissue of the ?65 Twin Reverb features 4… VOX AC30 CC2 The Vox AC-30 is quite possibly the most legendary guitar amp ever made, used by everyone from The Beatles and Rolling Stones to U2 and The Libertines. The AC30 CC2 updates the legendary amp with new and improved features – a guitarist’s dream amp, no less! The new VOX AC30 Custom Classic… Our Top 5 Modeling Amps Line 6 Spider IV 150 150-watts guitar amplifier combo with FX The new amplifier line includes many fresh features, innovations and updated highlights from its best-selling and award-winning predecessor, Spider III.  This 150-watts model is a great gigging amp full of great sounds and features. Fender G-DEC Junior Band included—all you need is, well, you! Our G-DEC series models are the world’s only all-in-one amps containing synthesized drum, bass, backing and teaching loops. The G-DEC Junior is a simplified, super-cute version of our award-winning original G-DEC. This 15-watt baby has an 8” speaker and simple interface with knobs for selecting various amp types, effects types, backing… Line 6 Spider IV 30 The 30-watt Spider IV combo features 12 amp models and seven Smart FX (three effects simultaneously). Each Spider IV preset, amp model and effect exceeds the renowned Line 6 standard of sound quality. Every aspect of the tone, from the guitar input to the three-quarter closed-back cabinet and Celestion® speakers, delivers exceptional definition, clarity and feel. VOX VT15 Sophisticated modeling technology combined with the VOX Valve Reactor power amp circuit that uses a 12AX7 vacuum tube results in sound quality that overwhelms the competition. Randall KH15 Kirk Hammett Practice Amp Kirk made it very clear from the start that he wanted to develop a full line of amplifiers that would provide fans his signature tone at any price point. The KH15 is equipped with a Clean, Overdrive and Boost mode and is the perfect practice amp for both the professional and beginning guitarist. … Roland CUBE Street – Black The Ultimate Amp for Travelling Musicians If you’re a musician who’s on the move, the new CUBE Street is for you. Whether you’re running to a rehearsal, lesson, street gig, or any situation that requires portability and versatility, the CUBE Street is a battery-powered marvel that’s ready to travel fast. The CUBE Street sounds amazing and is capable.

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~ by itsstecole on October 28, 2009.