Guitar/Buying an Amplifier
Amplifiers come in a wide variety of designs and your choice of amplifier should be based on the type of music you wish to play. Your local guitar dealer will let you test the different amplifiers they stock though they may only offer a limited range due to space restrictions. Its always a good idea to visit many dealers including pro-audio outlets to test amplifiers across the entire price range before committing yourself. This chapter will explain the difference between a tube amplifier and a solid state amplifier as well as exploring the variations on these two basic designs.
- 1 Tube, Solid State and Hybrid Amplifiers
- 2 Features
- 3 Wattage
- 4 Types of Unit
Tube, Solid State and Hybrid Amplifiers
Tube amplifiers produce a warm and fat tone that is popular with guitarists. New models are available from Marshall, Fender and Vox as well as other manufacturers many of whom have an equal reputation for quality. Vintage tube amplifiers are available to buy though maintaining them can be expensive especially with regards to the cost of replacement parts. The continuing popularity of vintage amplifiers from earlier decades has resulted in a market for reissues.
There are three operating modes for tube technology: Class A, Class B and Class AB.
- Pure Class A operates by having a full continuous current flowing through the tubes. The tubes are still fully powered even when there is no signal to divert to the speaker. This makes Pure Class A tubes expensive to run; similar to a car idling in neutral with the throttle pedal right down. Pure Class A responds very fast to an input signal with a tube saturation (distortion) that many guitarists find appealing. Many Pure Class A amplifiers use tubes with a low wattage to offset their inefficient operating mode thereby increasing the tubes lifespan and reducing running costs.
- Class A refers to an output design that doesn't use a Pure Class A single-ended output stage. Class A uses a pair of tubes or multiples terminating in a push-pull output stage.
- Class B is rarely used in guitar amplifiers due to crossover distortion issues so manufacturers instead use a combination of Class A and Class B known as Class AB.
- Class AB operates by using a pair of tubes. Whereas Pure Class A produces the entire waveform, positive and negative, with a single-ended output, Class AB produces the entire waveform using two tubes with one handling the positive voltage and the other the negative voltage. In comparison to Pure Class A, which uses a full continuous current to achieve a three hundred and sixty degree waveform and is always on at full power even when there is no signal present, Class AB uses a smaller amount of constant current to achieve the same always on state for the two tubes. Class AB is thereby more efficient with regards to power consumption and heat dissipation and the operating life of the tubes are greatly extended.
- A tube is based on vacuum technology and requires more energy than a solid state amplifier with the same wattage.
- Vacuum tubes are expensive and require replacement every one to four years depending on use.
- Amplifiers with tubes are generally heavier than solid state amplifiers due to the need for an output transformer.
- Tube amplifiers are usually more expensive than a solid state amplifier.
- Tubes require a warm-up period before they reach optimum performance.
Solid state amplifiers are very popular with beginners due to their affordability. Solid state amplifiers have a fast attack time and are immediately available for use when switched on. Solid state circuitry allows more volume to be applied to the output signal before clipping occurs which makes the amplifiers suitable for jazz or acoustic guitarists who may wish to retain a clean sound at high volumes. A solid state amplifier matched with good quality speakers can produce a wide frequency response. Some solid state amplifiers use field effect transistors (FET) on the preamp stage which at high gain produces a distortion similar to a tube amplifier.
Solid state amplifiers retain a tight low end while producing a full harmonic distortion at high gain which is desirable for the metal genre. This has resulted in a range of solid state amplifiers specifically designed for metal guitarists. Solid state amplifiers tend to be smaller and lighter than their equivalent tube amplifiers and these design factors allows manufacturers to build amplifiers weighing less than 10lbs which are capable of 150w clean RMS sound. Root Mean Square refers to continuous output as opposed to Peak measurement which is the wattage of an amplifier measured in a short burst.
Solid State amplifiers:
- A solid state amplifier requires less energy to power than an equivalent tube amplifier.
- Solid state circuitry needs minimum maintenance and there are no tubes to replace.
- Solid state amplifiers are more robust than tube amplifiers.
- Solid state amplifiers are available in an affordable price range.
- A solid state amplifier requires no warm-up.
All amplifiers have a preamp stage which boosts the signal from the guitar before it is sent to the power amplifier stage. Hybrid amplifiers are designed to utilize both tube and solid state technology and are available in the following configurations:
- tube preamp coupled with a solid state power amplifier
- solid state preamp coupled with a tube power amplifier
A tube preamp coupled with a solid state power amplifier outputs a tube tone with a fast attack. A solid state preamp coupled with a tube output provides solid state high gain with the warmth of tubes.
Design variations may include digital modeling, integrated tube/solid state stages or further additional tube or solid state stages in the signal path:
- The Vox Valvetronix signal path starts with solid state effects and preamp which sends the signal to a tube/solid state integrated output stage.
- The Line 6 Spider Valve signal path starts with digital tone processing followed by a tube preamp which sends the signal to a tube power amplifier.
- Input - accepts a quarter inch mono jack cable
- Power - off and on
- Volume - adjusts the volume
- Speaker - built-in or separate cabinet
- Gain - the amount of boost applied at the preamp stage
- Tone/Equalizer - treble and bass tone knobs or graphic equalizer
- Headphone socket - headphones can be used for private practice
- Channel selection - switch between clean and high gain
- Additional inputs - high sensitivity input for use with a low-output pickup and low sensitivity input for use with a high-output pickup
- Modeling - digital emulations of popular amplifiers and effects
- Onboard effects - built in proprietary effects such as chorus, delay and echo
- Effect loop - external effects can be plugged into the amplifier
- Line in - the audio signal bypasses the preamp stage and is sent directly to the power amplifier
- Line out - the output from the amplifier can be sent to another power amplifier or mixing desk
- Speaker out - standard on a separate amplifier and when found on a combo amplifier allows a different speaker to be used
- Foot-switch plug - an external foot-switch can be plugged in
- Impedance switch (on tube amplifiers only) - change the resistance, measured in ohms, of the amplifier to match speaker impedance
- Standby switch (on tube amplifiers only) - the standby switch has exactly the same function as the standby mode of a computer which removes the need to cold boot when taking a short break. Any technology that requires a time period to reach optimum working state benefits from this idea. Components are powered down while remaining in a ready state which saves energy and extends their operating life.
The wattage rating is the maximum volume that an amplifier is designed to output. An amplifier in the 30 to 50 watt range is loud enough for band use and home practice and is a common first purchase. If you are intending to buy a 100 watt amplifier you will need to consider using a rehearsal studio for practice. The list below outlines possible uses based on the amplifier's wattage rating.
Solid state amplifiers:
- 10-30W: home practice
- 30-50W: band practice, small club
- 50-100W: large venue
- 10-20W clean: home and band practice
- 10-20W overdriven: home and band practice
- 20-30W: band practice, small to medium club
- 30-50W: small to medium club
- 50-100W: extremely loud in confined spaces though diffuse in large halls
The function of a speaker is to convert an electrical signal into sound waves. This is achieved using an electromagnet called the voice coil which is attached to the speaker cone by a spring called the spider. The vibrations from the voice coil are transferred via the spider to the speaker cone.
A speaker cabinet will house either a single speaker or multiples. A two speaker configuration may utilize smaller speakers than a single speaker model e.g. 2x10″ instead of 1x12″. The main benefit of having multiple speakers is an increase in volume and bass response without sacrificing the higher frequencies. By having more speaker cones the speakers will move more air. For example, two 10″ speakers have a combined surface area of 157 sq.in. while one 12″ speaker has a surface area of 113 sq.in. A 4x10″ cab is often used for large combo amplifiers as it provides most of the bass response you would get from a 1x15″, but retains the high frequency that the 1x15″ cannot produce. Also, it will have increased power-handling capability, or more precisely, they split the amplifier output. Thus, given the same amplification head, a two speaker configuration will have a louder volume but only half the power to each speaker.
All else being equal, generally a low power speaker is louder at the same power as a high power speaker. This is known as speaker efficiency or sensitivity. A 25 watt speaker with a 10 watt amplifier will generally be louder than a 100 watt speaker on the same 10 watt amplifier. Thus, a multi speaker cab will allow the use of low power speakers with a high power amplifier.
To avoid damaging speakers it is commonly recommended that the speaker wattage should exceed the amplifier wattage. Damaging transient peaks or spikes that are the result of an amplifier outputting more wattage than the stated rating are negated by the higher headroom. It is not unusual to find a 150 watt speaker matched to a 100 watt amplifier or a 75 watt speaker matched to a 50 watt amplifier. Multiple speakers achieve the same effect by sharing the amplifier load between speakers which allows low wattage speakers to be used with a high wattage amplifier. If you are buying a combo amplifier the issue does not arise as the manufacturer has ensured that the amplifier and speaker are matched.
The speaker-out socket of an amplifier will have an impedance rating. You should only plug in speakers with the same impedance rating. Some amplifiers are equipped with a switch or dial which allows the impedance to be set at 4 ohm, 8 ohm or 16 ohm. Plugging in a speaker with a different impedance rating may cause damage to the amplifier.
Tube amplifiers are much more sensitive to speaker impedance. Any mismatch between the speaker impedance and the impedance set on the amplifier will cause a strain on the tubes and transformer. Never turn on a tube amplifier with no speakers connected. This might cause severe damage to the output transformer. Always turn off your tube amplifier before disconnecting the speaker. Some amplifiers have shorting jacks (e.g. Hiwatts), these may allow you to change speakers on the fly, but always at the amplifier side of the cable, never at the speaker side.
Types of Unit
Micro amplifiers do not exceed 10 watts. These amplifiers are known for their small size and portability. While some may have built-in speakers they are not suitable for band practice. Because micro amplifiers are low wattage solid state amplifiers that do not utilize FET circuitry they tend to distort very quickly. The Danelectro Honeytone and Vox amPlug respectively illustrates the differences between a micro amplifier and a headphone amplifier. The Danelectro Honeytone has a speaker and pots and resembles a miniature amplifier. The matchbox shape Vox amPlug is a headphone amplifier with rotary dials instead of pots and offers four model emulations including the Vox AC30.
These small portable battery powered amplifiers are designed for outdoor use where no mains power is available. The battery will normally provide up to six to ten hours use on one charge though buying a spare battery or ensuring that the amplifier can also be used with AC power will offset this limitation. Examples include the Pignose Hog 30 which has an 8″ speaker and a rating of 30 watts and the Vox DA5 which has a 6.5″ speaker and a rating of 5 watts.
Practice amplifiers are designed to be used at home and are not suitable for concerts or band rehearsals. The Vox DA5 and Epiphone Valve Junior are small practice amplifiers designed for home use and are easily transported and stored.
Small Venue amplifiers
Combo amplifiers are suitable for small venues. The standard combo consists of a 50 watt amplifier combined with one 12″ speaker with both components enclosed in a single cabinet. Some manufactures combine a 30 or 40 watt amplifier with two or four speakers. A tube or solid state amplifier with a minimum of 30 watts and good tone would be acceptable for professional use. Combo amplifiers are favored by many guitarists because of their compact form and matched amplifier and speakers. The Vox AC30 and Fender Twin are examples of combo amplifiers.
Large Venue amplifiers
Amplifiers that range from 50 to 100 watts are suitable for a large venue. A half stack consists of a separate amplifier head connected to one 4x12″ speaker cabinet and is a very common guitar rig. A full stack consists of a separate amplifier head with two speaker outs and two 4x12″ cabinets stacked vertically. A Marshall head and full-size cabinet are bulky and heavy items to transport. A full-sized Marshall cabinet has two handles, one on either side, and requires two people to lift and move it safely. Storage and transportation must be considered when buying a large guitar rig.
Heads, Cabinets, and Stacks
A half-stack consists of two components:
- head - amplifier
- cabinet - speakers
When purchasing the two components check that the impedance of the cabinet matches the impedance of the head. Make sure the cabs RMS rating is about the same as the head's power output at the impedance of the cab. Generally a single cabinet would have 4x12″ speakers though 1x15″, 2x12″ and 4x10″ cabinets are also available.