GUITAR
AMPS PAGE
SECTION 3: THE LIVE ELECTRIC
GUITAR REPRODUCTION AND AMPLIFICATION SYSTEM
Note: Links are shown in blue text and underlined
– select and click to open
WHY IS IT A “SYSTEM”?
Electronically
reproducing sound produced or created by a guitar requires a system, comprising a series of discrete
processes connected together in sequential or serial cascade – i.e. the
output from each stage in the process is the input to the next stage in the
process.
What
we actually hear is the end of the process – i.e. acoustic sound waves
generated by the loudspeaker. Everything else in between is irrelevant to our
ears because it is in electronic (electrical energy) form that we cannot hear.
So
it is important to understand what the system is and how it works.
SYSTEM STAGE 1: THE ELECTRIC GUITAR
The
electric guitar was invented nearly a century ago and has been constantly
evolving as the most popular musical instrument in the world.
A
comprehensive description of the instrument is at https://en.wikipedia.org/wiki/Electric_guitar
Guitar
History: https://www.youtube.com/watch?v=AjK4GVR1EcE
For
the purpose of explaining the evolution of the modern guitar see the history of
Martin Guitars, the oldest manufacturer in the USA.
Part
1 https://www.youtube.com/watch?v=Jv8WrYAMyvo
Part
2: https://www.youtube.com/watch?v=x2SSiZCM1LQ
The
guitar is an extension of the player’s mind and body and becomes a part of the
player as his/her means of musical expression.
Consequently
the choice of instrument is extremely personal and must be made with great care
– unless you want to try as many different versions of the instrument as you
can before settling on your instrument of choice.
Differences
between instruments are many and varied so it is wise to explore and research
before committing to purchase.
The
“sound” of an electric guitar is just one of the essential attributes.
Factors
such as body and neck timber species and characteristics, hollow body,
semi-hollow body, solid body, shape of body, size, weight, balance, neck
length, neck straightness and absence of warp or twist, number of frets,
fretboard width, string height above the pickups and neck, number of pickups,
type/construction of pickups, positioning of pickups, on-board controls,
position of output phono socket etc. are all important.
One
of the most important factors is the colour and finish.
ELECTRIC
GUITAR MANUFACTURE
Martin
USA Factory Tour https://www.youtube.com/watch?v=WES8A918wQc
Fender
USA Factory Tour https://www.youtube.com/watch?v=wbdodxel3jw
Fender
Custom Shop Factory Tour https://www.youtube.com/watch?v=odH_lc1QtTo
Gibson
USA Factory Tour https://www.youtube.com/watch?v=1zXJtkDstH0
A
good quality guitar will sound alive and vibrant - natural resonance adds colour and depth to
the sound and aids sustain. The sound should be pleasant and melodic. The sound
should not be harsh – unless you prefer it that way.
The
natural volume should be even across the neck and chords should have the same
tone across the arrange of notes.
To
set the stage, the My
Twangy Guitar website provides examples of around 225 different electric
guitar sounds from a range of guitars that includes solid body, semi-hollow
body and hollow-body designs in different makes and models.
THE PLAYING METHOD
To
coincide with the style of music being performed, the guitar may be played with
the fingers, finger-nails, thumb, finger picks, thumb
pick, a pick
or a plectrum, or a slide (steel guitar and Hawaiian
guitar) – and even drumsticks.
(6 min 17 secs in)
The dynamic range of the sound produced responds
to the amplitude of the string vibration, so the harder the string is plucked
the louder the sound and the higher the voltage output produced by the pickup.
This
is reflected in the playing style, which can range from be delicate, light,
moderate, medium, heavy, extra heavy and aggressive.
Pick
or Plectrum – what is the difference ? https://www.guitarsite.com/plectrum/
SYSTEM STAGE 2: STRINGS
Acoustic
Guitar Strings can be gut, nylon or steel.
Gut
strings: https://www.youtube.com/watch?v=ZbTTKMq5sno
and https://www.youtube.com/watch?v=kjVU-SRBMHA
Nylon
strings: https://www.youtube.com/watch?v=2IYlYZ5e5O4
and https://www.youtube.com/watch?v=JL4w3ixUDdw
Steel
strings: https://www.youtube.com/watch?v=5jSfnG8oOJI
and https://www.youtube.com/watch?v=pAHGQkeQfO0
Electric
guitar strings can be steel or stainless steel and come in various gauges,
namely light, medium and heavy.
Electric
guitar steel or stainless steel strings may be round or flat-wound.
Electric
guitar steel or stainless steel strings may be nylon wrapped or nylon dipped.
The
number of strings may be four or five for bass guitar, or six, seven, eight or
twelve for acoustic, lead or rhythm guitar.
Strings
are normally tuned to Concert Pitch – A440, but sometimes guitarists will raise
or lower the pitch for special effect.
Strings
should be grounded to the on-board electronics negative terminal.
SYSTEM STAGE 3: THE GUITAR PICKUP
The
first step is the pickup(s) located on or in the guitar.
The
pickup device is usually an electro-magnetic
device.
For
a more natural sound a piezo pickup or even a standard microphone may be used -
see Chet Atkins Medley
The
pickup generates an alternating current voltage that is supplied to the
amplifier or preamplifier via a flexible shielded cable.
Many
guitars provide more than one pickup to enable the sound tone to be varied. In
such cases selector switches and a simple mixing/blending circuit are provided
on-board the guitar.
Switching
enables series or parallel connection of pickups.
Usually
a simple single knob tone control is installed, providing a treble cut effect.
More
exotic designs include a battery powered solid state preamplifier, called an
“active” pickup.
Most
commercial amplifiers provide inputs for normal and active pickup feeds as
standard to accommodate high or low output from the guitar or preamp.
Pickups
are made in a wide range of designs and sound characteristics ranging from
bright, clean and accurate to dirty or muddy – see https://en.wikipedia.org/wiki/Pickup_(music_technology)
Their
position on the guitar body has a significant relationship with the output
voltage and the tone. The Bridge pickup delivers a harsh bright tone whereas
the Neck pickup delivers a mellow smooth sound. Other pickups may be installed
in between.
Some
pickups are mounted at right angles to the longitudinal axis of the strings
whereas others are set at an oblique angle.
The
physical distance between the pickup and strings is critical and should be
adjusted for best performance.
A
common configuration is in the popular and much copied Fender Stratocaster
guitar, that offers three pickups and a selector switch to provide a choice of
five tonal settings ranging from bright to mellow.
Multiple
pickups may be series or parallel connected.
One
problem that plagues guitarists is extraneous hum and noise pickup from
unshielded on-board electronics, including the pickup. Various pickup designs
have been invented in an attempt to minimise this irritating disturbance. The “Humbucker” pickup is one
form of anti-hum device.
Better
quality guitars use metal shielding around the electronics to prevent stray hum
and noise pickup. The shielding is sometimes simply a piece of aluminium foil
shaped to suit the guitar body cutout and ground by direct connection to the
phono socket ground terminal either by physical clamping or a wire. Shielding
may be retro-fitted.
By
definition, changing a pickup will change the sound.
Fender
provide a guide to selecting a pickup at their Stratocaster
catalogue page.
SYSTEM STAGE 4: CONNECTING CABLE
The
cable connecting the guitar to the amplifier must be shielded and grounded to
the amplifier chassis to prevent hum and noise pickup.
Nowadays
commercially manufactured high-quality cables are readily available at
affordable cost.
Many
styles and variants of cables are offered in the musical instrument market,
each supported by performance claims and various price points. Try before you
buy.
To
prevent hum pickup ensure the phono plugs have a metal shield around the cable
where it terminates to the plug. (Plastic sleeved plugs are OK for speaker
connectors)
It
is preferable for the cable to be sheathed with a tapered rubber sleeve at each
end to mechanically support the cable where it enters the phono plug connector
sleeve. This device helps prevent the cable fracturing mid-performance.
Some
cables are microphonic, which means they respond to unwanted impact noise. If
you have such a cable replace it.
It
is important to use densely shielded cable to prevent stray hum and noise
pickup from external sources.
An
alternative to using a connecting cable is a wireless
connection. This system requires a small short-range wireless
transmitter to be plugged into the phono socket installed in the guitar body –
or a short shielded cable installed between the guitar phono socket and a small
back transmitter unit.
This
method is popular because it offers freedom to the guitarist to move around a
stage while playing. But because it is wireless operated there will be some
change to the tone – comparable to the sound from FM radio.
In
both cases the receiver unit is mounted on or near the amplifier and connected
to the amplifier via another shielded cable. The receiver unit requires a
separate power supply.
SYSTEM STAGE 5: THE PREAMPLIFIER – STAND
ALONE OR ON-BOARD BUILT INTO THE AMPLIFIER
The
output voltage from a guitar is quite low – typically 100 to 300
millivolts. Similar to a dynamic
microphone or a vynil record player magnetic cartridge, but significantly lower
than an AM/FM/DAB tuner or a CD/DVD/Blue Ray Player.
The
core role of a preamplifier is to increase the signal voltage from the guitar
to that needed to fully drive the first stage of the tube Power Amplifier
section to maximum amplifier power output – usually set to between 0.7 and 1
volt.
That
means the minimum voltage gain for a pre-amp needs to be up to ten times its
input voltage – i.e. a gain of ten times. Multi-input preamps incur insertion
losses in the mixing circuits so the overall gain needs to be sufficient to
offset any losses.
Obviously
the output voltage from the guitar pickup(s) will be proportional to string
amplitude when picked or plucked, so a gain/volume control is necessary to
cater for different pickup performances and different playing styles. The
pre-amp gain control is usually set to provide maximum amplifier power when the
guitar volume control is set to maximum, to allow the guitarist to adjust
volume without having to touch the amplifier/preamp.
The
second function of a preamplifier is to provide some means of varying the tone
of the sound.
This
is usually achieved by including independent bass, midrange and treble Tone
Controls.
Bass,
treble and midrange tone controls add further insertion losses and require a
tube voltage amplifier stage to restore gain back to unity. Usually this means
another tube stage.
Standard
tone controls offer + or – 10 to 15db cut/boost at lowest and highest
frequencies of 30 to 50Hz and 10-15kHz.
Some
designs included a “presence” or mid boost control.
An
alternative to an additional stage tone control pre-amplifier is the Graphic Equaliser. They
come with a range of bands and boost/cut frequencies. A tube based Graphic
Equaliser requires a minimum of a single triode stage to provide 10-20 db boost
for each frequency – e.g. five twin triodes for a ten band unit. Consequently
solid state construction is a more practical solution. Solid state EQ’s are
readily available in up to 32 band options in both mono, two channel and stereo
modes. Note however that Graphic Equalisers provide “pyramid” boost/cut
profiles compared to the gentle slope of a standard logarithmic tone control.
Some
designs offer shelving
filter controls.
Some
designs, such as the Behringer
Ultra-Q T1951 tube EQ offer variable frequency and shelving settings for
each band.
The Realistic 12 Band Graphic Equaliser is typical
of the genre.
More
modern models by Phonic are shown at https://www.phonic.com/images/products-II/GEQ3102F/ds-GEQ1502F-GEQ3102F-en.pdf?type=file
EQ Pedals are also
readily available.
Note
that PA (Voice) Class EQ’s usually have a limited frequency range to inhibit
acoustic feedback in live performances. It is recommended that a full range EQ
(40Hz – 16kHz) be used to ensure full harmonics are amplified. This more
important if bass guitar is included in the mix.
Modern
solid state amplifiers often included “pyramid” controls that provide a high amplitude
narrow band boost or cut facility – similar to a multi-band graphic equalizer -
however this is usually not practicable with tube preamps because it means more
stages and more hum and noise.
The
primary disadvantage of on-board built-in pre-amplification effects and tone
controls is that the player cannot adjust them whilst playing. Pedals offer
also the advantage of being able to be easily replaced to obtain a different
tones or effects.
SYSTEM STAGE 6: OPTIONAL REVERB/ECHO
EFFECTS
Reverb
“Reverb”
was introduced in the 1960’s using a mechanical coil spring reverb unit adapted
from the Hammond Organ. These were widely used for “surf” music and are still
fitted to some OEM amplifiers.
One
of the earliest examples is the track “Baja” by the Chantays and The Ventures and The Astronauts.
Other
means of constructing reverb units include a light sensitive resistor (LDR)
supplied from an oscillating lamp light source that varies the light amplitude.
Another
used multiple delay stages - e.g. the Carvin Quad X hybrid
solid state/tube preamp.
The Spring Reverb unit
remains very popular in OEM amplifiers. It has a mechanical sound to it and is
often characterised by a distinctive
“whip” or ringing sound at the end of each note.
Good
examples are Bulldog and Footpatter and Tourqay and Misirlou – Pulp Fiction
and Yellow Jacket
Spring
Reverbs are very microphonic – i.e. pick up and amplify unwanted external
vibrations when in close proximity to other instruments.
Reverb
is explained further by Fender
Echo
Another
form of echo unit is the endless loop tape
echo units. This form of device uses standard ¼” tape endlessly rotating
over several selectable heads. By head selection and mixed gain a wide range of
delays can be achieved.
One early
example of the technique is from Santo and Johnny in an alternative take of Sleepwalk from 1959. The
playing technique is shown in this live TV recording.
A
popular band based on this technology is the UK band ‘The Shadows”. In this clip both the lead and rhythm guitars
use echo units, then the entire band has studio echo added as well. Other
examples of tape echo Shadows tracks are Apache and Sleepwalk and Blue Star and Geronimo and Man of Mystery and Wonderful Land and Going Home and Atlantis
The Klempt
Echolette was a fine example of a very functional popular tape echo unit.
Another
intro level tube based echo unit is the “Watkins
Copycat”. Was made in the UK.
The
tapes wear quickly so several spares may be needed to get through a full
performance.
For
practical reasons, this type of tape delay preamp is not viable to instal in
the main amplifier unit so it is advisable to use a stand-alone external unit.
Solid
state preamps offer all of this and more in a low cost compact format – but do
not provide the tube sound.
The Carvin Quad X preamp is
a hybrid solid state/tube unit that also offers various echo and reverb
effects.
Of
course studio created echo/delay is legendary, commencing very early in the
1950’s. This commentary explains the really great effect achieved on this
compilation track from Gene
Vincent in 1957 - Guitar by Cliff Gallup. The method
of creating the unique Gene Vincent sound is explained at https://www.soundonsound.com/forum/viewtopic.php?t=46533.
Another
famous pioneer was Sam
Phillips of Sun Studios with his innovative approach to creating studio
echo.
Studio
echo enhances the natural guitar sound and gives it realism. See Chet Atkins - Wheels and medley
SYSTEM STAGE 7: TREMOLO AND VIBRATO
EFFECTS
These
two important effects were fitted as a built-in standard feature on many
mainstream amplifiers from the 1950’s on.
One
of the earliest exponents was Duane Eddy with numerous tracks like Rebel Rouser and Forty Miles of Bad Road
and Ballad of Paladin
“Tremolo” modulates the frequency or amplitude. Created by
deflecting the string sideways or sliding up and down the fretboard. Fender perfected the electronic tremolo by
eliminating undesirable side-effects.
Duane Eddy – Rebel Rouser 1958
Link Wray - Rumble 1958
Tommy James and the
Chondelles - Crimson and
Clover 1968 (Note the tremolo on voice towards the end of the track)
Jodie Reynolds – Endless Sleep 1958
Jodie Reynolds – Fire of Love – 1958
The Viscounts – Harlem Nocturne – 1965
The Ventures – Yellow Bird (subtle tremolo with the odd vibrato) and Silver
City and Beyond the
Reef
The Smiths – How Soon Is Now?
“Vibrato” modulates the pitch.
This
live recording of Sleepwalk
on steel slide guitar from 1959 illustrates the technique.
Note: IMHO the original
recording of this track is truly a superb performance – musically and
electronically – one of the best tracks ever recorded. On CD is hard to beat –
just brilliant !!
For those interested
there is a detailed discussion about how they may have achieved the sound at https://bb.steelguitarforum.com/viewtopic.php?t=343343. Seems nobody actually knows.
My guess is that it may
have been directly recorded from the steel guitar with global studio echo added
later using multi-track recording. Sun Records had been doing this for several
years before that. This recording illustrates the importance of the high
amplitude asymmetrical peak signal and its contribution to the final sound.
Compare with the less dynamic sound from a Fender Strat - https://www.youtube.com/watch?v=HQjUZ8wZe8s
or https://www.youtube.com/watch?v=SEtIK2ZC9Pg
or the compressed sound on this
track https://www.youtube.com/watch?v=SS0MuXt7gwg
Mechanical
lever arms were fitted to most major brand models from the 1950’s on. The Grestch Country Gentleman
with Bigsby
lever/whammy bar is a fine
example of this feature.
Chet
Atkins – Dark Eyes
and MyTwangyGuitar – The
House is Rocking and Smoke
Demo
A
more modern design is shown by Jeff Beck in Apache and Sleepwalk
Differences
between these effects are explained at https://www.youtube.com/watch?v=_b8WfoAHILA
and https://www.youtube.com/watch?v=oKfqRGoUBus
and https://www.fender.com/articles/parts-and-accessories/pedal-board-primer-get-to-know-tremolo
For
a more modern example see Whammy Bar Tricks with John Petrucci of Dream Theater
SYSTEM STAGE 8: WAH AND WAH WAH EFFECTS
PEDALS
Early
pedals were based on an early Hammond Organ “swell pedal and were simply a foot
operated volume control installed between the guitar and the amp. This allowed
the player to vary the volume to modulate the signal volume (loudness).
One
of the best examples of this effect was used for the movie “Shaft” in 1971. Isaac Hayes and The Ventures.
The
original Wah pedal is said to have been created by Jim Dunlop in 1966
Later
versions vary the bass and treble tone mix – in other words a foot operated
tone control having the bass and treble controls connected inversely – i.e. max
treble min bass to max bass to min treble.
https://www.youtube.com/watch?v=8FK0z6GUqts
These
effects were introduced as an integrated feature in amplifiers in the 1960’s as
a deliberate method of creating a desirable form of distortion.
Explained
at https://www.youtube.com/watch?v=_ZxZfbkW86M
and https://prosoundhq.com/how-to-get-a-good-crunch-tone-amp-and-guitar-settings/
Modern
amplifiers either do not include these effects or use solid state circuitry to
achieve the result.
SYSTEM STAGE 10: COMPRESSION AND
LIMITING
Compression
and Limiting have been used
in radio broadcasting and recording for nearly a century and by evolution have
crept into the guitar industry to appear as a feature in guitar amplifiers to
produce particular effects.
Compression
and Limiting are widely used in AM/FM/DAB broadcasting to minimise modulation
bandwidth, in vynil recordings to reduce track amplitude and in CD/DVD/Blue Ray
recordings to reduce dynamic range to acceptable levels.
The
aural effect is to dull the sound as a result of attenuation or elimination of
high frequencies needed to create clean tone.
Compression
also increases the average musical signal power so at high volume will drive
the amplifier harder. Proceed at your own risk.
Refer
https://en.wikipedia.org/wiki/Dynamic_range_compression
The
sound of this magnificient track – I Can’t Stand the Rain
featuring Steve Cropper - is ruined by compression and limiting. Harmonics,
dynamic range and presence are subdued. The CD pressing is awesome. Note the
subtle vibrato in the background. Some other great tracks are on this album
too.
SYSTEM STAGE 11: THE AMPLIFIER
The Amplifier is designed and
constructed a system in its own right.
This is explained in Section 7.
Nominal power output ratings generally
increase in 5 Watts bands up to about 30 watts then 10 Watts steps to 100
Watts.
The
Amplifier and its Power Supply are normally constructed as an integrated unit
in guitar amplifiers offering power outputs from about 3-5 Watts rms – e.g. 1 x
6V6GT - to up to about 400 Watts rms – e.g. 8 x KT88 or 6550 tubes.
The
physical size and weight of the amplifier is very dependent on the choice of
power output and size of tubes. If all tubes are 7 or 9 pin miniature types the
amplifier can be in a compact form and deliver up to 20-25 Watts rms output
using 6BQ5, EL84, 6CZ5 or 6973 9 pin tubes. This class of tube is usually
reliable and does not present design issues other than preventing parasitic
oscillation.
Octal
socket tubes range in power output capabilities depending upon B+ voltage and
bottle diameter.
The
EL34/6CA7 pentode can be operated safely and offers a standard bottle for up to
100 Watts rms output at 800 VDC Plate volts, whereas the KT88 is significantly
larger in diameter, requires more chassis space and offers a large diameter
bottle for up to 100 Watts rms output at 600 VDC Plate volts. To avoid issues
with parasitic oscillation and thermal runaway the DC Screen Grid voltage
should always be kept as low as possible – ideally not more than 50% of the
Plate voltage.
This
class of tube is normally very reliable if not pushed, but when driven hard at
excessive voltages trouble is guaranteed.
To
ensure reliability it is not advisable to apply more than about 600 VDC to an
octal socket. Tracking between pins can develop over time.
For
appearance reasons a separate Amplifier head unit should match the same width
as the speaker cabinet it sits on. Cabinets typically come in 2 inch
increments, with 24 inches being a practical maximum for weight, handling,
transport and stage space. A 24 inch wide cabinet will accommodate speaker
drivers up to 18 inches diameter or 4 x 10 inch drivers. A 24 inch wide
Amplifier head will accommodate most standard Amplifier designs.
Beyond
that power level the chassis and transformers become too large and heavy to
handle conveniently so the Amplifier and Power Supply are separated into
discrete units connected together with insulated cables. This form of
construction is extreme and would be rarely seen in live work. The Wolff Insanity Brownout 1000 amplifier is one such
example.
19
inch rack mounting would be a logical way to go for a high-power unit but would
require even more space – one rack case for the Power Supply, one rack case for
the Amplifier and one rack case for the Pre-amplifier and driver tubes.
Other
alternatives using transmitting tubes require very high B+ voltages that become
impracticable for normal use.
In
this modern era, professional in-house PA systems will be all solid-state
because of the higher power to weight/size ratio of solid state amplifiers v
tube amplifiers.
Class
D solid-state amplifiers are now displacing their Class AB solid-state
predecessors for large installations but cannot deliver sound quality
comparable to tubes. This means that much of the effort that goes into
achieving the live tube sound by the guitarist is “lost in the system”.
Unfortunately the general trend to compressed digital sound, small size, low
weight and low cost is pushing tube amps out of reach of the dedicated
guitarist. Such is progress.
SYSTEM STAGE 12: THE LOUDSPEAKER
The
Loudspeaker could be the most important component in the entire system because
the sound it produces is the sound you actually hear.
Consequently,
the choice of loudspeaker must be made with care.
SYSTEM STAGE 13: APPEARANCE
It is an axiom of engineering practice that a
component or complete device must look attractive.
This rule also applies to guitar amplifiers.
No self-respecting guitarist will be seen
on-stage with a PA Amplifier, or PA Mixer Amplifier, or a basic open chassis
amp without a cabinet, as his or her amp choice – even when it will do the job
well. The guitar amplifier must look like a guitar amplifier. However history
shows it is OK to use a Bass amplifier so long as it looks the part.
If you are making the system yourself ensure
the amplifier chassis will aesthetically match the speaker cabinet requirements
to present a professional looking unit.
If it does not look good then it is unlikely
to work well, so abandon it and start again.
THE
FINAL SYSTEM CONFIGURATION
In
contrast, Eric Clapton offers a simpler system https://www.youtube.com/watch?v=ZoAp0xmI8gw
For
a cleaner system sound see https://www.youtube.com/watch?v=YhS8bCs12N8
and https://www.youtube.com/watch?v=Dyi2MKjs-kM
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH
HIGH-VOLTAGE - DEATH IS PERMANENT!!
Contact:
"electron"
Email: contact
This page is located at http://www.oestex.com/tubes/guitaramps/3_system.html
This page last modified 12 July 2023