S
stevieb
Just another guy, really.
Found it in Ampeg's J-20 User's Guide:
http://www.ampeg.com/pdf/J20_OM.PDF
I've cut and pasted it here, there is no copyright stamp on the manual, and I would think Ampeg would appreciate the plug. If you want to print out your own copy, you would do better to go to the link, rather than printing out this post- formatting is much better.
Important Information about Tubes and Tube Products:
A Brief History Of The Tube:
In 1883, Edison discovered that electrons would flow from a suspended filament when
enclosed in an evacuated lamp. Years later, in 1905, Fleming expanded on Edison’s discovery
and created the “Fleming Valve.” Then, in 1907, Dr. Lee de Forest added a third component
– the grid – to the “Fleming Valve” and the vacuum tube was a fact of life. The door to electronic
amplification was now open.
During World War II, data gleaned from their intensive research on the detectors used in radar
systems, led Bell Telephone Laboratories to the invention of the transistor. This reliable little
device gained quick support as the new component for amplification. The death of the vacuum
tube seemed imminent as designers, scientists, and engineers reveled in the idea of replacing
large, fragile glass tubes with these small, solid-state devices.
However, there were (and still are) many serious listeners who realized that the sound produced
by a “transistor” amplifier is significantly different from that produced by a tube amplifier
with identical design specifications. They considered the sound produced by these new
solid-state devices to be hard, brittle, and lifeless. It was determined that solid-state devices
produced a less musical set of harmonics than tubes. When pushed past their limits, they tend
to mute the tone and emphasize the distortion.
Tubes, on the other hand, produce a more musical set of harmonics, the intensity of which
can be controlled by the player. This characteristic adds warmth and definition to the sound
which has become the hallmark of tube amplifiers. When tubes are driven into clipping, the
harmonic overtones can be both sweet and pleasing or intense and penetrating, depending on
the musician’s musical taste and playing technique.
Over the years, application engineers have designed a number of outstanding solid-state
amplifiers that sound very, very good. Some use special circuitry which enables them to simulate
the distortion characteristics of a tube amplifier. However, the tube amplifier, still held in
the highest esteem by many musicians, offers a classic “vintage” sound in a contemporary
market.
Tube Types And Usage:
Tube amplifiers are based primarily on two types of tubes – preamplifier tubes and power
tubes. The tubes used in preamplifiers (12AX7, 12AU7, 12AT7, etc.) are smaller than the
power tubes. These tubes amplify the signal from your instrument and shape the sound. They
are inherently microphonic (they can mechanically pick up and transmit external noises). Since
these tubes are used in the critical first stages of a tube amplifier’s circuitry, it is very important
to use high-quality, low noise/low microphonic tubes for this application. Although tubes
of this quality may be difficult to find and typically cost more than “off-the-shelf” tubes, the
improvement in performance is worth the investment.
Preamplifier tubes are also used to drive the power tubes. When used in this application, a
12AX7 will produce a more distorted tone than a 12AT7, which produces a clearer, sweeter
sound. A 12AU7 is even cleaner and brighter than a 12AT7, giving more definition to the
sound. (In some cases it is possible to change the sound by changing the type of preamp
and/or driver tubes. When making any modification to your equipment, it is highly recommended
that you consult with a qualified service center.)
Important Information about Tubes and Tube Products:
J-20 Jet All-Tube Guitar Amplifier with Tremolo
7
The power tubes are the largest tubes used in an amplifier. These tubes convert the low-level,
conditioned signal from the preamplifier into a level that is sufficient to drive the speakers.
There are several types of power tubes available, each of which offers a different performance/
sound characteristic. For example, the EL34 power tube produces a great classic rock
sound. When an EL34 is driven into distortion, it produces a unique sound (“crunch”). When
compared to the 6L6, the EL34 distorts more quickly, exhibits a “looser” low-end response
and produces more harmonics at mid and high frequencies (“creamier” sound). These differences
become more noticeable at higher volumes.
The EL84 is similar to the EL34 but produces less output power. It can be easily driven into
distortion and is characterized by a smooth, sweet tone with excellent touch sensitivity.
6L6 tubes produce a big low-end thump and have a very good dynamic range. They offer a
more traditional “American Rock” sound. The 6V6 tubes produce a creamy sound with nice
distortion. On the other hand, the KT88 produces a big low-end but sounds more like an EL34
in the mid and high frequencies.
6550 power tubes are more rugged and stay cleaner-sounding even at full power. When they
do distort, the sound produced is more solid and has a tighter low end; more of a “heavy
metal” type distortion with lots of power.
Some tubes are available in matched sets. These tubes have been extensively tested for optimum
performance and longevity.
The Nature Of Tubes — Why (And When) To Replace Them:
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a
glass envelope or bubble. The tube’s longevity is based on a number of factors which include
how hard and often the amplifier is played, vibration from the speakers, road travel, repeated
set up and tear down, etc.
Any time you notice a change in your amplifier’s performance, check the tubes first.
If it’s been a while since the tubes were replaced and the sound from your amplifier lacks
punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes
probably need to be replaced. If your amplifier squeals, makes noise, loses gain, starts to
hum, lacks “sensitivity”, or feels as if it is working against you, the preamplifier tubes may
need to be replaced.
The power tubes are subjected to considerably more stress than the preamplifier tubes.
Consequently, they almost always fail/degrade first. If deteriorating power tubes aren’t
replaced they will ultimately fail. Depending on the failure mode, they may even cause severe
damage to the audio output transformer and/or other components in the amplifier. Replacing
the tubes before they fail completely has the potential to save you time, money and unwanted
trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more
than one) be replaced by a matched set. If you’re on the road a lot, we recommend that you
carry a spare matched set of replacement power tubes and their associated driver tubes.
After turning off the power and disconnecting the amplifier from the power source, carefully
check the tubes (in bright light) for cracks or white spots inside the glass or any other apparent
damage. Then, with the power on, view the tubes in a dark room. Look for preamplifier
tubes that do not glow at all or power tubes that glow excessively red.
Important Information About Tubes and Tube Products (continued):
J-20 Jet All-Tube Guitar Amplifier with Tremolo
8
Whenever you replace the power tube(s):
• Always have the amplifier’s bias voltage checked by a qualified service center. Improper
bias voltage will cause degradation in performance and possibly damage the tubes and/or
the amplifier. (See “The Importance of Proper Biasing”, below for more information).
• We highly recommend that you replace the driver tube(s) as well. The driver tube determines
the shape and amplitude of the signal applied to the power tube(s) and has to work
almost as hard as the power tube(s).
You can check your preamplifier tubes for microphonics by turning the amplifier on, turning up
the gain and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite).
You will be able to hear the tapping through your speakers, which is normal. It is not normal
for a tube to ring like a bell after it’s tapped. If it does ring then it’s microphonic and should be
replaced. Remember to use only high quality, low microphonic tubes in the preamplifier section.
Even though power tubes are rarely microphonic, you should check them anyway. The power
tubes can be checked for microphonics just like pre-amp tubes.
In the case of very high gain amps, you may be able to reduce the amount of noise generated
by simply swapping the preamp tubes around.
The Importance Of Proper Biasing:
For the best performance and longest tube life, proper biasing is imperative. Bias is the negative
voltage which is applied to the power tube’s control grid to set the level of idle current.
We cannot over emphasize the difference in warmth of tone and dynamic response that come
with proper biasing. If the bias is set too high (over biased), the sound from the amp will be
distorted at all levels. If the bias is set too low, (under biased) the power tubes will run hot (the
plates inside the tubes may glow red due to excessive heat) and the sound from the amplifier
will lack power and punch. The excessive heat greatly reduces tube life – from a few days to
as little as a few hours in extreme cases. Setting the bias on your amp is like setting the idle
on your car. If it’s too high or hot it’s running away with you and if it’s too low or cold it will
choke when you step on it.
The bias is adjusted at the factory in accordance with the type of power tube(s) installed in
your amplifier. It is important to point out that tubes of the same type and specification typically
exhibit different performance characteristics. Consequently, whenever power tubes are
replaced, the bias voltage must be checked (unless the amplifier is equipped with “self-biasing”
circuitry) and readjusted to accommodate the operating parameters of the replacement tubes.
Depending on the model and amplifier type, there may be hum balance controls, trim pots, or
bias adjustment controls on its rear panel. However, the bias adjustment should be performed
only by qualified service personnel with the proper, calibrated test equipment.
Important Information About Tubes and Tube Products (continued):
J-20 Jet All-Tube Guitar Amplifier with Tremolo
9
Important Information About Tubes and Tube Products (continued):
Survival Tips For Tube Amplifiers:
To prolong tube life, observe these tips and recommendations:
• Match the impedance of your speaker cabinet(s) to your amplifier. Improper impedance
matching will contribute to early tube degradation and may cause premature tube failure.
• Make sure the speaker(s) are properly connected prior to turning on the amplifier.
• After playing the amplifier, allow sufficient time for it to properly cool down prior to moving
it. A properly cooled amplifier prolongs tube life due to the internal components being less
susceptible to the damage caused by vibration.
• Allow the amplifier to warm up to room temperature before turning it on. The heat generated
by the tube elements can crack a cold glass housing.
• Replace the output tube(s) before the performance degrades or the tubes fail completely.
Replace the tube(s) on a regular basis (at least once per year or as often as every 4 to 6
months if you play long and hard every day).
• Always have the bias checked after replacing the output tubes (unless the amplifier is
equipped with “selfbiasing circuitry”). This should be done ONLY at a qualified service
center. Improper biasing could result in the tubes running too hot, which greatly reduces
the life of the tubes – or too cold, which results in distorted sound regardless of level
settings. Do not play the amplifier if it exhibits these symptoms – get the bias checked/
adjusted immediately to prevent tube failure and/or other damage.
• If the locating notch on the base of a power tube breaks off, replace the tube. This significantly
reduces the risk of damaging your amplifier by incorrectly inserting the tube.
• Protect the amplifier from dust and moisture. If liquid gets into the amplifier proper, or
if the amplifier is dropped or otherwise mechanically abused, have it checked out at an
authorized service center before using it.
• Proper maintenance and cleaning in combination with routine checkups by your authorized
service center will insure the best performance and longest life from your amplifier.
CAUTION: Tube replacement should be performed only by qualified service personnel
who are familiar with the dangers of hazardous voltages that are typically present in tube circuitry.
http://www.ampeg.com/pdf/J20_OM.PDF
I've cut and pasted it here, there is no copyright stamp on the manual, and I would think Ampeg would appreciate the plug. If you want to print out your own copy, you would do better to go to the link, rather than printing out this post- formatting is much better.
Important Information about Tubes and Tube Products:
A Brief History Of The Tube:
In 1883, Edison discovered that electrons would flow from a suspended filament when
enclosed in an evacuated lamp. Years later, in 1905, Fleming expanded on Edison’s discovery
and created the “Fleming Valve.” Then, in 1907, Dr. Lee de Forest added a third component
– the grid – to the “Fleming Valve” and the vacuum tube was a fact of life. The door to electronic
amplification was now open.
During World War II, data gleaned from their intensive research on the detectors used in radar
systems, led Bell Telephone Laboratories to the invention of the transistor. This reliable little
device gained quick support as the new component for amplification. The death of the vacuum
tube seemed imminent as designers, scientists, and engineers reveled in the idea of replacing
large, fragile glass tubes with these small, solid-state devices.
However, there were (and still are) many serious listeners who realized that the sound produced
by a “transistor” amplifier is significantly different from that produced by a tube amplifier
with identical design specifications. They considered the sound produced by these new
solid-state devices to be hard, brittle, and lifeless. It was determined that solid-state devices
produced a less musical set of harmonics than tubes. When pushed past their limits, they tend
to mute the tone and emphasize the distortion.
Tubes, on the other hand, produce a more musical set of harmonics, the intensity of which
can be controlled by the player. This characteristic adds warmth and definition to the sound
which has become the hallmark of tube amplifiers. When tubes are driven into clipping, the
harmonic overtones can be both sweet and pleasing or intense and penetrating, depending on
the musician’s musical taste and playing technique.
Over the years, application engineers have designed a number of outstanding solid-state
amplifiers that sound very, very good. Some use special circuitry which enables them to simulate
the distortion characteristics of a tube amplifier. However, the tube amplifier, still held in
the highest esteem by many musicians, offers a classic “vintage” sound in a contemporary
market.
Tube Types And Usage:
Tube amplifiers are based primarily on two types of tubes – preamplifier tubes and power
tubes. The tubes used in preamplifiers (12AX7, 12AU7, 12AT7, etc.) are smaller than the
power tubes. These tubes amplify the signal from your instrument and shape the sound. They
are inherently microphonic (they can mechanically pick up and transmit external noises). Since
these tubes are used in the critical first stages of a tube amplifier’s circuitry, it is very important
to use high-quality, low noise/low microphonic tubes for this application. Although tubes
of this quality may be difficult to find and typically cost more than “off-the-shelf” tubes, the
improvement in performance is worth the investment.
Preamplifier tubes are also used to drive the power tubes. When used in this application, a
12AX7 will produce a more distorted tone than a 12AT7, which produces a clearer, sweeter
sound. A 12AU7 is even cleaner and brighter than a 12AT7, giving more definition to the
sound. (In some cases it is possible to change the sound by changing the type of preamp
and/or driver tubes. When making any modification to your equipment, it is highly recommended
that you consult with a qualified service center.)
Important Information about Tubes and Tube Products:
J-20 Jet All-Tube Guitar Amplifier with Tremolo
7
The power tubes are the largest tubes used in an amplifier. These tubes convert the low-level,
conditioned signal from the preamplifier into a level that is sufficient to drive the speakers.
There are several types of power tubes available, each of which offers a different performance/
sound characteristic. For example, the EL34 power tube produces a great classic rock
sound. When an EL34 is driven into distortion, it produces a unique sound (“crunch”). When
compared to the 6L6, the EL34 distorts more quickly, exhibits a “looser” low-end response
and produces more harmonics at mid and high frequencies (“creamier” sound). These differences
become more noticeable at higher volumes.
The EL84 is similar to the EL34 but produces less output power. It can be easily driven into
distortion and is characterized by a smooth, sweet tone with excellent touch sensitivity.
6L6 tubes produce a big low-end thump and have a very good dynamic range. They offer a
more traditional “American Rock” sound. The 6V6 tubes produce a creamy sound with nice
distortion. On the other hand, the KT88 produces a big low-end but sounds more like an EL34
in the mid and high frequencies.
6550 power tubes are more rugged and stay cleaner-sounding even at full power. When they
do distort, the sound produced is more solid and has a tighter low end; more of a “heavy
metal” type distortion with lots of power.
Some tubes are available in matched sets. These tubes have been extensively tested for optimum
performance and longevity.
The Nature Of Tubes — Why (And When) To Replace Them:
Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a
glass envelope or bubble. The tube’s longevity is based on a number of factors which include
how hard and often the amplifier is played, vibration from the speakers, road travel, repeated
set up and tear down, etc.
Any time you notice a change in your amplifier’s performance, check the tubes first.
If it’s been a while since the tubes were replaced and the sound from your amplifier lacks
punch, fades in and out, loses highs or lows or produces unusual sounds, the power tubes
probably need to be replaced. If your amplifier squeals, makes noise, loses gain, starts to
hum, lacks “sensitivity”, or feels as if it is working against you, the preamplifier tubes may
need to be replaced.
The power tubes are subjected to considerably more stress than the preamplifier tubes.
Consequently, they almost always fail/degrade first. If deteriorating power tubes aren’t
replaced they will ultimately fail. Depending on the failure mode, they may even cause severe
damage to the audio output transformer and/or other components in the amplifier. Replacing
the tubes before they fail completely has the potential to save you time, money and unwanted
trouble. Since power tubes work together in an amplifier, it is crucial that they (if there is more
than one) be replaced by a matched set. If you’re on the road a lot, we recommend that you
carry a spare matched set of replacement power tubes and their associated driver tubes.
After turning off the power and disconnecting the amplifier from the power source, carefully
check the tubes (in bright light) for cracks or white spots inside the glass or any other apparent
damage. Then, with the power on, view the tubes in a dark room. Look for preamplifier
tubes that do not glow at all or power tubes that glow excessively red.
Important Information About Tubes and Tube Products (continued):
J-20 Jet All-Tube Guitar Amplifier with Tremolo
8
Whenever you replace the power tube(s):
• Always have the amplifier’s bias voltage checked by a qualified service center. Improper
bias voltage will cause degradation in performance and possibly damage the tubes and/or
the amplifier. (See “The Importance of Proper Biasing”, below for more information).
• We highly recommend that you replace the driver tube(s) as well. The driver tube determines
the shape and amplitude of the signal applied to the power tube(s) and has to work
almost as hard as the power tube(s).
You can check your preamplifier tubes for microphonics by turning the amplifier on, turning up
the gain and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite).
You will be able to hear the tapping through your speakers, which is normal. It is not normal
for a tube to ring like a bell after it’s tapped. If it does ring then it’s microphonic and should be
replaced. Remember to use only high quality, low microphonic tubes in the preamplifier section.
Even though power tubes are rarely microphonic, you should check them anyway. The power
tubes can be checked for microphonics just like pre-amp tubes.
In the case of very high gain amps, you may be able to reduce the amount of noise generated
by simply swapping the preamp tubes around.
The Importance Of Proper Biasing:
For the best performance and longest tube life, proper biasing is imperative. Bias is the negative
voltage which is applied to the power tube’s control grid to set the level of idle current.
We cannot over emphasize the difference in warmth of tone and dynamic response that come
with proper biasing. If the bias is set too high (over biased), the sound from the amp will be
distorted at all levels. If the bias is set too low, (under biased) the power tubes will run hot (the
plates inside the tubes may glow red due to excessive heat) and the sound from the amplifier
will lack power and punch. The excessive heat greatly reduces tube life – from a few days to
as little as a few hours in extreme cases. Setting the bias on your amp is like setting the idle
on your car. If it’s too high or hot it’s running away with you and if it’s too low or cold it will
choke when you step on it.
The bias is adjusted at the factory in accordance with the type of power tube(s) installed in
your amplifier. It is important to point out that tubes of the same type and specification typically
exhibit different performance characteristics. Consequently, whenever power tubes are
replaced, the bias voltage must be checked (unless the amplifier is equipped with “self-biasing”
circuitry) and readjusted to accommodate the operating parameters of the replacement tubes.
Depending on the model and amplifier type, there may be hum balance controls, trim pots, or
bias adjustment controls on its rear panel. However, the bias adjustment should be performed
only by qualified service personnel with the proper, calibrated test equipment.
Important Information About Tubes and Tube Products (continued):
J-20 Jet All-Tube Guitar Amplifier with Tremolo
9
Important Information About Tubes and Tube Products (continued):
Survival Tips For Tube Amplifiers:
To prolong tube life, observe these tips and recommendations:
• Match the impedance of your speaker cabinet(s) to your amplifier. Improper impedance
matching will contribute to early tube degradation and may cause premature tube failure.
• Make sure the speaker(s) are properly connected prior to turning on the amplifier.
• After playing the amplifier, allow sufficient time for it to properly cool down prior to moving
it. A properly cooled amplifier prolongs tube life due to the internal components being less
susceptible to the damage caused by vibration.
• Allow the amplifier to warm up to room temperature before turning it on. The heat generated
by the tube elements can crack a cold glass housing.
• Replace the output tube(s) before the performance degrades or the tubes fail completely.
Replace the tube(s) on a regular basis (at least once per year or as often as every 4 to 6
months if you play long and hard every day).
• Always have the bias checked after replacing the output tubes (unless the amplifier is
equipped with “selfbiasing circuitry”). This should be done ONLY at a qualified service
center. Improper biasing could result in the tubes running too hot, which greatly reduces
the life of the tubes – or too cold, which results in distorted sound regardless of level
settings. Do not play the amplifier if it exhibits these symptoms – get the bias checked/
adjusted immediately to prevent tube failure and/or other damage.
• If the locating notch on the base of a power tube breaks off, replace the tube. This significantly
reduces the risk of damaging your amplifier by incorrectly inserting the tube.
• Protect the amplifier from dust and moisture. If liquid gets into the amplifier proper, or
if the amplifier is dropped or otherwise mechanically abused, have it checked out at an
authorized service center before using it.
• Proper maintenance and cleaning in combination with routine checkups by your authorized
service center will insure the best performance and longest life from your amplifier.
CAUTION: Tube replacement should be performed only by qualified service personnel
who are familiar with the dangers of hazardous voltages that are typically present in tube circuitry.