6
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's 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 amplifica-
tion. 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 driv-
en 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 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 (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 per-
formance is worth the investment.
Preamplifier tubes are also used to drive the power tubes. When used in this appli-
cation, 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, giv-
ing 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 modifica-
tion to your equipment, it is highly recommended that you consult with a qualified
service center.)
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 dis-
tortion 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 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.
The 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.
Tubes are made up of a number of fragile mechanical components that are vacu-
um-sealed in a glass envelope or bubble. The tube's longevity is based on a num-
ber 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 amplifi-
er 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 com-
ponents 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.
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 section below entitled, "The Importance of
Proper Biasing", for more information on this subject).
• 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 speak-
ers, 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.
IMPORTANT INFORMATION ABOUT TUBES AND TUBE PRODUCTS:
A BRIEF HISTORY OF THE TUBE:
TUBE TYPES AND USAGE:
THE NATURE OF TUBES - WHY (AND WHEN) TO REPLACE THEM:
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Power
Speaker Specifications
Performance
Weight and Dimensions
