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Ampeg SVT-VR Manual

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SVT-VR Bass Guitar Amplifier
11
A brief history of the tube:
In 1883, Thomas Edison discovered that electrons would flow from a suspended filament when enclosed
in an evacuated lamp. Years later, in 1905, John Ambrose 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
“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 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 may 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 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 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 the instrument and shape the sound. They are inherently microphonic (mechani-
cally 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 that any replacements are 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. It is possible to change the sound
of the amplifier 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
… 11 …


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