Stacks
Stacks Fast Facts
When more flexibilty is required than a standard combo amp, musicians turn to stacks. A stack is simply a seperate amp set on top of a seperate speaker cabinet or "cab." The benefit in doing this is the individual amp and cab components are much better when designed independently as well as more powerful. They can be used in series when more power is required or if you are looking for the flexibility of mixing and matching amps and speakers.
A Brief History of Multi Stage Amps and Stacks
Larger and more complex amps have many stages of tube amplification: preamp stages, signal-processing stages, and power amp stages.
The Preamp
The preamp stage is much like a mixer in a PA system, which must amplify
an incoming mike or guitar signal to line-level strength before the signal
can be processed with effects for tonal shaping. Likewise, a tube amp
must preamplify a guitar’s signal so that it can be further processed.
This is the first gain stage of the tube amp.
Signal-processing stages
An example of a signal-processing stage is the reverb section, where the
signal is diverted through a reverb spring system and then returned by
another gain stage, and finally blended with the original signal. Tone
controls and second gain stages (often employed for an overdrive effect)
are other examples of signal-processing stages.
Power-amp stages
The power-amp section takes the preamp’s signal and amplifies it many
times to a level that can drive the speakers. All tube amps with power
ratings of 10 watts or more employ a push/pull power amp design. This
means that the power tubes work as a team to amplify the signal and drive
the speakers. (Practically all transistor amps employ a push/pull configuration
as well.) The output tubes all share in the sound, so for maximum efficiency
it is desirable to use tubes that operate as similarly as possible. Also,
for efficiency, use power tubes of the same make – manufacturer’s specifications
for tubes bearing the same stock number may vary over a broad range. And,
if one power tube is bad, it is advisable to change all of them. Having
one fresh, powerful tube and three old ones, for example, can create an
imbalance in the push/pull effect, resulting in inefficient operation.
The power-amp section is only as strong as its weakest link. So, if one
tube out of four is faulty or varies from the others in its performance
character, the overall sound of the amp will be limited.
Since a tube is a mechanical assembly of parts that forms an electronic device, it is subject to some mechanical problems and limitations. Tubes wear out in direct proportion to how hard they are worked (due to the circuit design) and how often and loud you play your amp. Vibration and jarring shorten the useful life of the tube as well. Ideally, a tube could be built so that no vibration existed between its mechanical elements. However, in practice this doesn’t happen. So when the tube is vibrated (usually by the speaker), the elements shake, resulting in an additional signal being amplified. This phenomenon is commonly referred to as microphonics. The construction methods and materials used in the chassis and cabinet may actually serve to create pleasing microphonics, which give the amp a distinctive desirable sound. However, adverse tube microphonics can be a big problem when the elements of the tube rattle or ring, producing a signal all by itself. A tube with this problem is unsuitable for use in music amps, much like a faulty guitar pickup or a bad microphone is undesirable for most musical purposes.
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