Page 13 - Triwatt Custom Lead Amp Builder's Guide

Distortion
The overdriven sound of a tube power amplifier is highly desirable, with many different
output stage designs to produce the variety of trademark sounds heard on modern
recordings. The only problem is that a tube power amplifier is only capable of producing
this sound at one volume (usually, fairly loud!).
There are probably 3 distinctly identifiable types of tube power amplifiers used:
Leo Fender's classic early designs used 6V6 tubes, and later, the higher powered 6L6's. This
gave a characteristic full and punchy sound, suitable for many styles of the day, and later.
Steel and country players like the chime-like clean sounds, and blues players were quick to
discover the classic way it breaks up when pushed hard. At really high overdrive, though,
the sound becomes quite dirty, with bass in particular sounding flabby.
Marshall designs started as Fender copies, but soon switched to EL34 output tubes, possibly
for local supply reasons. Anyway, the rest is history. These tubes exhibit a softer overdrive
transition, and maintain clarity even at high overdrive levels. They also have a limited
middle response, giving rise to the famous Marshall crunch sound. The lower powered
EL84 tubes have similar characteristics.
Vox AC30 (and the more popular top boost model) uses a Class AB power amplifier design,
with the tubes biased ‘hot’, so while this operates in class A at lower levels, it is a class AB
design. There's no negative feedback in the power amp either, so this gives a different
sound, often described as a sweeter overdrive. Listen to Brian May's sounds for plenty of
good examples.
The Fender and Marshall designs use class AB for their output designs, biased with the
tubes almost off with no signal. This is more efficient (more watts per tube), and better for
tube life. When you play, tubes take turns handling each half of the signal. This leads to
some (unwanted) distortion as the tubes cross over. Class A designs are rare in medium to
high power guitar amps, but true class A has the tubes operating at half power, with no
signal applied. When you play, the tube fluctuates between full and no power, so there is no
switching to add unwanted distortion. This is a very superficial explanation; please read
elsewhere on the Internet for more detailed descriptions.
Wide Dynamic Range A plucked guitar string requires a wide dynamic range
to handle the initial peak, and then cleanly amplify the decaying string
vibrations. Some poor designs do not have this capability in their preamp stages, let
alone the power amp to handle this. Pre-amplifier stages need generous power rails,
and should not have gain stages which cause the initial plucked part of the string
sustain envelope to be clipped.
Instrument Speakers Unlike hi-fi speakers, which are designed to keep the
coil entirely within the magnetic field to maximize linearity, instrument speakers are
designed to have the coil partially leave the magnetic field at the extremes of cone
travel. This is partly to protect the speaker, but also produces a ‘soft-clipping’ effect

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