11.5

Effects and effect plug-ins 127
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tube(s) through bias voltage, because tubes always amplify only a half-wave of
an alternating circuit. A class-A circuit may be put together using only a few
components and sounds very "warm" because of its constantly asymmetrical
characteristic curve shape (some even harmonics appear). This amplifier,
however has the disadvantage of low efficiency or low output performance
and a comparatively high liability of erosion. The latter has been left out of the
modeling, of course...
 Class A/B, high power: In this case, each half-wave has its own tube, which
(almost) doubles the efficiency. Class A/B router amps are more complicated
to implement (and calibrate). Compared to Class A, the sound characteristics
include descriptions such as "sovereign" and "powerful", but somewhat
"colder", because they result in almost exclusively odd harmonics. Using
positive and negative feedback via the output transformer, an additional
"sound design" is often added here. For this reason, VANDAL's A/B power
amp features more juice in its lows and more bite in its highs.
 Presence: On many amplifiers, this pot is located next to the EQ section,
although a presence boost actually takes place inside the power amp circuit.
Most push-pull amps use negative feedback (from the output transformer
back to the power amp's input) to linearize the amplification process.
Lowpass-filtering this feedback signal and mixing it (anti-phased) with the input
results in boosting the mid and treble region. Using presence, the sound takes
on a livelier and more up-front character.
 SAG control: Many older tube amps use rectifier tubes to transform AC
current to DC (instead of conventional semiconductor diodes used today).
However, a tube is a high-resistance component, and can't produce an even
current flow during steep load changes. This "sagging" feature of a
cranked-up tube amp is the acoustical result of these short-lived interruptions.
Moderate sagging is initially perceived in the attack; it sounds somewhat
compressed, but in a "lively" way. If the effect is even stronger, it changes the
entire signal. Besides the dynamics, the harmonics spectrum also changes,
because operating point of the tubes is shifted.
 Sagging occurs in this form only in push-pull end phases; in principle, Class A
amps always draw constant (maximum) current from the mains.
 Intensive sagging will result in creating less "presence", as the overall signal
gain in the power amplifier decreases, thereby generating only small voltage
excursions to feed back on the power amp's input.
Preamp channels
The guitar amp is set up with three channels in all preamp
configurations (Classic, British, Modern High Gain).