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EFFECTS MENU 761
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).
 Clean: This channel should create no distortions. If the incoming channel is "hot
enough", then it will be distorted in a typical tube fashion. The model will use only half of
a double triode; in principle, a simple amplification takes place at the "working level".
 Crunch: Amplification stage number, circuit and, last but not least, the resulting sound
all vary:
- Two amplification stages are used in the "Classic" preamp setting. This model
uses a long-established design that is similar to the classic Fender amps or the
first Marshalls. The "classic" preamp delivers saturated lows that are only slightly
dampened by the second stage. As a result, this circuit delivers the typical
"brown" sounds of older vintage amps.
- The "British" switch is inspired by dual input stages, such as Marshall "Plexi"
variants (JTM, "Super Lead", etc.). The signal is sent to half of each triode circuit,
and these favor different frequency ranges ("warm", "bright", etc). The "British"
circuit in the VANDAL amp uses a set mixing ratio for both tube portions. In