64
8 BLOCKS GUIDE
ADVANCED PAGE: PREAMP SECTION
Definition
– This control is a basic “tilt EQ” which adds highs/cuts lows, or vice versa. It is located at the amp
type’s input, so its effect is heard before preamp distortion or a front-end tone stack.
Low Cut Freq
– This control filters the lows at the input to the amp.
Hi Cut Freq
– This filters the highs at the very end of the preamp simulation. Experiment with this to fine-tune
your tone. For example, some of the higher gain amp types are characterized by fairly heavy filtering after the
preamp stage. Increase for a brighter tone or decrease for a darker tone.
Tonestack Type
– By default, the Bass, Mid and Treble controls operate as a “passive” tonestack: they simulate
exactly the frequency and phase response of the classic passive tonestacks found in the original amplifiers many
amp types are based on. This lets you change this behavior from “PASSIVE” to “ACTIVE”, or to substitute the
passive tonestack of another amp.
Selecting a substitute tonestack allows you to mix and match amps and tone stacks to create your own hybrids.
This allows you to use, for example, a Plexi-type tonestack on a Blackface amp model.
Selecting the “ACTIVE” type gives each tone c/– 12 dB boost/cut operation for up to twice the range of
a typical amplifier. Since the active tone controls are more sensitive, small adjustments have bigger effects. For
example, full PASSIVE treble for a high-gain British amp would be equivalent to only +5.0 dB ACTIVE, leaving 7 dB
of additional headroom! Active tone controls do not interact like those of a typical amplifier, so when you adjust
the treble, the mid and bass are not affected. This can make dialing in a certain tone easier and quicker than it
might be with a PASSIVE tonestack.
Tonestack Freq
– Sets the center frequency of the tone controls to determine their effect on the sound. This
control works whether you are using Active, Passive, or substitute tone stacks. This parameter defaults to
an appropriate value whenever you change the amp TYPE, but it can then be changed as desired. But if you
subsequently change the Tonestack Type, the Tonestack Frequency will not necessarily be correct anymore.
Tonestack Location
– Lets you change the location of the tone stack. “PRE” places the tone stack at the input
to the virtual preamp, “POST” places the stack between the preamp and power amp. “MID” places it between the
last two triode stages, and “END” places it after the power amp (which is physically impossible with a real amp).
Preamp Sag
— Turning this ON causes the amp block to behave like an integrated tube head or combo amp.
Turning this OFF simulates a separate preamp and power amp.
Triode 1 Plate Freq, Triode 2 Plate Freq
— These parameters set the cutoff frequency of the last two triodes
in the chain. Many amps have a capacitor across this triode’s plate resistor. This capacitor is used to smooth the
response and reduce noise. You can adjust the amount of capacitance, and the resulting frequency, using these
parameters.
Bias Excursion
— The higher the value, the more the bias shifts when the virtual power tubes are overdriven.
Bias excursion pushes a power amp from Class-AB operation towards Class-B operation, which can result in
crossover distortion. A little goes a long way, but too much can lead to what is referred to as “blocking distortion”
which can make an amp sound unpleasant.
