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8 BLOCKS GUIDE
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POWER SUPPLY PAGE
Supply Sag
– This controls virtual power amp dynamics. Higher settings simulate higher power supply
impedance, and thus greater tube plate voltage “droop,” for a more compressed feel. This control interacts with
the Master and will have little effect if the power amp is not being “pushed hard”. As Master is increased, the
power amp draws more current from its power supply and the Supply Sag control will have more effect.
IMPORTANT: Turning Supply Sag fully counterclockwise defeats power amp simulation for a single
amp block! This allows it to be used into a real tube power amp without globally disabling Power
Amp Modeling in SETUP : Global : Config. With Supply Sag at 0, Master works as a simple volume,
Depth is deactivated, and Presence turns into a simple shelving filter.
B+ Time Constant
– This interacts with the Supply Sag control because it makes the virtual power supply
response slower or faster. When the supply is fast the amp will sag rapidly accentuating the pick attack and
compressing after. Most guitar players like this, but setting it too fast will cause excessive AC ripple and ghost
notes. For convenience the virtual power supply voltage (B+) is shown as a meter on this page when the Supply
Sag control is selected. (The meter shows dB, relative to the idle voltage.)
Supply Type, AC Line Frequency
– These select between AC and DC virtual power supply types. AC
rectification and resulting supply ripple are modeled, and the line frequency is also selectable. Note that as with
a real tube amp, the AC Supply can cause “ghost notes” when Supply Sag is low and B+ Time Constant is high.
Lower B+ Time Constant values will make the amp feel “faster,” but too low can also cause ghost notes.
Variac
– This sets the relative AC line voltage into the amp simulation. Volume varies greatly when a Variac is
used with a real amp, but the virtual variac compensates for this.
Screen Frequency, Screen Q
– These set the resonant frequency of the virtual power tubes’ screen filter, and
the Q of that filter.
Level
– Duplicated control from the Tone page.
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SPEAKER PAGE
These parameters shape the virtual speaker impedance curve and the resulting resonances in the virtual power
amp. Amp and speaker interaction affects tone by causing an increase in power amp response at certain
frequencies. Note that setting Negative Feedback greater than “0” flattens the effect of the response curve.
LF Resonance Frequency, LF Resonance Q, LF Resonance
– Guitar loudspeakers have strong low-frequency
resonance. This shifts up slightly when the speaker is mounted in an enclosure. This resonance causes an
increase in the power amplifier response due to the finite output impedance of the power amp.
HF Resonance Frequency, HF Resonance
– A loudspeaker voice-coil presents an inductive load to the power
amp at high frequencies. This inductive load, in conjunction with the output transformer capacitance, creates a
high-frequency resonance at the specified frequency.
HF Slope
– This parameter allows fine adjustment of the high-frequency impedance of the virtual voice coil
(which affects the slope of the impedance curve). A speaker voice coil is “semi-inductive” due to eddy current
losses in the motor. This presents an impedance to the power amp that is neither fully inductive nor fully resistive.
The amount of resistive loss varies by brand and type. Reducing Slope simulates a speaker that is less inductive,
increasing Slope simulates a speaker that is more inductive. Typical speakers range from 3.0 to 4.5 with the
median being about 3.7. Lower values yield greater midrange while higher values are more scooped and sizzly.
Speaker Drive
– This simulates distortion and gentle compression caused by pushing a speaker too far. It
interacts with the Master which determines how hard the actual power amp is pushing. Don’t overlook this when
striving for “vintage” tones as it helps make edge-of-breakup tones sound like an old, well-played amp.
