Amp block

User contributions

Amp modeling developments

• The amp modeling in the Axe-Fx II is called G2 and Virtual Vaccuum Modeling modeling (see the Fractal Audio website). Part of it is ported to Standard/Ultra firmware 11.
• Cliff: "Almost all the amps in the AFXII are based on actual amps." Source1 Source2
• Cliff: "As you listen to clips from modelers what you start to recognize is a certain "stationary" aspect to the tone compared to the every-changing tonality of a tube amp. Another thing is finger response. With a good tube amp you can vary the tone quite a bit just by how you fret the note and attack it. Modelers tend to make every note sound the same. So I tested some hypotheses and came to the conclusion that it's because a real vacuum tube has a transfer function that is not static. The transfer function is dependent on time, frequency and amplitude. Where you really hear it is in the in-between regions where the tube is just starting to distort. At first I tried some dynamic transfer functions but that was a lesson in futility. So then I created the VVT stuff. In VVT there is an actual vacuum-tube replica in software. You enter the values of the resistors and capacitors on the grid, cathode, etc. and it behaves just like a tube complete with Miller effect, cathode memory, etc. The problem is that it requires an obscene amount of horsepower so the only solution was a dedicated DSP. The other big part of the G2 sound is the output transformer modeling. The OT distorts and as it distorts its inductance decreases which changes the bandwidth and loop characteristics." Source
• Firmware 5:
  • Cliff: "The new firmware (V5) is all about dynamics. The power amp modeling was totally rewritten. If you listen to those early VH albums you can hear the amps knock and ring when he hits them hard. Lots of work went into discovering why that happens and replicating it. It's due to several factors: the power supply sagging, the bias point shifting as the supply sags, the screens ringing and power compression in the speakers" Source
  • "Much improved grid modeling in Amp block preamp and power amp stages. New modeling very accurately replicates grid conduction and resulting bias excursion. This results in a more dynamic, thicker and bouncier tone. The power tube grid conduction parameters are exposed to the user in the GUI. The Bias Excursion parameter controls how much the grid voltage droops when the grids conduct. The Excursion Time and Recovery Time parameters control the time constants associated with the excursion."
  • "Added dynamics processing to Amp block. A new tab, “DYN”, in the amp block, allows adjusting various parameters of the dynamics processor along with several other parameters related to amp dynamics. The Dynamics parameter controls the amount of dynamics processing and models the interaction between the power amp, power supply and loudspeaker under high power-level conditions. The Dynamics Time parameter (ADV tab) controls the time constant of the associated processing. The Level parameter is duplicated on the DYN page for convenience."
• Firmware 6:
  • "Reworked power amp modeling based on new research. The power amp modeling has been totally rewritten based on “amp matching” studies. This includes improved output transformer saturation modeling."
  • "Most amp models have been matched to their respective physical amps. Exceptions are those models which don’t have a physical counterpart, i.e. FAS MODERN, etc. The matching data is integral to the amp block and is transparent to the user. This has increased the size of the firmware slightly and also uses the master DSP for part of the calculations which increases the CPU load associated with the amp block slightly."
  • Cliff: "Almost everything has been reworked. Almost all the "real" amps are "matched" to the actual amp. This involved much more than just Tone Matching. It also included gain matching, harmonic content matching, tone-stack matching, etc." Source
• Firmware 7.00:
  • "The dynamics processing of the amp block was totally rewritten for this release. A complex set of formulas was developed that completely describe the various voltages in a tube amp. Unlike other modelers that simply model an amps dynamics as a first-order compressor, the Axe-Fx II now accurately models the complex interaction of the power tubes with the surrounding circuitry including the power supply and screen voltage network. You may notice a difference in the feel of the various amp models. Please refer to the descriptions below to understand the operation of the various controls. Doing so will enable you to adjust the dynamics to your personal preferences."
  • "Improved preamp modeling. Harmonics now move more with input level which results in a more open and less congested tone."
• Firmware 8.00:
  • "Improved phase inverter modeling provides “juicier” tone when PI is driven hard (MSTR set high)."
  • "Doubled resolution of internal amp matching data."
  • "Added input matching data to many amp models. While this may not be audible in many cases, especially for higher gain amps, it does affect the feel."
  • Cliff: "If you set the Global to 7.xx the modeling code is IDENTICAL to the actual 7.00 firmware. All I do is call the old function instead. However, the presence calculation is based on the new (and far more accurate) calculations." Source
• Firmware 8.01a:
  • "Added global Modeling Version parameter which allows one to select between modeling versions. This parameter is global and is in the Global menu."
• Firmware 9:
  • "New power amp modeling with improved dynamic response. This new modeling features improved transformer/plate interaction modeling resulting in better feel and a punchier response. The Supply Sag parameter is more responsive as a result. Additionally, crossover and transformer hysteresis distortion modeling is improved resulting in more overtones when playing softly. This improves controlled feedback performance and yields a more aggressive tone at lower Power Tube Bias settings. The Global menu allows the choice of Version 9.xx, 8.xx or 7.xx modeling to suit individual tastes. Note that Version 9.xx is slightly quieter so don’t be swayed by Fletcher-Munson effects when evaluating differences."
  • "Greatly improved cathode follower modeling. The cathode follower modeling now varies the amount of distortion in addition to compression. This results in a more dynamic attack, improved feel and more “punch” and “thunk” (since this also creates low frequency energy into the power amp). The amount of cathode follower affect is controlled, as always, by the COMP parameter in the Amp block. Note that the higher the COMP value, the more effective distortion on sustained notes. Therefore as you increase COMP, you may want to decrease Drive. Also note that excessive values can result in pumping or blocking distortion. Note that the “cathode follower effect” occurs even in common cathode stages as well so even amps that don’t have cathode followers may exhibit some cathode follower-like effect."
• Firmware 10:
  • "Implemented “Multi-point Iterative Matching and Impedance Correction” technology (MIMICTM P.A.F.) to amp models. MIMIC applies analytic signals to an amplifier and captures the fine nuances of each amp at various points in the circuit and corrects each model vs. its theoretical implementation. In some cases the difference can be substantial, in other cases the difference is minimal. This depends on the layout of the amp and the various parasitics involved. MIMIC has the advantage of applying these corrections at the appropriate location in the amp model rather than as just an output EQ so that the various controls of the model behave virtually identically to the actual. For example, the Modern modes of a Dual Rectifier are highly sensitive to Master Volume with the tone becoming thicker as the MV is increased. MIMIC preserves this behavior rather than just getting louder as the Master Volume is increased. The equalization correction portion of MIMIC processing can be turned off in the Advanced menu tab of the Amp block, if desired. Note that the nonlinear correction and other aspects of MIMIC are integral to a model and cannot be turned off. In many cases the equalization correction can be subtle and many not be immediately audible when switched on or off."
  • "Updated numerous amp model details in light of MIMIC's identification of deviations between the models and actual amps."
  • "Tweaked power amp modeling slightly to increase even-order harmonics. This makes most models “sweeter”."
  • "Improved triode modeling removes “glare” from distortion yielding greater clarity and string separation."
  • "Improved power tube modeling gives more punch and pop, especially to tones that rely on power amp distortion. Additionally this provides a more dynamic response, better touch sensitivity and improved pick attack."

Preamp and power amp modeling

• The pre-amp and power amp sections of the amp sim cannot be split.
• The power amp can be disabled though, either globally (Global menu) or per preset (turn Sag to zero). Note that turning off power amp simulation doesn't work well with amp sims that fully rely on power amp distortion such as the Wrecker sim.

What happened to Global Amps

• The Global Amps feature in the Standard/Ultra is part of Global Blocks in the Axe-Fx II.

Using 1 or 2 Amp blocks (HiRes mode)

• Each preset can have up to two Amp blocks. One DSP is devoted entirely to the Amp block(s). Adding a second Amp block just adds about 2% CPU usage. Cliff: "There is actually a small amount of processing for the amp blocks done on the master DSP. That, along with inter-DSP communications, uses about 2% of the master DSP." Source
• Each Amp block can use the X/Y feature to switch between sets of parameters.
• When using a single Amp block in a preset, the Amp block (automatically!) runs at double the internal sampling frequency (firmware 3.03 and higher). Main benefit of this is less aliasing in high gain models. "Amp block now has high-res mode. In this mode the internal sampling rate is doubled so as to provide greater fidelity and resistance to aliasing. This mode is automatic and is selected whenever there is only amp block in the layout grid. Adding a second amp block will revert to normal resolution. Note that switching between presets with differing number of amp blocks may introduce an additional delay as a “soft reset” of the amp blocks must be done whenever changing the resolution." And Cliff: "The oversampling rate is cut in half when running two amps. It's probably not noticeable. Even when running at half, it's as fast or faster than every other product available." Source
• You can use two Amp blocks simultaneously by using two grid rows.
• You can switch between Amp 1 and Amp 2 using MIDI CCs. Or morph between them using an expression pedal: add a Mixer block, input both signal chains into it, use the modifier menu to go from 0-100% on one amp and 100-0% on the other, adjust the curve responses for a smooth crossfade.

Audio gap when entering an Amp block's Edit mode

• If you press Edit in the Amp block (or using X/Y on the front panel), there will be a short gap in the audio stream because of processing tasks. Source

Resetting Amp block settings

• Doubleclicking Bypass resets an Amp block. This will reset all parameters to default. If firmware earlier than 6 is installed, the amp type wll be set back to 59 Bassman. If firmware 6 or later is installed, the current amp type stays the same.

Tips for adjusting amp gain

• Here's a list of things you can do to increase or decrease an amp's gain range. Also check this Wicked Wiki thread.
  • Each amp has a Drive parameter controlling the amount of preamp gain. You can assign an external controller (expression pedal) to vary the gain.
  • Adjust the global Amp Gain parameter, see Global menu (front panel).
  • Use the Input Trim parameter in the Amp block. You can use this parameter as a substitute for the Lead Drive control on a Mesa Mark amplifier or other amps which have two Drive/Gain controls.
  • Increase Master Volume for more power amp distortion.
  • Increase Mstr Vol Trim on the advanced parameters page (firmware 10).
  • Use Boost in the Amp block. Also, switching on Bright often increases gain.
  • Adjust Transformer Match and Drive, or engage the Saturation switch.
• Insert a Drive block before the amp and set it to FET Boost or Tape Dist (clean boost) and attach a pedal to its Drive parameter. Or set it to TS808 or Tube Drive block with Drive all the way down and Level maxed ("old school" trick that helps tighten up the low end). Alternatively use a NullFilter block before the amp instead of a Drive block, with lots of dBs to boost the amp's input.
  • Jay Mitchell: "Cascade one amp block into another. Turn off "Sag" in the first one. Now you've got an extra preamp feeding your amp, which opens up an incredible spectrum of gain staging. For example, think Twin Reverb preamp, with Plexi tonestack set to "post," feeding a Plexi 2 with default settings. The possibilities exceed anything one person could hope to explore in a lifetime. You can get it awfully close. You want to minimize the effect of the preamp in Amp 2. To do this, set the 2nd Amp's Bright to off, MV to a high value and find a neutral setting for the tone controls in the 2nd Amp. Then use Drive in the 2nd Amp for your MV. The amp types you choose for this arrangement will make a huge difference, as will quite a few parameter settings."
  • Jay Mitchell: "Start with Tape drive, set the clipping mode to "HV tube", Drive moderate, Level as appropriate for the amp block it's driving, and you'll have another tube gain stage, complete with EQ."
  • Note that, unlike with the Standard and Ultra, adjusting the Input Level (I/O menu) does not affect the gain!
  • Cliff: "If you want less distortion on low notes there are several ways to achieve this: 1) Use the Low Cut, 2) Increase Definition, 3) Increase Xfrmr LF. The first two reduce low frequency content going into the preamp, the last one reduces it going into the power amp (there's a hidden low-cut between the preamp and power amp but the user doesn't have access to this). So it depends on where you are getting the distortion from. If it's mostly preamp distortion, use #1 or #2. Otherwise use #3. You can add bass back with the Depth knob or in the EQ page. The default settings are accurate for the amp being modeled. As amps become more modern, it seems people's taste agree with yours and many modern amps feature aggressive low-cut and then add bass back in the power amp. So what you are doing is "modernizing" your amp. For example, the HBE has a very high low-cut and then adds bass back with a fixed Depth circuit." Source

TYPE page

AMP TYPE

• Firmware 10: "When in the Type page of the Amp block, the A,B, and C Quick-Control knobs now control Drive, MV and Level, respectively. This allows for quicker auditioning of the various models."

PRE page

DRIVE, MASTER VOLUME

• Vintage amps don't have separate gain (drive) and master volume controls. That's why Master defaults to "10" in these models (firmware 10 and later). Use Drive for volume and gain.
• Cliff: "For clean tones the Drive control should be set fairly low and the Master set very high. On a real "Blackface", for instance, the Master is essentially maxed since that amp has no master volume. A Blackface typically achieves full power at around 10-11 o'clock on the volume (Drive). It's also insanely loud. Beyond that everything starts to saturate and clip. If you set the Master low and the Drive high, for clean tones, the low end will tend to get muddy. Good cleans are obtained with little, if any, preamp distortion and a nice amount of power amp distortion. Power amp distortion has a much different character and tends to be glassy and bouncy. Preamp distortion is rougher and more compressed."
• Cliff: "The real key is to adjust the relative amounts of each. You want to balance preamp and power amp distortion for the best tone. What I do is start with the MV low and turn up the Drive until I get the desired amount of gain and sustain. Then turn up the MV until I get the desired compression. Then fine-tune each." Source
• Cliff: "The amp block is always the place to set your volume. The Level control is repeated at several places in the amp block menus for convenience so you don't have to keep switching pages. The Master Volume (MV) DOES affect the tone. It sets the level into the power amp simulation. The Level control has no affect on the tone. For MV amps, i.e a 5150, adjust the MV until the desired amount of power amp distortion is obtained. Most MV amps rely on preamp distortion and don't produce much power amp distortion. If you turn the MV up too high on them the tone will get muddy and flubby. Non-MV amps rely primarily on power amp distortion so you need the level into the power amp to be hot enough to push the power amp into distortion." Source
• Cliff: "I just start low and bring it up until I get the desired compression. Then I chug the E string and if it's too buzzy or flubby I drop it down a bit. For tight, high-gain stuff you want to keep it low. For liquid, spongy tones you want to set it higher". Source
• Cliff: "In most cases the knobs do translate. Usually within 10%." Source
• Firmware 10:
  • "Amp models now default to a starting Master Volume setting when selected. Also, the proper setting for non-MV amps is now a Master Volume setting of 10.0. Non-MV amps, therefore, will default to a value of 10.0 when selected. If more MV drive is desired for non-MV amps, the new MSTR VOL TRIM parameter in the Advanced GUI page can be used to increase (or decrease) the Master Volume. The starting MV value for non-MV amps is roughly the “sweet spot” for the amp. This is the point where the power amp starts to contribute to the tone and feel of the amp. Decreasing the MV will typically cause the amp to get brighter and less compressed and increasing the MV will cause the amp to get more midrange focus and more compressed. As always, your ears should be your guide."
  • "Improved “Drive Stack” accuracy in Amp block. This allows for near exact control behavior for the Drive control over the full range of operation."
  • "Firmware 10: "New MSTR VOL TRIM parameter in the Advanced GUI page can be used to increase (or decrease) the Master Volume."

BASS, MID, TREBLE

• Firmware 6: "In general most knobs now behave exactly like the actual amp when possible. In a few instances there may be minor discrepancies between the knob position of the model and actual amp due to programming constraints and/or peculiarities of the actual amp (such as poor potentiometer tolerance). Due to variations in presence circuit topologies the taper of the Presence parameter, in particular, may vary between the model and the actual amp. In other words, a different setting on the model may be required to achieve the same response as the actual amp. In most cases however, the Drive, Treble, Mid, and Bass knobs will be accurate to within 10% of the actual amp."
• Cliff: "In most cases the knobs do translate. Usually within 10%." Source

FAT

• Added in firmware 6. Located under the Mid knob. When engaged it shifts the center frequency of the tone stack down thereby “fattening” the tone. It's similar to the Fat switch on a Mesa Boogie amp.
• Cliff: "The Fat switch simply alters the tone stack treble capacitor. So the effect depends on the location of the tone stack." Source
• Firmware 9.00: "Now always display text. When the switch is active the text is highlighted."

BRT (Bright)

• Turning up the amp's Drive or Master may decrease the impact of the Bright switch, depending on the amp type.
• With some amp types, such as Plexi, the Bright switch has a large impact on the amount of gain.
• It's possible to change the effect of the Bright switch on the tone. In the Advanced parameters adjust the Bright Cap value.
• Firmware 9.00: "Now always display text. When the switch is active the text is highlighted."

COMP

• Firmware 7.00: "Added compression modeling to amp block preamp section. A new parameter, “COMP”, controls the amount of compression. Most models default to zero as they do not have measureable compression. Other models have a non-zero default value which matches the amp’s preamp compression characteristics. The time constant of the dynamics can be set in the Advanced menu with the PREAMP DYN TIME parameter. Preamp compression can be used to emphasize pick attack which is useful for certain musical styles. Use caution when dialing extreme values as this can cause excessive pumping."

SAT (Saturation)

• Cliff: "It switches in a zener diode clipping stage right before the tone stack. This is the "Arrendondo Mod"." Source
• This parameter is enabled by default in the Cameron 2 amp model. Try it with amps like Plexi, JCM800, Friedman and Mesa Mark models.
• Firmware 9.00: the SAT switch function has been added to the Pre page in the Amp block menu under the COMP knob.

BOOST

• Sometimes enabling Boost works better than turning up preamp gain. It's a clean boost so it'll increase the gain of all frequencies. To enable the Boost, scroll to Type in the AMP block's properties and press Enter.
• Firmware 9: the BOOST switch is now a dedicated knob and also modifiable so it can be activated remotely.

PWR page

PRESENCE, HICUT

• If Damping is zero, Presence turns into a HiCut control.
• Cliff: "A tube amp's presence control is basically a type of treble control. It affects a higher range of frequencies and operates on a different principle but the net effect is an increase in high frequencies. There is also a slight increase in distortion in the higher frequencies since the power amp becomes less linear for those frequencies. Source
• Important when switching off power amp modeling: check the Presence setting. Cliff: "If you turn off power amp modeling always check the presence control. It changes from a "classic" control to a shelving type where 5.00 is neutral. I just spent an hour trying to figure out why this preamp model I am working on wasn't matching. Forgot to set the presence control to 5.00." Source
• Firmware 8.00: "Authentic Presence control modeling. The Presence control in the Amp block now behaves like the actual amp rather than an idealized version. The Presence Frequency parameter is now a frequency multiplier rather than an absolute frequency as the frequency of the presence circuit depends on the Presence control position. The Presence Frequency parameter works by scaling the value of the virtual presence circuit’s capacitor value. NOTE: Any presets created with earlier firmware versions will have the Presence Frequency parameter reset to 1.0. Setting the Pres/Depth Type parameter to Active or Active Pres will override the authentic modeling and implement an ideal presence circuit with fixed center frequency."
•  Cliff about HiCut: "It is dependent upon Damping, just like a real amp. Hi Cut is modeling the Miller capacitance at the input to the Phase Inverter. The more negative feedback, the less the Miller capacitance." Source

DEPTH

• Cliff: "The Depth knob adjusts automatically only if the actual amp has no depth control. Some amps have a fixed depth circuit, equivalent to fixing the Depth knob in a certain position. If that is the case the model automatically pre-sets the Depth knob to the appropriate position when the type is selected. If the amp has a Depth knob (sometimes call Resonance, Girth, etc.), then the model does NOT pre-set the Depth control." Source

DYN PRES (Dynamic Presence)

• This models the output transformer leakage inductance that results in a brightening of the tone when the power amp is pushed. This control is set to a default value when the model is selected corresponding to the real amp, if applicable. Increasing this value results in a brighter response as the virtual power amp is pushed. When playing softly or at lower gains, the influence of this control is lessened. Note that this only affects the power amp modeling and is dependent on the degree of power amp overdrive. This control can also be set negative to cause the tone to darken when playing hard. This control can also be used to help “dial in” the sweet spot of an amp model. As the MV is increased an amp becomes more liquid, compressed and easier to play. However, the highs may get overly compressed causing the amp to sound too dark. The Dynamic Presence control allows you to get the desired power amp drive and liquid feeling and then bring the highs back without affecting the rest of the spectrum.

DYN DEPTH (Dynamic Depth)

• Analogous to the Dynamic Presence control, this increases or decreases low frequencies when the virtual power amp is being pushed. While real amps don’t display this behavior, it is a valuable tone-shaping tool.

LEVEL

• The Level parameter in the Amp block lends itself very well to controlling the overall volume level of the preset, to match it to other presets and to prevent clipping the digital signal. Also see Drive and Master.

SPKR page

Low and High Frequency Resonance controls

• Cliff: "There are certain aspects that simply can't be modeled and require user intervention. For example, a speaker has a low-frequency resonance. A tube amp will create a higher output at that resonant frequency. The Axe-Fx has no way of knowing what that resonant frequency is and defaults to a value that is common for the speakers that are typically used with that amp. However, if you drive that speaker through a solid-state amp you won't excite the resonance unless you adjust the Speaker Resonant Frequency to match it."
• Cliff: "The SRF parameter (low resonance frequency) makes a difference in certain circumstances: medium-gain with lots of power amp breakup and high-gain into a traditional guitar cab. For example, a Deluxe Reverb easily gets into power amp breakup with a Drive of 5.0. Play a G chord and listen to the low notes breaking up. Adjust the SRF and you'll hear the character change pretty dramatically. This applies to both FRFR and traditional speaker applications. For high-gain amps into a traditional cab the SRF has a definite effect for palm-mutes and diad chugging. As you excite the resonant frequency the speaker excursion increases and you get much more punch. This even applies a bit to low and medium gain amps if you rely on that punch as part of your tone. Note, however, that the punch is rarely audible to the audience or captured during recording. One way to find the SRF is to put a Filter block after the amp block. Set the type to Peaking, Q to 5 or so and Gain to 10 dB. Start with a Freq. of 50 Hz. Play some chugga-chugga and slowly adjust the Freq. until you hear and feel the cabinet resonate. Make a note of the frequency. Remove the filter block and set the amp block SRF to match. 4x12s typically have an SRF of between 80 and 120. Open back cabs are typically a bit lower."
• Cliff goes into detail in this thread.
• Cliff: "The speaker tab is not an EQ. It allows you to adjust the impedance that the virtual speaker presents to the virtual power tubes. In most cases the resulting EQ is quite different than the impedance curve since negative feedback flattens the response. If you turn the damping all the way down then the EQ will be close to the impedance curve (but still influenced by the transformer).". Source
• Jay Mitchell (answering the question "What is the effect on the emulation if this value is set incorrectly?"): "You'll get the response of the amp driving a speaker with a resonant frequency other than that of the speaker that is represented by the IR you're playing through. Whether that difference is even audible depends on the resonant frequency itself (much below 80 Hz has relatively little effect on guitar frequencies) and the virtual design of the amp sim (e.g., higher values for Damp will reduce the effect). The justification for getting this "right" is getting the closest possible match to the physical cab you're simulating. There's nothing to say it won't sound better set to some other frequency."
• Jay Mitchell: "The term "resonant" is unfortunate, because there is no resonant peak in the electroacoustic response of most cone transducers - including guitar speakers - under normal conditions. There is an interaction between the source impedance of an amplifier and the load impedance presented by the loudspeaker. Both impedances can vary with frequency, but frequency-dependent variations in the load impedance are much greater. What is the effect of this interaction? In the most extreme case of a high source impedance - say, 3 ohms, which could be the case with a Class A SE tube amp with no negative feedback - there will be a response peak of just over 3dB at the "resonant frequency" of the speaker. In a less extreme (and more common) case - say, 1 ohm source impedance - this response peak will be reduced to about 1.23dB. In comparison to the other response peaks and dips in a guitar amp/cab system, the effect is a subtle one. Another relevant item: if you are looking for the "right" frequency (IOW, the actual resonant frequency of the cab represented by the sim), you cannot identify it with an IR of the speaker. The response of the speaker is influenced by many variables, and resonant frequency is just one of them. Anything you do via use of the Axe-Fx alone, while it may be pleasing to you, is unconnected to the actual resonant frequency of the speaker. As one person has already suggested here, choosing the fundamental of a note on the guitar may produce a desireable result. In the end, this is another form of EQ which adds a small response peak at a relatively low frequency. The higher you have the "Damp" parameter set in the amp sim, the less effect this parameter will have."
• Cliff: "That parameter was added in 5.02. It is called HI RES but operates somewhat differently than the old HI RES parameter. Reduce this value to achieve a smoother tone. The old Axe-Fx is equivalent to around 4-5. Also note that the old Axe-Fx had more HI CUT by default. You may want to reduce the HI CUT FREQ to emulate that "polished" tone." Source
• Forum member Smilefan's explanation of the resonance controls (source): "Impedance is the electrical characteristics of the amp/speaker hardware that restricts the flow of power to the speaker. Impedance is a complex issue. When you see an “8 ohm” speaker, that’s a great oversimplification. A speaker’s impedance is different at different frequencies. As a speaker’s impedance changes, it changes the task of the amp, asking for more or less current flow. The greater the impedance, the more current and voltage are required from the amp to drive the speaker. This interaction has a great effect on sound and performance. The controls you have in the Amp model function like an integrated 3 band Parametric EQ. Except it allows you to precisely tailor the output impedance of amp to the speakers at a wide range of frequencies. There are two major areas where output impedance affects the way a loudspeaker sounds. Frequency response and controlling the motion of the speaker cone. So variations in output impedance produce a noticeable affect on what frequencies the amp’s tonal and drive characteristics will be displayed thru the speaker, and the playing feel of the amp thru the motion of the speaker cone.
• More information about resonance in this thread.
• Jay Mitchell: "Putting a cone transducer in a sealed enclosure adds stiffness to the speaker's suspension. The effect of this added stiffness is to increase the resonant frequency. The smaller the enclosure, the greater the added stiffness, and therefore the greater the increase in the resonant frequency. It is not uncommon for the small per-speaker volume in a typical 4x12 to increase the resonant frequency by quite a bit more than 10 Hz. A speaker in an open-back cabinet will see a small-to-nonexistent increase in its resonant frequency, and there may be additional, secondary resonances. A ported enclosure has two resonant frequencies, ideally (when the enclosure is correctly tuned for the speaker) of equal magnitude. The Axe-Fx speaker parameters are not capable of accounting for the second resonant frequency of a ported cab or small secondary resonances, but these effects are usually not sonically relevant." Source
• Jay MItchell: "Here's the simple part: the output impedance of the amplifier is in series with the load impedance presented by the loudspeaker. The combination of the two is a frequency-dependent voltage divider. In the physical world, this relationship always takes care of itself. In the virtual world, it never does. It must be consciously addressed by the code-writer and/or the user." Source
• Jay Mitchell: "All electric motors - including loudspeakers - function in reverse (IOW, as generators) as well. You can actually use a cone transducer as a microphone. For this reason, anything that affects the mechanical motion of a transducer also affects the electrical load (aka "impedance") that it presents to the amplifier. As I pointed out earlier in this thread, placing a cone transducer in a sealed box adds to the spring constant of the suspension. This has the effect of increasing the resonant frequency, which is the frequency at which the electrical impedance reaches its peak value. Internal reflections in an undamped cab can also cause small artifacts in the impedance vs. frequency characteristic of a speaker. As an informational item, the term "resonance" is woefully misunderstood in this context. You cannot identify the "resonant" frequency of a speaker/cab combination via any listening exercise. In most cases, there is no amplitude response peak at the resonant frequency. Impedance vs frequency information is not available from the IR of a speaker, either. It is a separate data set without which the detailed interactions between an amp and speaker - which profoundly affect the sound and feel of guitar amps - cannot be accurately simulated. Source
• Cliff: "LF: in general the Q is between 2 and 2.5. The Hi Freq is usually between 1 and 1.5 kHz. Hi Freq sets the critical frequency (or corner frequency) of the inductive portion of the loudspeaker's response. The critical frequency is the frequency at which the reactive component of the impedance is equal to the resistive component. This is found by fc = R/(2*pi*L). For a typical speaker R is around 6 ohms and L is around 0.75 mH. Therefore fc = 1270. Jensens tend to have higher inductance so that would move this value down. Eminence speakers tend to have lower inductance so that would move this value up. Celestion does not publish their values so I used Eminence values when calculating the defaults. You'll notice the Marshally stuff has fc around 1500 which is consistent with a typical Eminence copy of a Greenback. => You cannot obtain speaker impedance via audio stimulus and microphone measurement. Impedance is defined as voltage divided by current so you need to measure the current vs. applied voltage across the frequency range of interest. I have the equipment to do it, and have measured many speakers, but the average person do esn't have the equipment nor the knowledge to use the data. The influence of speaker impedance is generally not that great. The exception are amps with no negative feedback. In these cases the speaker impedance has a much more pronounced effect on the overall response. These amps include Vox, Matchless and most other "Class-A" designs. As soon as you add negative feedback the response flattens considerably. However... Presence and Depth reduce negative feedback so if you dial significant amounts of those in then the speaker impedance becomes a factor again. All-in-all you only have to be in the ballpark. 1500 Hz is a good starting point for Hi Freq. Adjust up or down slightly by ear. I don't believe that 3000 Hz is accurate. I've never seen a speaker that would have the corner frequency that far out. => IMHO, the stock settings are accurate. As I explained a few posts up I wouldn't set Hi Freq outside the range of 1.0 to 1.6 kHz. Vibroverb model is an exception (800 Hz) since it had a more voice coil inductance. => I call it critical frequency since it is similar to the critical or corner frequency of a filter. I had to come up with some way of setting the loudspeaker inductance relative to the resistance. Frequency seemed to make more sense. I thought about an inductance parameter but figured that would be too nebulous. At the default settings the impedance rise of the simulated voice coil matches very close with published data. I have overlaid the modeled impedance curve with published data and it is a very good fit. For example, take the JCM800 model. The graph on the SPKR page has a scale of +20 dB at the top. Look at the response at 2kHz. It's roughly 1/4 of full-scale which equates to 5 dB. If we look at the impedance curve for a typical 8-ohm speaker we see that the impedance at 2 kHz is roughly 13 ohms. For a 6.5 ohm voice coil (typical) this means that the voltage at the speaker is 6 dB higher at 2 kHz. Pretty darn close to what the graph is showing. While there is no high-frequency resonance in the speaker itself, a resonance IS formed due to the winding capacitance of the transformer. This capacitance resonates with the voice-coil inductance. Jay likes to brag about his experience yet he seems to miss some pretty basic concepts. => The negative feedback is set in the Advanced menu. The SPKR page only sets the impedance curve of the speaker/OT combo. The values chosen are prototypical for the speaker used with the modeled amp. You should not need to vary these parameters much IMO. I only ever vary Low Freq and High Freq. Whenever I'm matching an amp I adjust Low Freq to match the resonance of my reference cabinet. I occasionally vary Hi Freq to get more or less midrange bite." Source
•  Firmware 10: "Removed Mid Freq parameters from Speaker tab of amp block. MIMIC renders these controls irrelevant and better results are obtained by using any of the EQ resources."

SPKR DRV (Speaker Drive)

• This parameter simulates speaker breakup. It interacts with Master Volume. If you crank it, you'll get the sound of a blown speaker.
• Cliff: "The range of the speaker drive parameter is far greater than you would be able to push any real speaker before it self-destructed. If it doesn't sound good set that high, simply turn it down." Source
• Firmware 9.01: "Speaker Drive in Amp block now defaults to zero when changing model type."

XFMR DRV (Transformer Drive)

• Cliff: "Transformer Drive is exclusive to the II. It models the core saturation in the output transformer. The Drive increases the amount of core saturation." Source
• Cliff: "Don't overlook this when striving for "vintage" tones. I was playing around with this last night and it's very powerful in making edge-of-breakup tones sound like an old, well-played amp (if that's your thing)." (...) "Fenders, Vox, and other models where you crank the Master." "Yes, basically inverse to core size. The higher you set the drive the more it saturates the virtual transformer's core. It doesn't affect the B+, that's done with the Sag parameter." Source
• Firmware 6: "Since the new output transformer modeling is improved, more effective and more important to the tone, this parameter, XFRMR DRIVE, has replaced the SPKR DRIVE parameter on the DYN (Dyanmics) page of the amp block."
• Cliff: "The size of the transformer is dictated by the necessary power handling. You can simulate smaller/larger transformers by adjusting the Transformer Drive parameter."Source

EQ page

Graphic EQ

• The GEQ in the Amp block is fixed in position at the very output.
• Press Enter to reset all sliders to 0dB.
• Cliff: "The speaker tab is not an EQ. It allows you to adjust the impedance that the virtual speaker presents to the virtual power tubes. In most cases the resulting EQ is quite different than the impedance curve since negative feedback flattens the response. If you turn the damping all the way down then the EQ will be close to the impedance curve (but still influenced by the transformer)." Source
• Firmware 10: "Changed outermost bands in all graphic EQs to shelving types"

DYN page

SUPPLY SAG / MAINS IMP. (SAG)

• Turning Sag to zero disables power amp simulation for the preset. The Sag control has no effect at all when Power amp simulation is switched off in the Global menu.
• Cliff: "Supply Sag models the power supply resistance. This includes the power transformer, rectifier and any other resistances before the filter caps. The higher the resistance, the more the supply droops when current is pulled from it by the power tubes. The more the supply droops, the spongier the feel." Source
• Firmware 10: "Note that high values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes."
• Firmware 7.00: "Note that there are two dynamics controls for the power amp section. SUPPLY SAG controls how much the virtual power supply sags. This is a complex interaction between the master volume (MSTR), transformer matching (XFRMR MATCH) and screen network. Depending upon the amp you may even feel the screen voltage bounce if the screen network is underdamped (amps with chokes can often be underdamped). The screen network parameters are automatically set when the model is selected and cannot be altered by the user. DYNAMICS is an idealized dynamic range processor which controls the power amp response independently of the aforementioned parameters although it is still somewhat dependent on master volume. In general, the more heavily driven the power amp section, the more effect the SUPPLY SAG and DYNAMICS controls have."

DYNAMICS

• Firmware 5: "Added dynamics processing to Amp block. A new tab, “DYN”, in the amp block, allows adjusting various parameters of the dynamics processor along with several other parameters related to amp dynamics. The Dynamics parameter controls the amount of dynamics processing and models the interaction between the power amp, power supply and loudspeaker under high power-level conditions. The Dynamics Time parameter (ADV tab) controls the time constant of the associated processing. The Level parameter is duplicated on the DYN page for convenience."
• Cliff (about firmware 6): "V6 is much more open, like a real tube amp. If you like the compression of the old firmware turn up the Dynamics knob in the Amp block. It defaults to zero now as the new modeling handles all the sag naturally rather than using the "compressor around the waveshaper" technique found in other products." Source
• Firmware 7:
  • "The dynamics processing of the amp block was totally rewritten for this release. A complex set of formulas was developed that completely describe the various voltages in a tube amp. Unlike other modelers that simply model an amps dynamics as a first-order compressor, the Axe-Fx II now accurately models the complex interaction of the power tubes with the surrounding circuitry including the power supply and screen voltage network. You may notice a difference in the feel of the various amp models. Please refer to the descriptions below to understand the operation of the various controls. Doing so will enable you to adjust the dynamics to your personal preferences."
  • "The DYNAMICS control in the Amp block now allows negative values. Negative values cause dynamic range expansion while positive values work as before and cause dynamic range reduction. Use caution when dialing in extreme values as this can cause unwanted distortion."
  • "Note that there are two dynamics controls for the power amp section. SUPPLY SAG controls how much the virtual power supply sags. This is a complex interaction between the master volume (MSTR), transformer matching (XFRMR MATCH) and screen network. Depending upon the amp you may even feel the screen voltage bounce if the screen network is underdamped (amps with chokes can often be underdamped). The screen network parameters are automatically set when the model is selected and cannot be altered by the user. DYNAMICS is an idealized dynamic range processor which controls the power amp response independently of the aforementioned parameters although it is still somewhat dependent on master volume. In general, the more heavily driven the power amp section, the more effect the SUPPLY SAG and DYNAMICS controls have."
• Cliff (firmware 7): "The default is zero." Source

PICK ATTACK

• Firmware 9: "Controls a sophisticated dynamic range processor that operates on leading edge transients. Negative values reduce pick attack while positive values enhance it."

THUNK

• Firmware 10: "Added “Thunk” control to amp block. This parameter allows adding “weight” to tones by simulating the very low-frequency interaction of a speaker cabinet with the guitar. Higher values simulate closer proximity of the guitar to the cabinet."

XFRMR MATCH (Transformer Match)

• Firmware 3.0: "This is an extremely powerful parameter that sets the relative output transformer primary impedance which in turn controls how easily the power tubes are driven into clipping. The higher the Master Volume setting the more pronounced the effect of this parameter. Decreasing the matching causes the power tubes to clip later and therefore the phase inverter and grid clipping becomes more predominant. Increasing the matching causes the power tubes to clip sooner. At lower settings the speaker resonance will be more pronounced, at higher settings the speaker resonance will be less pronounced. For optimum results bring up the Master until the desired amount of power amp distortion is achieved, then adjust the matching until the character of the distortion is as desired. The various LF and HF resonance parameters interact strongly with this parameter so be sure to experiment with those as well when crafting your ideal tone. The value of this parameter is relative to the actual transformer matching which is set internally and not directly exposed. The value is reset to 1.0 whenever they amp type is selected."
• Cliff: "Very powerful control. Use in moderation. It changes the turns ratio of the virtual output transformer. Primary impedance is a function of turns ratio. As you increase the turns ratio you increase the impedance by the square of the turns ratio: Zp = N^2 * Zs. An easier description: Increasing Transformer Match -> Thick. Decreasing Transformer Match -> Scooped." Source
• Cliff: "One of the most powerful controls in the Amp Block Is Transformer Match. If you want a more "open" sound and feel, turn it down. If you want more compressed sound and feel, turn it up. A little goes a long way. Note that this control has more or less effect depending upon the setting of the Master Volume. Transformer Match has more influence at higher MV values and vice-versa. If you turn TM down, you may want to turn MV up to compensate and vice-versa. Turning it way up (around 2.0), for example, simulates the sound of running an 8-ohm speaker on the 4-ohm tap."
• Firmware 6.01: "As a result of amp matching tests, the Transformer Match internal values have been reduced. This may be detectable as a slightly more open and less compressed tone. If you desire the slightly more compressed sound of Version 6.00, this can be obtained by increasing the Transformer Match to a value of 1.1."

Source

• Cliff: "Don't overlook this parameter when your MV is set high. It is extremely powerful. A little in either direction can make a big difference. If you want a more open tone, turn it down slightly. If you want more compression and sustain, turn it up a bit. This parameter is essentially a "turns ratio" for the OT." Source
• Cliff: "Higher values are "warmer" but more compressed. Lower values are more open but harsher. Only small adjustments are needed. Transformer Match is the single most powerful advanced parameter when dealing with non-MV amps (i.e. when you have the MV cranked)." Source
• Cliff: "The most powerful advanced parameter is Transformer Match. When people try different tube brands or rebias their amp to use a different type of tube they make all kinds of hyperbolic claims about those tubes but it isn't really the tube that made the difference. Well it is but it's not because the tube is doing something special. It's simply because the tube has a different transconductance (gain). Amp designers choose an OT turns ratio such that the amp is "matched" to the load. However "matched" is a nebulous term since tube gains vary, speaker impedance is variable and bias point is adjustable. Therefore there is no absolute turns ratio that ensures perfect matching. Matching implies that the swing at the power tube grids just pushes the plates to the rails. If the output transformer is undermatched, the grids will clip before the plates hit the rails and vice-versa. Designers also select the turns ratio based on personal preference. Some designers prefer undermatched OT since this gives a more "open" sound, while others prefer overmatched since this gives more touch response. For example, a Trainwreck is highly overmatched. For a given OT, if the tubes have higher gain than originally then this effectively overmatches the OT and vice-versa. Now this matters most for non-MV amps that get their distortion from the power amp, i.e. old Marshall, Fender, etc. So... if you are going to experiment with any advanced parameter, start with Transformer Match. A little bit in either direction can make a big difference. Note that the Transformer Match parameter is relative to the internal value.". Source
• Cliff: "Transformer match has nothing to do with the physical size of the transformer. It is the turns ratio. The higher the turns ratio (higher Transformer Match) the higher the reflected impedance from the speaker and vice-versa. The higher the value the sooner the power tubes distort. The optimum turns ratio is such that the maximum power can be obtained. Tube amps tend to be slightly undermatched though since the speaker impedance is not constant. This varies with the make/model of amp and is encoded in the model data. The size of the transformer is dictated by the necessary power handling. You can simulate smaller/larger transformers by adjusting the Transformer Drive parameter." Source

ADV page

INPUT TRIM

• This parameter lets you adjust the range of gain of the amp. It's the same thing as the Amp Gain parameter in the Global menu but Input Trim operates per preset.
• Cliff: "They (Amp Gain and Input Trim) are basically the same thing. The global amp gain has a smaller range as it's designed to be for fine-tuning between guitars whereas the local trim allows you to radically alter the response of the model. The local trim is equivalent to -20 to +20 dB." Source
• Input Trim can be attached to a controller, using the Modifier menu, for a variable boost.
• Adam: "You might want to convert the Input Trim parameter to dB if you're used to thinking of it that way. As a rule of thumb, every 2x multiplier = +6dB boost. In other words, Input Trim = 4.0 produces a +12dB boost. Here's a handy calculator." Source

MSTR VOL TRIM (Master Volume Trim)

• See DRIVE, MASTER VOLUME.

LOW CUT FREQ, HI CUT FREQ

• Low Cut Freq controls the amount of lows the amp sim sees AT THE INPUT. Ranges from 10-1000Hz, with the lowest setting basically letting all the lows you feed it in. The main practical use for this is to tighten up a tubby bass end. Somewhere between 10-150Hz is generally where it will sound best for standard guitar tones. I sweep this by ear in that range to get a good balance of bass punch and defined string snap. 
• Hi Cut Freq is a low-pass filter positioned at the end of the preamp section that will Chop all frequencies above the value you select. Ranges from 2000-20000Hz. This will make your top end sound smooth and silky, the lower the value, brilliant and defined, the higher the value. Try changing values from stock when you want to fine tune a sound. I have found it useful for putting a vintage sounding ‘sheen’ on overly crisp patches (dropping the stock setting down).

DEFINITION

• Firmware 6: "Added Definition control to Amp block. This parameter allows changing the fundamental character of the amp from vintage to modern or vice-versa. Positive values increase the amount of upper overtone saturation whilst negative values reinforce lower harmonics."

CHARACTER, CHARACTER FREQ

• These two parameters control extremely powerful “inverse homomorphic filters”. When playing softly these dynamic filters have little effect on the sound. As the amount of distortion increases, the influence of these filters increases. The Character Frequency control sets the center frequency of the filters while the Character control sets how pronounced the effect is. For example, to darken the tone when playing harder, one might set the frequency to 10 kHz and the amount to -5. Setting the amount to +5 will make the tone brighter when playing hard. The amount defaults to zero whenever an amp type is selected.

TONESTACK TYPE

• The tonestack is the set of tone controls for an amplifier. Use Global > Config to set the default for all amps: active or passive. Use the Advanced page in the Amp block to select a different tonestack for an amp.
• "Passive" means the default stack for the amp is selected. So if you select a 59 Bassman amp model, you get the 59 Bassman tone-stack. If you select ODS-100, you get the Skyline tone-stack. You can over-ride the default tone-stack by selecting Active (for a generic active EQ) or selecting a tone-stack from the other amp models. IOW, if you select a 59 Bassman amp model and set the tone-stack to "Passive" it's the same as setting the tone-stack to "Bassguy".
• Forum member DonPetersen generated a frequency curve graph of the tonestack of each amp type in the Standard/Ultra, using white noise. You can view the graphs in this thread. Note that the graphs for the Axe-Fx II models will be different.
• Firmware 6: "Reworked most tone stacks based on amp matching results. In general most knobs now behave exactly like the actual amp when possible. In a few instances there may be minor discrepancies between the knob position of the model and actual amp due to programming constraints and/or peculiarities of the actual amp (such as poor potentiometer tolerance). Due to variations in presence circuit topologies the taper of the Presence parameter, in particular, may vary between the model and the actual amp. In other words, a different setting on the model may be required to achieve the same response as the actual amp. In most cases however, the Drive, Treble, Mid, and Bass knobs will be accurate to within 10% of the actual amp."
• Cliff: "In most cases the knobs do translate. Usually within 10%." Source

TONE LOCATIONS

• With some amp sims, such as the Lonestar, moving the tonestack results in a loss of volume.

GRID MODELING, Grid Excursion

• Firmware 4.00 added the Grid Modeling parameter. This answers requests from users to be able to remove "real world" high frequency fizz from tones.
• Cliff: "I prefer the "fizz" on probably because I grew up with it. I just like the extra grit. When you're playing in a group context that grit seems to make the guitar cut better and fills out the sound. Without it things sound sterile. I spent months trying to capture that. One day one of my employees came by the lab while I was working on the new algorithms. I was trying to explain the grit to him that I heard in my JCM800. "Hear that sizzle on top of the notes? Hear that raspy, bacon frying sound? That's what modelers are missing." So I spent months figuring out where that came from and how to replicate it. All IMHO..." Source
• Cliff: "The input of a tube looks like a diode. If the grid voltage exceeds the cathode voltage by a few volts it starts to conduct. This does two things: 1. It clips the waveform. 2. It shifts the bias since it it charges the coupling cap between the stages. A little grid conduction actually fattens up the tone. Too much can sound "farty"." Source
• Firmware 5: "Much improved grid modeling in Amp block preamp and power amp stages. New modeling very accurately replicates grid conduction and resulting bias excursion. This results in a more dynamic, thicker and bouncier tone. The power tube grid conduction parameters are exposed to the user in the GUI. The Bias Excursion parameter controls how much the grid voltage droops when the grids conduct. The Excursion Time and Recovery Time parameters control the time constants associated with the excursion."
• Firmware 6.01: "Changed Grid Modeling switch in amp block so that Off turns off ALL grid modeling including preamp tubes."
• Firmware 10: "Removed grid excursion parameters. These parameters can still be accessed via Axe-Edit."

PWR TUBE TYPE

• Firmware 10: "Added Tube Type parameter to amp block. This allows selecting Tetrode (i.e. 6L6, KT66, etc.) or Pentode (i.e. EL34, 6BQ5, etc.) power tube types. The type defaults to the appropriate value when a model is chosen but may be overridden by the user."

DAMPING

• Adjusting this parameter affects the amp's volume level. If the power amp is saturated, both damp and level must be increased to maintain level the same.
• Cliff: "The Axe-Fx attempts to normalize the volume as you change the damping. (...) However, if you are driving the "power amp" hard the equation falls apart because it assumes linear operation. Therefore there may be some volume change. This is done since otherwise you would constantly have to adjust your output volume as you change the damping. Unfortunately it is impossible to predict how saturated the power amp is since that depends on input level. The compensation isn't perfect, the idea is to minimize the volume fluctuations since without compensation the volume would fluctuate wildly."
• Cliff: "Note that when you set Damping to 0 that Presence becomes a Hi-Cut so if you have the Presence turned up you'll lose high end when you turn Damping to 0. Also, Depth does not work at a Damping of 0 since Depth modifies the feedback and there is no feedback." Source

PWR SUPPLY TYPE

• Firmware 10: "Amp block power supply modeling now models AC rectification and resulting supply ripple (if Pwr Supply Type is set to ‘AC’). The power supply type can be selected between AC and DC with the Pwr Supply Type parameter. The line frequency can be selected with the AC Line Freq parameter. Note that high values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes."

B+ TIME CONST

• Cliff: "It's both attack and release. B+ Time Constant is the time constant associated with the Supply Sag parameter. The power tubes draw current from the supply. The supply has a finite resistance. As the power tubes draw more current the supply voltage droops. The rate of change of the droop and recovery is dictated by the supply capacitance. The product of the resistance and capacitance is the time constant. It's typically around 10 ms. You can vary this using the B+ Time Constant parameter. It is not a simply compression though. As the supply sags, the headroom is reduced but many other things happen. One thing that happens is that the screen voltage droops. The screen voltage is derived from the B+. However the screen has it's own dynamic response, which is often 2nd-order since there is often a filter choke. If you listen carefully to the models with a filter choke you can hear the screen voltage "bounce" when you hit a power chord. The damping of the screen filter is not exposed to the user. When the screen voltage droops, the power tube gain decreases. It effectively shifts the bias point. There is quiescent draw from the supply as well. As you increase the bias (Power Tube Bias) the quiescent draw increases which decreases available headroom. The Axe-Fx II does not model all this stuff with compressors, like other products do. It actually uses a differential equation for the supply and the current from the power tubes. It then solves the equation at each sample instant to find the supply voltage and screen voltage." Source
• Cliff: "The effect of lower B+ is equivalent to increasing Transformer Match. A lower B+ means the plates clip sooner which is the same as increasing the turns ratio on the transformer. This is assuming that you rebias since typically lower the B+ affects the bias." Source
• Firmware 10: "Note that high values of Sag along with low B+ Time Constant values can cause “ghost notes” when the supply type is AC (as in a real amp). Lower B+ Time Constant values will make the amp feel “faster” but too low can cause ghost notes."

AC LINE FREQ

• See PWR SUPPLY TYPE.

TRIODE PLATE FREQ

• Cliff: "It sets the cutoff frequency of the resistor/cap combination on the plate of the last triode stage (the previous stages are not user adjustable). Most amps have no cap on the last stage but a few do. You can vary this parameter to simulate increasing/decreasing the capacitor value. The frequency is only approximate since the actual frequency varies with the bias point/cathode impedance/drive/etc."
• Groovenut: "It will give you some control over the high harmonics that are created during clipping. The cap in question forms a low pass filter with the plate resistor on the triode stage. In English, it will allow you to control the buzziness that sometimes occurs with higher gain settings. It can also serve as a gain dependent tone control of sorts." Source
• Firmware 2.0 and up also expose Triode1 Plate Freq. and Triode2 Plate Freq. Release notes: "This parameter sets the cutoff frequency of the plate impedance for the next-to-last triode 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 this parameter. The last triode plate capacitor is also exposed: Triode2 Plate Freq."

MV LOCATION (Master Volume Location)

• Cliff: "Most amps are Pre-PI, including Dumbles. Post-PI is rare and often does as a mod. This causes the PI to distort rather than the power tubes. It is a harsher sound." Source

AMP VOICING

• Firmware 3.0: "This parameter voices the amp to a variety of tonal styles. Voicings take the guesswork out of mix engineering by automatically crafting the tone like a professional engineer would. Choose “Neutral” for the raw amp sound. Choose one of the other voicings to rapidly achieve a mix-ready tone."
• Voicing adjusts the PWR AMP LOCUT and HICUT parameters and applies some parametric EQ.
• Firmware 10: Amp Voicing has been moved to the Advanced tab.

TRIODE HARDNESS

• Firmware 2.0: "This parameter controls how sharply the triodes enter saturation and can be used to simulate softer or harder tubes. The default value is 5.0 and is set to this value whenever the type is changed. The effect of this is subtle and most apparent at edge of breakup. Lower values give softer saturation, higher values give a more aggressive breakup. NOTE: Existing presets should be checked as this value may load to a value of other than 5.0 depeding upon what version of firmware was originally used to create the preset. All factory presets have been reworked as the factory presets will load to 0.0."
• Firmware 6.02: "When resetting an amp, this value defaults to 0.0. To achieve the sound of 6.00 firmware, set this to 5.0. Note that all presets created prior to Version 6.02 will have this value set to 0.0. You may override this value by setting the desired value and then saving the preset."
• Firmware 10: "The Triode Hardness parameter operates differently than before and also defaults to an appropriate value when the model type is selected. Existing presets are automatically updated upon recall."

PREAMP BIAS

• Firmware 7: "Exposed the bias point of the last tube stage in the preamp modeling. This parameter, called PREAMP BIAS, sets the bias point of the last triode (cathode follower not counted). Depending on the bias points of the previous stages increasing or decreasing this value can alter both the harmonic content and the attack characteristics. Typically, if the previous stage has a negative bias then increasing this value will be more noticeable and vice-versa. This value is set to a default value for the model whenever the type is changed but can be overridden by the user."

TREM/MIX page

TREM FREQ, TREM DEPTH (Bias Tremolo)

• Firmware 10: "Added Bias Tremolo to Amp block. This is a true bias tremolo and works by varying the bias of the virtual power tubes. The tremolo action is therefore different than other types of tremolo and the amount of tremolo varies with a multitude of variables, most importantly the tremolo is “self-ducking” and decreases at higher signal amplitudes. Note particularly that bias tremolo is a somewhat crude tremolo circuit and it’s interaction with the power amp depends on many things including damping, bias, etc. On some amps high values of bias trem depth can result in excessive crossover distortion. On other amps the amount of tremolo can vary greatly between loud and soft playing. All this, however, is part of the allure of bias tremolo as it results in a particularly “organic” sound. Control of the bias tremolo is afforded by the Trem Freq and Trem Depth parameters. A modifier can be attached to Trem Depth to facilitate engaging and disengaging the tremolo via footswitch or for other applications."