Amp block

User contributions

Amp modeling developments

• The amp modeling in the Axe-Fx II is called G2 and Virtual Vaccuum Modeling modeling (see Fractal Audio website). Part of it was 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."
• 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."
  • Cliff: "The amp models in V10 sound exactly like their real-world counterparts. If you don't like the model, then you won't like the amp. The models are statistically matched to the amps in both frequency response and distortion profile. In some cases this gives a certain sizzle. Some may not like this but it is accurate." Source
  • Cliff: "The amp modeling in V10 is very similar to V9. There are only a few changes to the algorithms. The big difference between V9 and V10 is the inclusion of the MIMIC data and many of the cabs were remixed. Of those two the remixed cabs will be far more audible. So to get that V9 sound the most important thing is to go to the cab block and select the equivalent V9 cab(s). These are indicated by "(V9)" in the name. For example, if you preset is using "1x12 Brit G12H30" it has a remixed cab. Therefore change the cab to "1x12 Brit G12H30 (V9)". Source
• Firmware 11:
  • "Improved pre-amp algorithms."
  • "Improved cathode follower algorithm. There are three parameters exposed for the cathode follower algorithm: Cathode Comp (which is also the COMP knob), Cathode Time and Cathode Ratio. Cathode Comp sets the amount of compression. Cathode Time sets the attack time of the compressor. Cathode Ratio sets the maximum amount of compression with lower values giving more compression."
  • "Improved power amp algorithms. New algorithms yields smoother highs and more open sound."
• Cliff: "If an amp has just "Tone" then that's mapped to Treble. Leave bass and mid at noon." And: "If they don't have a presence/hi-cut that should be set to zero." Source
• Cliff: "We have 50+ real amps here. These amps have been MIMIC'd and the models sound exactly like the actual amps. There are, however, a bunch of models in the Axe-Fx II that are not matched to any real amp. They are purely theoretical. They sound great as well. In the early days of the Axe-Fx all the models were based on theoretical analysis. Then we got into matching the models to real amps. Some of those early models were inaccurate but still sounded great. So rather than replace them we simply added new models and left the old ones. Das Metal was one of these theoretical models that sounds great. Instead of replacing it with matched VH4 models we added models. Best of both worlds." Source
• Firmware 12:
  • "Improved Amp block power supply current draw modeling which improves “feel”."
  • "Improved Amp block phase-inverter-to-power-tube-grid interaction modeling which improves transient response resulting in faster attack. This also improves the clarity of notes especially when using heavily overdriven power amp settings."
  • "Added Global “Modeling Version” parameter to select between Version 11.xx or Version 12.xx power amp modelling."
  • "The Amp block “Grid Modeling” parameter has been renamed and repurposed. The new name is “Modeling Mode” and offers the following choices:
    • Authentic – Replicates a tube amplifier with the utmost accuracy.
    • Grid Cond Off – Turns off grid conduction modeling in the power amp simulation. This reduces blocking distortion.
    • Smooth – Sets Triode Hardness to the minimum value essentially creating an ideal preamp and turns off grid conduction modeling in the power amp. This removes most of the “nasty” distortion that tube amps create.
    • Ideal – Removes most of the “warts” from the modeling which includes grid conduction, output transformer distortion, bias shifting and AC power supply modeling. Supply sag, screen voltage effects and crossover distortion are still modeled.
    • Ideal/Smooth – Same as #3 but also sets Triode Hardness to its lowest value. This is in essence an ideal preamp plus an ideal power amp.
  • The audibility of these settings is dependent upon the particular amp model and various parameters. The use of these modes in conjunction with other parameters can yield idealized tones not achievable with real tube amps. The first two choices duplicate the previous behavior of the deprecated “Grid Modeling” parameter so no changes are required to existing presets. Note that when “Ideal” is chosen the global Modeling Version parameter is irrelevant."

MIMIC

• See MIMIC.

Splitting pre-amp and power amp

• The pre-amp and power amp sections of an amp sim cannot be split.
• The power amp modeling can be disabled though, either globally (Global menu) or per preset (turn Sag to zero). Turning off power amp simulation doesn't work well with amp sims which fully rely on power amp distortion such as the Wrecker sim.
• If you only need power amp simulation, for example when using your own pre-amp, use the Tube Pre model. It uses the Vintage tonestack which is flat when the tone controls are at noon. Source

What happened to Global Amps

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

Using a single or dual Amp blocks (HiRes/Normal resolution)

• Each preset can have up to two Amp blocks. One DSP is devoted entirely to the Amp block(s). 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, the Amp block (automatically!) runs at double the internal sampling frequency. Main benefit of this is less aliasing in high gain models. Firmware release notes: "In high-res 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 And: *I don't hear a difference either but some claim they can. A single amp block runs the amp simulation at 16x oversampling. Two amp blocks run each simulation at 8x." 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.
• Cliff: "Don't put amp blocks in series. Especially when one is clean and the other is high-gain. Always put them in parallel and set the bypass mode to mute." Source
• When using dual Amp blocks (simultaneously), adding a Drive block before one of them may cause latency. Try adding a (bypassed!) 2nd Drive block to the other row. Source
• Switching between two "amp routings". There are various ways to switch between two amps. For example by using scenes or XY. If the routings also need to contain things like a Drive block or Compressor, here's how you do it, courtesy of Bakerman (source):
  1. Set IA switch to Vol/Pan 1 Bypass.
  2. At Axe-FX I/O: CTRL page set Vol/Pan 2 to the same CC# as Vol/Pan 1.
  3. Begin one path with Vol/Pan 1, the other path with Vol/Pan 2.
  4. Make these settings:
  5. Vol/Pan 1: Volume 10, Bypass mode = mute
  6. Vol/Pan 2: Volume 0, Bypass mode = thru
  7. Store the Vol/Pan blocks both on or both off per scene. You'll get signal from #1 only when they're engaged, #2 only when they're bypassed.
• When using a single Amp block in a preset, use Amp 1, or you might experience a "pop" when switching to that preset.

Audio gap when editing an Amp block

• If you press Edit in the Amp block or use X/Y, 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 completely. This will reset ALL block parameters.
• Selecting another amp type and then the previous one again, will reset all Amp block parameters to their default values for that particular model, except:
  • Input Drive
  • Bass, Middle, Treble
  • Presence/HiCut
  • Level
  • Balance
  • Input Select
  • Voicing
  • Pwr Supply Type
  • Trem Freq
  • Bypass Mode

How to adjust 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 one or more Drive parameters 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.
  • Increase Master Volume for more power amp distortion.
  • Increase Mstr Vol Trim on the Advanced parameters page.
  • Use Boost in the Amp block. Also, switching on Bright often increases gain.
  • Engage the Saturation switch.
  • Increase Level on the IN/GTE page (firmware 11 and newer).
  • 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 (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."
  • Unlike the Standard and Ultra, adjusting Input Level in I/O does not affect 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

About fizz

• Cliff: "Fizz is desirable. Almost all high gain amps use a lot of feedback on the last triode stage to make it as "hard" as possible. This is typically done by using no cathode cap or a small cathode cap. You never see big cathode caps on the last stage of a high-gain amp. This gives lots of negative feedback and makes it clip harder. Another technique is to use negative feedback to the grid. This is done in 5150-based designs which makes the stage clip even harder. The last stage of a 5150 basically looks like a diode clipper. The harder the clipping the more overtones that are created and the better the tone cuts. Without those overtones your sound is lost in the mix. Amp designers know what they are doing. Trust them." Source
• Cliff: "Fizz on the decay is natural and it's what tube amps do. If you don't like it you can reduce the Triode Hardness but then it won't cut in the mix as well." Source
• Cliff: "The "fizziness" of clipping is determined by how "hard" the clipping is. There are three primary places that clipping occurs in a tube amp: the preamp tubes, the phase inverter and the power tube plates. Preamp tube clipping can range from soft to hard depending upon the design. Phase inverter (PI) clipping, which is actually the power tubes grids clipping, is very hard. Power tube clipping ranges from soft to hard depending upon the amount of negative feedback in the power amp. Preamp tube clipping is comprised of cutoff, which is soft, plus saturation, which tends to be hard. Actual saturation rarely occurs because most preamp stages are designed such that the grid clips before the tube enters saturation. Grid clipping is hard. Local negative feedback is used in the form of cathode caps to shape the response of a preamp stage. If there is no cathode cap then there is negative feedback at all frequencies which increases the hardness of the clipping. The last stage usually dominates the clipping. Some amps have no cathode cap on this stage, e.g. JCM800, and therefore have hard preamp clipping. The Axe-Fx II does not expose the negative feedback settings for the preamp stages to the user, these are hard-coded. Reducing the Triode Hardness parameter will soften the clipping more-or-less depending upon the particular amp model. In a typical tube amp the power tubes start to clip right about the same time the PI/grid clipping occurs. This is intentional so as to get the most power from the tubes. However some amps are intentionally mismatched as the designer's intent was to get more power tube clipping than PI clipping (i.e. Trainwrecks). The Transformer Match parameter adjusts the relative onset of power tube vs. PI/grid clipping. Lower values will cause the PI/grid clipping to occur before power tube clipping. Higher values will cause the power tubes to clip before the PI. Note that the power tube plates follow the impedance curve of the speaker so while the PI/grid may be designed to start clipping first, this only occurs in the midrange. At frequencies above 1 kHz or so the power tubes clip first since the voltage on the plates increases as a function of the speaker impedance. The first thing to clip tends to dominate as once you enter clipping the effect of clipping elsewhere is diminished. Negative feedback around the power amp attempts to linearize the transfer function. The more negative feedback the more the power amp is linearized. However this also causes the clipping to become harder. A power amp with no negative feedback will go into clipping softly. As you increase the negative feedback the "knee" gets sharper. The Damping parameter is the negative feedback control. Higher values give more feedback and harder clipping. Presence and Depth work by modifying the negative feedback. As you increase them the feedback gets less so by turning up the Presence you get softer clipping in the power amp. Therefore to decrease the hardness of the power amp clipping: reduce Damping, increase Presence, increase Transformer Match. To reduce preamp clipping hardness reduce Triode Hardness. There is no parameter exposed to adjust the PI hardness. HOWEVER, the relative hardness of clipping is not all that audible. You have to listen closely. The IR is far more important in the final result. Some IRs let through a lot more high frequencies and therefore sound more fizzy. Furthermore overanalyzing this is inadvisable. Many amps are specifically designed to clip hard as this gives a more aggressive tone that fits better in the mix. Some amps actually attempt to increase the hardness of the clipping as much as possible by using diode clipping or using very high values of negative feedback (i.e. Modded Marshalls, Camerons, 5150 III). Listening at low levels fools your ear. Our ears are more sensitive to midrange at low listening levels. This means we hear the clipping differently than when listening at the actual level the real amp would be generating. Source

Comparing real controls with the model

• Cliff: "If the amp has no Master Volume, set the MV to 10 (the model will default to 10 when you select it). If the amp has no midrange control, set the MID to 5.00. If the amp only has a "Tone" control, set Bass and Mid to noon and the Treble control is your tone control." Source
• If the real amp has two gain controls, the one which is the closest to the 1/4" input is modeled as Input Drive in the model. The other one is Overdrive.
• If the real amp has two inputs (f.e. low and high) the model is based on the high input. Set Input Trim to 0.500 to get the equivalent of using the low input. 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

INPUT DRIVE, OVERDRIVE, MASTER VOLUME

• Some models have Input Drive and Overdrive controls. Input Drive is the same thing as Drive in earlier firmware versions.
• Vintage amps don't have separate gain (drive) and master volume controls. Master Volume defaults to "10" in these models. Use Input 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."
  • "New MSTR VOL TRIM parameter in the Advanced GUI page can be used to increase (or decrease) the Master Volume."
  • Cliff: "Just multiplies the MV by the amount. You only need to use it if you want more power amp drive and your MV is already at 10. IOW, if MV is 10 and you set MV Trim to 2.0 then the MV will be 20." Source
• Firmware 10.10:
  • "The Amp block now differentiates amps that have both Input Drive and Overdrive controls, i.e. Mesa Mark series, Dumble, etc. When a model is selected for amps of this type, the menu shows both controls. For other types the menu shows only the Input Drive control (which was formerly called simply “Drive”). The Overdrive control defaults to noon when amps with this control are selected. As such, any presets based on these amps may need to be updated as this control was not present previously and the amount of drive may differ now. Note that these two controls are applied to the appropriate point in the circuit for the amp being modeled, i.e. for Dumble-style amps the Overdrive is prior to the last triode stage, in Mesa Mark amps the Overdrive is applied prior to the third triode." Models featuring Input Drive and Overdrive are: Bludojai Lead, ODS-100 Lead, CarolAnn OD2, Fox, USA IIC+, USA Lead, Fryette. Cliff: "Input Drive increases the gain amount as you rotate the knob clockwise. As the gain increases the tone is shifted from a treble and upper mid emphasis, which produces an up front sparkling tone, to a lower mid and bass emphasis, which produces a thick meaty tone. Overdrive increases the gain amount as you rotate the knob clockwise but with no alteration of the tonal balance. Different combinations of Input Drive and Overdrive settings will have a dramatic effect on the response of the amplifier and the personality of your instrument. It is easy to get familiar with the action of these controls and you’ll be amazed with your ability to make any guitar sound mellow, fat, soulful or aggressive." Source
  • Cliff: "In a typical amp Input Drive is called various names (Drive, Volume, Gain, etc). It is the knob closest to the input jack. In many cases this potentiometer has a bright cap on it so the frequency response will be dependent on the knob position. In some amps there is also a second drive control. This is your Overdrive knob. It does not have a bright cap so it only affects the gain." Source
• Cliffs presentation on Axe-Fest 2013: MV is the most important Amp block control for tone. You have to find the sweet spot. Start at 3 and increase until desired compression is reached. Stock presets are set to sweet spots, subjectively (based on the guitar used and personal opinion). Do not use MV for volume and don't turn it up too much (unless it's a non-MV amp). If an amp has Input Drive and Overdrive controls, use Input Drive for tone shaping and Overdrive as a flat gain control.

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.
• Firmware 9: the BOOST switch is now a dedicated knob and also modifiable so it can be activated remotely.
• Firmware 10.10: for amp models which have Input Drive and Overdrive controls, Boost is now located in the Adv menu.
• Enabling Boost is the same as setting Input Trim at 4.

BASS, MID, TREB

• These are the usual tone controls: Bass, Middle and Treble. The exact behavior depends on the amp model.
• 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

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."
• Firmware 11: "Improved cathode follower algorithm. There are three parameters exposed for the cathode follower algorithm: Cathode Comp (which is also the COMP knob), Cathode Time and Cathode Ratio. Cathode Comp sets the amount of compression. Cathode Time sets the attack time of the compressor. Cathode Ratio sets the maximum amount of compression with lower values giving more compression."

CUT

• Firmware 10.12: "When active this reduces the amount of low frequencies into the amp simulation. This can be used to achieve a “tighter” tone or to reduce low-end “flub”. This is similar to increasing the Low Cut frequency but still retains some low end so it doesn’t get thin."
• Cut is a first-order shelving filter (high-pass) at 120 Hz. Adam Cook: "You can use a Filter block before the Amp set to Shelving if you want to add more flexibility to what the Cut switch is doing in the Amp block." Source
• Cliff: "120 Hz is where most amp designers put it. A typical cathode bypass has the pole at approx. 85 Hz. Assuming 6 dB gain reduction that puts the center frequency at 120 Hz." Source
• Cliff: "The bass cut switch is before the distortion so it will change the feel and breakup characteristics. The bass cut is basically intended to give you that Tube Screamer with Drive on 0 sound without having to use a dedicated Drive block." Source

FAT

• Located under the Mid knob. When engaged it shifts the center frequency of the tone stack 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
• Cliff: "The Fat Switch multiplies the tone stack treble cap by four. Depending upon the type of tone stack, tone control settings, position, etc., etc. the effect can be more or less noticeable." Source

BRT (Bright)

• The bright cap in real amps is used to compensate for guitars with weak pickup loads. Treble increases as gain is decreased. It changes the mids which affects gain too.
• Cliff: "The Bright switch always controls the bright cap on the input volume". Source
• Turning up the amp's Drive or Master may decrease the impact of the Bright switch, depending on the amp type.
• It's possible to change the effect of the Bright switch on the tone. In the Advanced parameters adjust the Bright Cap value.
• Some amp types, such as Plexi, the Bright switch is set at a very high value. It has a large impact on the amount of gain.
• Cliff: "If an amp doesn't have bright switch the operation of of the models' bright switch is undefined. I chose what I considered a reasonable value for the bright cap but if that doesn't satisfy the user then they are free to change it to a different value." Source

SAT (Saturation)

• Cliff: "It switches in a zener diode clipping stage right before the tone stack. This is the so-called Jose Arrendondo Mod." Source
• Enabling it decreases power amp smoothing which results in meaner distortion.
• This parameter is enabled by default in certain models such as Cameron ch. 2 amp and 5153 Blue and Red. Try it with amp types such as Plexi, JCM800, Friedman and Mesa Mark.

PWR page

PRESENCE / HICUT

• These are tone controls for the power amp section.
• 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. 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: "HiCut 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
• From the MIMIC whitepaper: "Note however that the taper of the presence (and depth) control can deviate from the actual amp. In our tests we found that the presence control on many amps did nothing for the first 80% of its rotation and all the action occurred in the last 20%. We feel that this design anomaly is undesirable and therefore did not model that aspect."

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
• Cliff: "Depth does not work at a Damping of 0 since Depth modifies the feedback and there is no feedback." Source
• From the MIMIC whitepaper: "Note however that the taper of the presence (and depth) control can deviate from the actual amp. In our tests we found that the presence control on many amps did nothing for the first 80% of its rotation and all the action occurred in the last 20%. We feel that this design anomaly is undesirable and therefore did not model that aspect."

DYN PRES (Dynamic Presence)

• Release notes: "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."
• Another way to look at it: a distortion-dependent treble filter.

DYN DEPTH (Dynamic Depth)

• Release notes: "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."
• Another way to look at it: a distortion-dependent bass filter.

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. Adjusting Level has NO impact on the tone.

SPKR page

Speaker impedance: Low and High Frequency Resonance, Transformer parameters

• 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
• 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.
• 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."
• 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 5.00: "The value of this parameter sets the “corner frequency” of the impedance rise due to voice-coil inductance (technically this is a “semi-inductance”). The actual impedance seen by the virtual power tubes is then internally calculated based on the transformer and power tube parameters. Typical guitar speakers have a corner frequency between 1 kHz and 2 kHz. This value is preset based on the model but the user can override the value as desired. Many speaker manufacturers publish impedance data for their drivers which can be used as a reference point. Lower values give more midrange emphasis. For convenience, the transformer low-cut and high-cut frequencies are now present on the SPKR page and their influence on the open-loop response is reflected in the impedance graph."
• Firmware 5.02: "HF Resonance: this control is similar to the previous control but only changes the slope of the resonance. The default value is consistent with the typical “semi-inductance” of a speaker voice-coil. Varying this value will change the high-frequency load presented to the virtual power tubes."
• 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."
• Cliff (firmware 10): "The default LF Resonance is based on the cab most likely to be used with that amp. For example, if you select a Twin Reverb model it will use the resonance data from the Twin Reverb amp that we used as the reference (we measured all the cabs as well as shooting IRs). Your 1960A cab probably has its resonance around 80 Hz so ideally you would want to do exactly what you did. The ultimate solution would be to measure the LF resonance but that requires special test equipment. I've found that you can usually find it by ear though. As you adjust the LF resonance you'll hear the cabinet sympathize. There is no way around this at the moment as no modeler can measure the impedance of a guitar cabinet (despite any claims to the contrary). However, I have an idea for the next generation of products that solves this. Note that this is not applicable when using FRFR and IRs."
• Cliff: "The resonant frequency goes up when mounted in a cabinet. I would estimate that it is going to be around 100 Hz. It doesn't need to be spot-on. If you are within 10% you'll be fine. If you want to be anal about it you can use an impedance analyzer. This is what we use: Dayton Audio DATS Dayton Audio Test System 390-806." Source
• Cliff: " Guitar folklore has it that SRV and Joe Walsh intentionally mismatched their speaker impedance. I imagine others have done this. The general idea is you plug an 8-ohm speaker into the 4-ohm jack or vice-versa. The Axe-Fx allows you to replicate this behavior using the Transformer Match control. To simulate plugging an 8-ohm speaker into the 4-ohm jack set Transformer Match to twice it's current setting (i.e. 2.0). For the other way around set it to half (i.e. 0.5). And you don't have to worry about frying your OT. BTW, for this to be audible the Master Volume needs to be set very high so that you are clipping the (virtual) output tubes. Most amps set the impedance ratio of the OT so as to get maximum power from the power tubes (within the SOA). Some amps intentionally mismatch the OT to give more control over the distortion by riding the volume control, i.e. Trainwrecks. Most amps actually slightly undermatch the OT as the speaker impedance is greater than nominal outside of the midband. Trainwrecks are overmatched.Source and Source2
• Cliff: " Most guitar speakers are roughly the same when it comes to the high-frequency reactive behavior. The impedance increases starting around 1 kHz and then increases at 3-4 dB/octave. This is due to the voice coil inductance. A pure inductance would increase at 6 dB/oct. but there are eddy current losses that make the voice coil look "semi-inductive". The Axe-Fx II models this with a high-order lossy inductor model. The low-frequency response of guitar speakers, however, varies greatly between speakers of different makes and models. This low-frequency response is a sharp resonance typically in the range of 50-150 Hz. The magnitude of this resonance varies from several to 20 times the nominal impedance.

The impedance of a speaker influences the response of a tube amp since a tube power amp is essentially a transconductance amplifier. It creates a current for an applied voltage. This current in turn creates a voltage across the speaker terminals that is dependent upon the impedance of the speaker. Therefore the power amp will resonate at the resonant frequency of the speaker. This causes certain notes to become emphasized as they excite the resonant frequency. Negative feedback around the power amp will reduce the amount of resonance but not all amps use negative feedback (i.e. Vox). The increased voltage amplitude at the resonant frequency also causes the power amp to clip sooner at the resonant frequency. Think of it this way: if the power tubes are swinging, say, 200 V at the midrange frequencies, they will swing X times more at the resonant frequency where X is the ratio of the resonant impedance to the nominal impedance. So if the resonant impedance is 10 times the nominal impedance the power tubes will want to swing 2000 volts. This is impossible so they will clip. For high-gain tones this can cause the tone to sound muddy or feel spongy. For lower gain tones this can thicken the tone and make it feel, well, more spongy. Cabinet/speaker IR data does not contain the impedance information. The only way to obtain impedance data is to measure the current vs. voltage vs. frequency (despite what modeler advertising literature would like you to think). The Axe-Fx uses default values of LF Resonant frequency and impedance for each amp model. For models based on combo amps these values are derived from measurements of the actual amp's speaker. For models based on amp heads the values are based on measurements of the cabinet most likely to be used with that head. You can adjust the frequency and impedance to suit your taste. Reducing the impedance (Low Res) will reduce the bass response and can give tighter bass. Raising the impedance will increase the bass response and can give a fuller sound. Altering the frequency (Low Freq) will change the frequencies at which the power amp resonates and tuning this to the key you are playing in can be an effective strategy, e.g. set it to 82 Hz if playing in E. Don't be afraid to try drastic settings. Try turning Low Res all the way to zero. Compensate by adding some bass with the Bass knob or the EQ section. As I mentioned earlier the LF Resonance will cause the power amp to clip earlier than it will when amplifying midrange frequencies. Turning down the Master Volume will increase the headroom in the power amp and reduce this clipping. Furthermore the Transformer Match also influences when the power amp clips. So there is a relationship between LF Res, MV and Transformer Match. Many manufacturers publish impedance data for their speakers. Eminence and Jensen and probably others publish detailed impedance data. You can look at the impedance plots and set the resonance parameters to match (roughly). The Low Res parameter is indicated from 0 to 10 and sets the resonance in dB from 0 to 24 dB (dB is a ratio of powers so it's not really the proper units for this but that's semantics). For example, the Jensen P12N has resonant frequency of about 100 Hz so you would set Low Freq to 100 Hz. The impedance at this frequency is about 40 ohms. To get the Low Res amount use the formula (20 * log10(Zr/Rdc)) / 2.4 where Zr is the impedance at the resonant frequency and Rdc is the DC resistance. For this speaker Low Res is then (20 * log10(40/6.2)) / 2.4 = 6.7. A power amp isn't perfect though. Winding resistance in the output transformer increases Rdc, typically by a couple ohms. Therefore our above example would become (20 * log10(40/8.2)) / 2.4 = 5.7. The exact value isn't overly critical though and all this is subtle nuances. The resonance Q is a bit more difficult to calculate. It is derived from the bandwidth at the points where the impedance "gain" is the square root of the resonance impedance gain. IOW, if the impedance is, say, 10 times the nominal impedance then the bandwidth is given by the frequencies where the response is 3.16 times the nominal impedance. For our example the resonance gain is 5 (40 / 8 = 5). So the bandwidth is the frequencies at which the impedance equals sqrt(5) * 8 = 18. From the graph this is approximately 75 Hz and 130 Hz. Q is defined as f0 / bw so our resulting Q is 100/60 = 1.67. Most speakers have a Q of around 2.0 or so. Again the exact value isn't overly critical and don't be afraid to try extreme settings (you can't break anything). Finally, just because real speakers behave like this doesn't mean we have to adhere to this behavior. Perhaps a better speaker has no resonance (Low and High Res are zero), or maybe the Q is a lot lower or higher. In our virtual world we can design a speaker that is impossible to construct in the physical universe. tl;dr version: Mess with Low Freq and Res if you want, or not." Source

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
• Firmware 10.10: "Transformer distortion modeling is now independent of transformer match value. Before the amount of distortion was also dependent on the match value making adjustment more difficult."

EQ page

Graphic EQ

• The GEQ in the Amp block is fixed in position at the very output of the preamp, so it's between the preamp and power amp stages.
• Press Enter to reset all sliders to 0 dB.
• Firmware 10: "Changed outermost bands in all graphic EQs to shelving types."
• Firmware 11: "Added EQ Type parameter to Amp block. This allows selecting between an 8-band, 7-band or 5-band EQ. The 7-band and 8-band types emulate popular graphic EQ pedals. The 5-band type emulates the response of the on-board EQ in the Mesa Boogie Mark series amplifiers. Note that 5- and 7-band types are non-constant-Q designs whereas the other types are constant-Q designs. When selecting amp models based on Mesa amps the type automatically changes to 5-band."

DYN page

SUPPLY SAG / MAINS IMP. (SAG)

• Turning Sag to 0 disables the entire 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 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."
• 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."

DYNAMICS

• The control affects compression. Adjust this to make the amp sound tighter or looser.
• 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."
• 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."

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)

• This parameter controls power amp clipping. Similar to adjusting MV and to mismatching a real amp and speaker. Decrease it to make the sound broader.
• 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." 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
• Cliff: "The internal default value is based on the amp that was modeled and an assumed speaker voice coil resistance of 0.8 times the nominal impedance. I.e., if the speaker is rated at 8 ohms the assumed voice coil resistance is 6.4 ohms. Some speakers are slightly below this, others are above. 16-ohm Celestion Greenbacks, for example, are about 12 ohms so that would 0.75 times the nominal impedance. To simulate this you would reduce the matching to 0.75/0.8 = 0.9375. If you find yourself lowering this value consistently then your Master Volume is too high (assuming it's a MV amp). If it's a non-MV amp and you still find yourself lowering this value then you'll probably find the tone harsh or too scooped at loud volumes. In general I find people set the MV too high on MV amps. I think they don't realize that most MV amps achieve full volume around 2-4 on the MV knob and then it's just compression after that. Amp makers are partly to blame here as they do this on purpose to make their amps seem louder than they really are. Of course the sweet spot is that point at which the power amp starts to compress so you want to set the MV high enough to get into the sweet spot. It's a psychological thing. People always like a more "open" sound even though they don't really understand what makes a tone "open". When you lower the Transformer Match you reduce the power tube compression of the lows and highs. The problem is humans naturally gravitate to this to the point that they will make the tone excessively "open" and then it doesn't fit in the mix. I do not recommend deviating much from 0.9 - 1.1. Of course there are no rules. With real amps some people like that more open sound and achieve it by plugging their cab into the higher impedance output, i.e. plugging an 8-ohm cab into the 16-ohm jack. This would be equivalent to setting the match to 0.5. SRV liked it the other way round IIRC which would equate to a match value of 2.0." Source

• Cliff: "Guitar folklore has it that SRV and Joe Walsh intentionally mismatched their speaker impedance. I imagine others have done this. The general idea is you plug an 8-ohm speaker into the 4-ohm jack or vice-versa. The Axe-Fx allows you to replicate this behavior using the Transformer Match control. To simulate plugging an 8-ohm speaker into the 4-ohm jack set Transformer Match to twice it's current setting (i.e. 2.0). For the other way around set it to half (i.e. 0.5). And you don't have to worry about frying your OT." Source

LEVEL

• Duplicate of Level on the PWR page.

ADV page

BOOST

• Duplicate of the control on the PRE page.

MODELING MODE

• The Amp block “Grid Modeling” parameter has been renamed and repurposed. The new name is “Modeling Mode” and offers the following choices:
  • Authentic – Replicates a tube amplifier with the utmost accuracy.
  • Grid Cond Off – Turns off grid conduction modeling in the power amp simulation. This reduces blocking distortion.
  • Smooth – Sets Triode Hardness to the minimum value essentially creating an ideal preamp and turns off grid conduction modeling in the power amp. This removes most of the “nasty” distortion that tube amps create.
  • Ideal – Removes most of the “warts” from the modeling which includes grid conduction, output transformer distortion, bias shifting and AC power supply modeling. Supply sag, screen voltage effects and crossover distortion are still modeled.
  • Ideal/Smooth – Same as #3 but also sets Triode Hardness to its lowest value. This is in essence an ideal preamp plus an ideal power amp.
• The audibility of these settings is dependent upon the particular amp model and various parameters. The use of these modes in conjunction with other parameters can yield idealized tones not achievable with real tube amps. The first two choices duplicate the previous behavior of the deprecated “Grid Modeling” parameter so no changes are required to existing presets. Note that when “Ideal” is chosen the global Modeling Version parameter is irrelevant.
• Switching modes may emit a loud pop. Cliff: "No way around this. Turn the volume down before switching modes." Source
• Cliff talking about the old Grid Modeling parameter: "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

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
• If the real amp has two inputs (f.e. low and high) the model is based on the high input. Set Input Trim to 0.500 to get the equivalent of using the low input. 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
• Cliff: "Input Trim is something you shouldn't play with normally unless you want to deviate from the actual amp. Input Trim allows you to reduce or increase the gain of the virtual amps input buffer. This is analogous to changing the type of tube for V1 in an actual amp. Some people like less gain for V1 so will replace a 12AX7 with a 12AT7. Some people want a little more gain." Source
• Enabling Boost is equivalent to setting Input Trim at 4.
• Search the Amp List page using the keyword "trim" for information regarding specific amp types.

MSTR VOL TRIM (Master Volume Trim)

• See INPUT DRIVE, OVERDRIVE, MASTER VOLUME.

LOW CUT FREQ, HI CUT FREQ

• Low Cut Freq controls the amount of lows the amp sim sees. It's a blocking filter at the input (before distortion). 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. It's an adjustable version of the Cut switch on the PRE page.
• Hi Cut Freq is a low-pass 2nd order 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.

DEFINITION

• 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. It's a treble boost/cut (tilt EQ).

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.
• This control is similar to Dynamic Presence and Dynamic Depth but the frequency is adjustable.

BRIGHT CAP

• See BRT (Bright).

MV CAP

• Firmware 11.01: "Exposed “MV Cap” parameter in Amp block. This parameter sets the value of the bright cap across the Master Volume pot and is located in the Advanced menu."
• Cliff: "Setting it to 1 pF defeats it." Source

TONESTACK TYPE, FREQ, LOCATION

• 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".
• 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
• Cliff: "Whenever you set the bass and treble to zero the tone stack becomes basically "flat" and the mid becomes a volume control. Most tone stacks behave this way." Source
• With some amp sims, such as the Lonestar, moving the tonestack results in a loss of volume.

PWR TUBE TYPE, PWR TUBE BIAS

• 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."
• Cliff: "The only possible parameter I can see adjusting to thicken clean tones is Power Tube Bias. The higher the value the less crossover distortion. Note that many other modelers don't even model crossover distortion. Small amounts of crossover distortion add aggression to high-gain sounds. In fact it is said that EVH liked to bias his amps cold to get that extra bite. The infamous HM-2 pedal intentionally adds crossover distortion. However crossover distortion can make clean tones sound thin. Many amps renowned for their clean tones run the power tubes hot so there is little or no crossover distortion. These amps are commonly (and mistakenly) referred to as "Class A" but they are really cathode biased Class AB with the tubes biased hot. Many people bias Fender-type amps hot to get warmer clean tones (at the expense of tube life). The default bias value in most cases is such that the bias point is roughly 60% of full-power ("Class A" types not included). If you increase the bias value to 0.5 or so you'll be running the virtual tubes at around 75% of full power and clean tones will sound warmer but you will lose that sizzle on high-gain tones." Source
• Cliff: "Changing the power tube between pentode and tetrode doesn't change the sound in the same way actually changing tubes would because it only changes the distortion curves. It does not change the transconductance so the transformer matching is constant. When you put different power tubes in an amp the difference in tone isn't due to some inherent difference in the "sound" of the tubes. It's mainly due to the different transconductances. An EL34 has more than twice the transconductance of a 6L6. This means that the plate current will be twice as great for a given grid voltage. This makes EL34s sound "more midrangey" and 6L6s sound "tighter" or "fuller". The truth is that if you bias them correctly and compensate for the difference in transconductance you will hear very little difference. Unfortunately you can't compensate for the transconductance easily in a real amp without changing the gain of the phase inverter and/or putting in a different output transformer." Source
• Cliff: "The Advanced menu of the amp block has a parameter called "Pwr Tube Bias". This sets the quiescent operating point of the virtual power tubes. What is the quiescent operating point you ask? It is the amount of idle current flowing through the virtual tubes when no signal is present. Power tubes are basically nonlinear controlled current sources. They can only sink current (current flow in one direction only) so to get an alternating signal you have to have some amount of idle current then you modulate that. The push-pull power amp was invented to increase efficiency by allowing lower idle currents. The name "push-pull" refers to the fact that one tube is responsible for mainly the positive portion of the waveform and the other for the negative. Some overlap of the responsibility is required since power tubes are nonlinear and without that overlap crossover distortion occurs. Setting the idle current, or bias as it's commonly known, affects the resulting transfer function of the power amp. Too little bias and there can be excessive crossover distortion. So-called "Class A" amps bias the tubes quite high. The following graph depicts the transfer function of the Axe-Fx II for five different bias settings: 0, 0.2, 0.4, 0.6 and 0.8. These are the red, green, blue, cyan and black traces, respectively. (see source URL for graphic) Note the severe crossover distortion at zero bias. At a bias of 0.4 the transfer function is almost a perfectly straight line. At 0.8 the response gets softer. Amps like AC-30s run the bias around 0.7 which gives them the soft distortion characteristics. Note that this applies to power amp distortion which only occurs when the power amp is driven hard (Master Volume set high). Some amps are intentionally biased cold to generate crossover distortion. Small amounts add an aggressive distortion. Some amps (i.e. Boogies) are intentionally biased cold to avoid having to set the bias and thereby reducing maintenance and warranty costs. Negative feedback around the power amp (Damping) further linearizes the power amp. So the transfer functions depicted are only accurate when Damping is zero. In conclusion, Power Tube Bias is a powerful parameter that can allow you to fine-tune the power amp distortion characteristics to your particular style. Source

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."
• AC = Alternating Current. DC = Direct Current.
• Cliff: "The most notable thing would be the ghost notes. There is also a slight difference in feel." Source
• Cliff: "Those old amps make ghost notes. My 100W Plexi has some ghost notes that are louder than the fundamental. The easiest way to eliminate them (if you don't like/want them) is to simply set the Supply Type to DC. However, IMO, the ghost notes are a large part of the character of these designs and removing them isn't desirable. Don't over-analyze it. Recognize that certain designs produce ghost notes and embrace it." Source

MAINS IMP. (SAG)

• Duplicate of the Sag control on the DYN page.

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

• 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.

PWR AMP LOCUT, HICUT

• These settings are adjusted automatically when changing the Amp Voicing parameter.

DYNAMICS TIME

• See DYN page.

TRIODE HARDNESS

• 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.
• 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."
• Cliff: "What the parameter does now is control the asymmetry of the triode model. The higher the value, the more asymmetrical the clipping. A value of zero would be completely symmetrical which will decrease the even harmonics and increase the odd harmonics. Triodes are very asymmetrical. The default values are rounded to the nearest 0.5 based on the MIMIC measurement for that amp." Source
• Cliff: "Lower values will have less even and more odd harmonics. The smoothness/harshness of distortion is a function of the ratio of even to odd harmonics. The more symmetrical the clipping, the more odd and less even harmonics." Source
• Cliff: " The default hardness value is based on the tubes that were in the amp being modeled. Most tubes fall in the range of 8 to 9. Perhaps old Mullards or Gold Lions or whatever are softer and would be equivalent to 5 or less. I don't know, I've never tested any. In general lower values will sound softer (naturally) but have less note separation. Higher values will give a more aggressive distortion and better note separation. There are no rules. Adjust the value to your personal preference. I doubt a real tube would ever be able to get to a value of zero but that doesn't mean it isn't a useable sound." Source
• Cliff: "If you are right on the edge of breakup the triode hardness is very powerful as it controls the harmonic series. Higher values will cause the overtone series to have a less steep decay and will increase perceived "sparkle". Together with the preamp bias you can control how chimey and "round" the tone is (preamp bias effectively controls the ratio of even/odd harmonics)." Source
• Cliff: "Triode Hardness at zero gives a smoother distortion with reduced upper harmonics. However if you carefully compare a real tube preamp with the Axe-Fx models you'll clearly hear the difference as you reduce Triode Hardness. It's even more apparent when you compare the distortion spectrum. It's yet even more apparent when you use measurement techniques that learn the proper value." Source

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."
• Cliff: "The further you move away from (roughly) zero the more even harmonics are introduced. It's an asymmetric transfer function, so you have to experiment." Source

USE MIMIC

• Instructs the Axe-Fx to use or not use the MIMIC data for the selected model.

TREM/MIX page

TREM FREQ, TREM DEPTH (Bias Tremolo)

• Firmware 10: "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."
• Cliff: "If the power tubes are being overdriven the bias tremolo can add lots of crossover distortion."
• To decrease crossover distortion, set Modeling to Ideal/Smooth.