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Common amp mods and their respective Amp Block parameters.

biggness

Power User
I'm wanting to know what parameters I need to touch on when approaching "modding" certain amp models in the AxeFx.

We all know that Plexi's, and JCM800's have been modded throughout history with great success, with some even becoming their own industry leading entities.

So the Jose Mod is essentially the Saturation Switch in the AxeFx. I got that one.

What about choke mods? Transformer mods? Voicing switches? Extra gain stages?

I know "mods" is a generic blanket term, and technically speaking, changing any parameter outside of the BMT controls is a mod. I'm just looking for those parameters that would definitely be affectd by those with wizard soldering skills.

Example:

What in the world would one do to take a stock JCM800 and end up with this beast?

 

mixermang

Inspired
i was about to say that tone sounds like all preamp gain staging insanity with a bunch of input filtering. 100% Freidman tone

power tube bias would be mod #1, that's power tube grid bias knob, FAS has the factory settings hot hot, for the mesas i bring them down to ~0.3ish but that's for a Recto that is already distorting below MV 2.00, the low bias on the edge of instability is where the good preamp distortion is, somehow power tube grid bias has a drastic effect on preamp distortion breakup characteristic even with MV down at zero.

for cathode biased amps you can scale the value of the bias cap and how fast it lets the power tube bias shift, that's in cathode resistance knob, once you have cathode resistance on, the cathode time constant knob acts like a sag control for bias shift. recto is fixed bias but i swear to god if there was one knob in AFX3 that should be labeled "realism" it should be cathode resistance knob. once there is a cap on the power tube bias controlling the bias excursion and you stretch out the time constant to match or exceeded B+ time, something connects in there to homogenize the breakup and bias shift and tone in a really satisfying way, it all comes together somehow

negative feedback mod is a big one, that's in negative feedback knob. once you have negative feedback you can use depth and presence, or change where depth freq and presence freq are centered with depth/presence freq (advanced)

gain bright caps are an easy mod, that's in preamp bright cap knob. you can change the cap value but you can't change how it's done, just straight capacitor value, vs resistor in series or parallel. the bright cap knob works but i always thought it's a little harsh. a recto has a 1nF cap on the red modern gain pot and that's way way too much on the model

master volume bright caps are a thing too, recto irl has a master volume bright cap but the model is done with the loop bypassed so no master cap, not sure what the value on the amp is but sometimes i like to use ~20pF, same thing with the gain bright cap knob though, it works but it feels way too harsh, it's too easy to make the phase inverter overload and get spitty, then it's no good

it's not an amp mod but you can literally design your own speakers in the speaker page, you can change the amount of low boost and high boost, and you can change where the low boost and high boost are centered. if you drop speaker HF freq down lower it can fix a lot of clarity problems that the tonestack couldn't help you with. and if you have mush problems with high master volume you can move the LF freq down lower to move the flub out of the way to put the flub where you want it, or turn down LF resonance and keep the screaming high master volume tone but without any flub.

i don't know much about chokes but a big irl amp upgrade is a bigger mercury magnetics output transformer, that's in the Xformer match knob.

Turning it up is undermatching (smaller transformer), turning it down is overmatching (bigger transformer). turning it up is overmatching (bigger transformer), turning it down is undermatching (smaller transformer). overmatching makes it sound stronger/bigger but undermatching makes it respond more intimately and will distort (saturate) easier, it becomes a whole extra layer of distortion texture when it saturates, you can exaggerate output transformer distortion with Xformer drive knob to hear what it's doing easier.

check out the Tech Notes in the News section
 
Last edited:

biggness

Power User
i was about to say that tone sounds like all preamp gain staging insanity with a bunch of input filtering. 100% Freidman tone

power tube bias would be mod #1, that's power tube grid bias knob, FAS has the factory settings hot hot, for the mesas i bring them down to ~0.3ish but that's for a Recto that is already distorting below MV 2.00, the low bias on the edge of instability is where the good preamp distortion is, somehow power tube grid bias has a drastic effect on preamp distortion breakup characteristic even with MV down at zero.

for cathode biased amps you can scale the value of the bias cap and how fast it lets the power tube bias shift, that's in cathode resistance knob, once you have cathode resistance on, the cathode time constant knob acts like a sag control for bias shift. recto is fixed bias but i swear to god if there was one knob in AFX3 that should be labeled "realism" it should be cathode resistance knob. once there is a cap on the power tube bias controlling the bias excursion and you stretch out the time constant to match or exceeded B+ time, something connects in there to homogenize the breakup and bias shift and tone in a really satisfying way, it all comes together somehow

negative feedback mod is a big one, that's in negative feedback knob. once you have negative feedback you can use depth and presence, or change where depth freq and presence freq are centered with depth/presence freq (advanced)

gain bright caps are an easy mod, that's in preamp bright cap knob. you can change the cap value but you can't change how it's done, just straight capacitor value, vs resistor in series or parallel. the bright cap knob works but i always thought it's a little harsh. a recto has a 1nF cap on the red modern gain pot and that's way way too much on the model

master volume bright caps are a thing too, recto irl has a master volume bright cap but the model is done with the loop bypassed so no master cap, not sure what the value on the amp is but sometimes i like to use ~20pF, same thing with the gain bright cap knob though, it works but it feels way too harsh, it's too easy to make the phase inverter overload and get spitty, then it's no good

it's not an amp mod but you can literally design your own speakers in the speaker page, you can change the amount of low boost and high boost, and you can change where the low boost and high boost are centered. if you drop speaker HF freq down lower it can fix a lot of clarity problems that the tonestack couldn't help you with. and if you have mush problems with high master volume you can move the LF freq down lower to move the flub out of the way to put the flub where you want it, or turn down LF resonance and keep the screaming high master volume tone but without any flub.

i don't know much about chokes but a big irl amp upgrade is a bigger mercury magnetics output transformer, that's in the Xformer match knob.

turning it up is overmatching (bigger transformer), turning it down is undermatching (smaller transformer). overmatching makes it sound stronger/bigger but undermatching makes it respond more intimately and will distort (saturate) easier, it becomes a whole extra layer of distortion texture when it saturates, you can exaggerate output transformer distortion with Xformer drive knob to hear what it's doing easier.

check out the Tech Notes in the News section
Wow, what an impressive sharing of knowledge. Much appreciated, and added to the notes archives.

Do you have any presets to share that display some of your mods? Maybe stock amp in Scene 1 and and your modded amp in Scene 2? I'd love to take a gander.
 

Dave Merrill

Fractal Fanatic
Wow, what an impressive sharing of knowledge. Much appreciated, and added to the notes archives.

Do you have any presets to share that display some of your mods? Maybe stock amp in Scene 1 and and your modded amp in Scene 2? I'd love to take a gander.
I was thinking that too, especiallya demo of the cathode resistance/time constant mods.
 

MackieFX

Inspired
i was about to say that tone sounds like all preamp gain staging insanity with a bunch of input filtering. 100% Freidman tone

power tube bias would be mod #1, that's power tube grid bias knob, FAS has the factory settings hot hot, for the mesas i bring them down to ~0.3ish but that's for a Recto that is already distorting below MV 2.00, the low bias on the edge of instability is where the good preamp distortion is, somehow power tube grid bias has a drastic effect on preamp distortion breakup characteristic even with MV down at zero.

for cathode biased amps you can scale the value of the bias cap and how fast it lets the power tube bias shift, that's in cathode resistance knob, once you have cathode resistance on, the cathode time constant knob acts like a sag control for bias shift. recto is fixed bias but i swear to god if there was one knob in AFX3 that should be labeled "realism" it should be cathode resistance knob. once there is a cap on the power tube bias controlling the bias excursion and you stretch out the time constant to match or exceeded B+ time, something connects in there to homogenize the breakup and bias shift and tone in a really satisfying way, it all comes together somehow

negative feedback mod is a big one, that's in negative feedback knob. once you have negative feedback you can use depth and presence, or change where depth freq and presence freq are centered with depth/presence freq (advanced)

gain bright caps are an easy mod, that's in preamp bright cap knob. you can change the cap value but you can't change how it's done, just straight capacitor value, vs resistor in series or parallel. the bright cap knob works but i always thought it's a little harsh. a recto has a 1nF cap on the red modern gain pot and that's way way too much on the model

master volume bright caps are a thing too, recto irl has a master volume bright cap but the model is done with the loop bypassed so no master cap, not sure what the value on the amp is but sometimes i like to use ~20pF, same thing with the gain bright cap knob though, it works but it feels way too harsh, it's too easy to make the phase inverter overload and get spitty, then it's no good

it's not an amp mod but you can literally design your own speakers in the speaker page, you can change the amount of low boost and high boost, and you can change where the low boost and high boost are centered. if you drop speaker HF freq down lower it can fix a lot of clarity problems that the tonestack couldn't help you with. and if you have mush problems with high master volume you can move the LF freq down lower to move the flub out of the way to put the flub where you want it, or turn down LF resonance and keep the screaming high master volume tone but without any flub.

i don't know much about chokes but a big irl amp upgrade is a bigger mercury magnetics output transformer, that's in the Xformer match knob.

turning it up is overmatching (bigger transformer), turning it down is undermatching (smaller transformer). overmatching makes it sound stronger/bigger but undermatching makes it respond more intimately and will distort (saturate) easier, it becomes a whole extra layer of distortion texture when it saturates, you can exaggerate output transformer distortion with Xformer drive knob to hear what it's doing easier.

check out the Tech Notes in the News section
Wow, seems like you know a lot - got YouTube channel? You should!
 

biggness

Power User
i was about to say...
Alrighty, MixedMango, I applied some of what you shared to a Friedman BE-100, as the tone in the video I shared was the "goal" and based off of that layout.

It's pretty daggum gnarly. :oops: This an awesomely tight metal/rock tone, with lots of clarity and note separation. All without using a drive block before the amp. I'd put it up against any high gain fire breather now.

I included a preset with Scene 1 as the base tone, matching the BMT knob positions of the modded amp in Scene 2 to showcase the stark contrast.

Do you see anything I am overlooking, or went too heavy on?

Thanks again!
 

Attachments

  • MixerMods.syx
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ak49climber

Inspired
Until this thread I didn’t pay much attention to the bias settings for tubes. I have been struggling with the rectifier models for a while now and turning the bias down to .3 made a huge difference!
 
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