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Pre-amp vs power amp distortion in AxeFxII

urowinner

Member
I've been diving into learning about tone and came across an interesting passage in "Creative Guitar 1: Cutting Edge Techniques" by Guthrie Govan:

"It's worth bearing in mind that there are two distinct kinds of overdrive. The first is generated by the pre-amp (the tone shaping part of the amp where the gain and tone controls live) and is characterized by a fuzzy sound quality that will be familiar to anyone who's tried to make their amp sound nasty at low volume levels. The second is the result of overdriving the power amp (the part of the amp responsible for overall volume level, courtesy of the Master Volume Knob). This type of overdrive tends to sound more musical - the guitar still feels responsive and you still get all the sustain you could ask for, but the overall timbre is a lot smoother, with none of the buzzy abrasiveness of pre-amp distortion"

Many of you probably already know the information in the previous paragraph, but for me it seems a very helpful way of understanding what I'm trying to dial in on the AxeFxII.
Just thought there might be others out there who might benefit from Guthrie's explanation of overdrive.

Please let me know if there is more to the picture that I'm unaware of. Since the AxeFx models so many different amps, it doesn't take long to discover that different amp models respond differently to drive vs master volume settings.

Would you agree that it's fair to approach tone on the AxeFx from the idea that "more fuzz=more Drive knob and smoother distortion=more master volume knob"?
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Would you agree that it's fair to approach tone on the AxeFx from the idea that "more fuzz=more Drive knob and smoother distortion=more master volume knob"?
That's true of the Axe-Fx AND real amps. Be aware that modern, heavy sounds are mostly preamp distortion though. Guthrie doesn't use a lot of gain. High levels of power amp gain can get flubby real fast.

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.
 
There's really no definitive answer for that. Amps are all designed differently. Look at the 5150. A lot of people attribute a good chunk of it's tone to the crossover distortion that occurs in the poweramp from the low bias. That's just one amp. Different tubes and circuits react differently to adding more juice. You should notice this in the amps in your Axe-FX as well. A lot of the amps have very different gain characteristics.
 

endgroove

Inspired
Would you agree that it's fair to approach tone on the AxeFx from the idea that "more fuzz=more Drive knob and smoother distortion=more master volume knob"?
Yeah, it's fair to approach it this way.

You probably understand this already, but I think it boils down to two very important things: the structure of your gain and the structure of your (for lack of a better word) "eq". These will profoundly affect the ultimate outcome of your distortion's tone. You've touched on the former with preamp vs. power-amp distortion. Check out this excellent discussion:

http://forum.fractalaudio.com/axe-fx-ii-discussion/45541-lost-notes-chord-voicings-axe-ii-4.html

for the consequences of the latter.
 

nikki-k

Experienced
I am sorry to sidebar this thread, but I feel a need to reassert my wish: Selectable tube types.
This thread brings up the difference s of preamp vs power tube distortion. Cliff quite succinctly outlines a great foundation for utilizing this information. A next step is to realize the effect different power tube types can have. We do have Power Tube Hardness and Bias Point settings, but there is (quite) a difference between, say, a 6L6 and an EL34. As Cliff stated, pushing the power tubes harder (can) will result in a more compressed tone, and flubbiness (dependent on various factors). Tube choice here is part of that; if I wanted a tighter rhythm tone, for instance, I do not think I would choose to drive an EL34 *set* very hard, and might opt for a set of 6L6s instead. Sidenote for this sidebar: I have no idea of the extent to which power tubes are modeled in the Axe-Fx; I do not know if there is a distinction between EL34, 6L6, KT88, etc being made, or anything *deeper*. This is a wish based purely upon speculation. No offense is intended toward Cliff, Fractal, or the OP in this thread.

Further, being able to choose to run those Pentode tubes in a (pseudo) Triode mode would be an extra bonus. As Pentode mode was described as being implemented after Triode mode had been used previously for amp models, I am once again speculating, based upon the information presented.

Both of these aspects will have definite affect with the resulting tone.
 

Steck

Member
I am sorry to sidebar this thread
I was just pondering a similar thought this afternoon, which was whether the WIKI had any info about the specific tubes in each of the amp models (without me digging around on the internet in search of schematics and then making notes, etc). The reason why I was thinking this is that in some cases if we wanted to try and replicate a particular tone with an amp that isn't modeled "directly" (e.g. already present in the list of amps), choosing an amp that is similar (EL34 outputs vs 6L6, and so forth) might get us down the road.
 

Dancing Frog

Inspired
I was just pondering a similar thought this afternoon, which was whether the WIKI had any info about the specific tubes in each of the amp models (without me digging around on the internet in search of schematics and then making notes, etc). The reason why I was thinking this is that in some cases if we wanted to try and replicate a particular tone with an amp that isn't modeled "directly" (e.g. already present in the list of amps), choosing an amp that is similar (EL34 outputs vs 6L6, and so forth) might get us down the road.
There is a lot more involved in trying emulate an amp that isn't modeled directly than looking at the compliment of tubes. It's a good place to start though. I think more in terms of architecture when I am working on trying to replicate a tone. There are a lot of amps that share a similar architecture in the preamp and power amp, because they all share a common ancestry. It would be interesting if someone would take a stab at perhaps a family tree. That might be more useful.
 
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endgroove

Inspired
I am sorry to sidebar this thread, but I feel a need to reassert my wish: Selectable tube types.
Forgive me if I don't understand this completely, but isn't this already more or less possible by taking the tone stack of one amp, and putting it on the power amp of another? (e.g. Recto tone stack / AC30 power amp)

I guess this raises a question (for me anyway): Are systemic power amp variables other than power tubes (e.g. phase inverter, output transformer, power transformer, choke, filter caps) able to be dialed in to be the same from model to model? In other words if non-power tube parameters were "universal" could you program the Axe to do what you're asking?

Yes, less easy than a drop-down menu... But possible?

Just thinking out loud.
 

jesussaddle

Power User
That's true of the Axe-Fx AND real amps. Be aware that modern, heavy sounds are mostly preamp distortion though. Guthrie doesn't use a lot of gain. High levels of power amp gain can get flubby real fast.

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.
Brilliantly put. Didn't have that level of clarity on that topic, so thank you!
 

jon

Fractal Fanatic
a tonestack is neither a preamp nor power section of an amp, but the amp's 'voicing'. well, technically, it IS part of the preamp section, but to answer your question, no it would not be the same.

+1. My wish since I started using the axe was to have the ability to change power tube sections - it's what I was doing for years with real amps as well - using the THD yellow jackets. I found that my favorite tube was the EL84, and then the kt88, and I didn't really care much for the 6L6 and el34's, but I'm more into mild crunch, not searing distortion. YMMV, but it's an interesting thing to change the power tubes and see what it is about each that fascinates you....
 

jon

Fractal Fanatic
Would you agree that it's fair to approach tone on the AxeFx from the idea that "more fuzz=more Drive knob and smoother distortion=more master volume knob"?
I'd change that slightly to more fizz = drive knob and smoother overdrive = master volume.

Also keep in mind that a master volume does not really give you the option of getting power tube saturation at low volumes - that's a marketing trick used by amp manufacturers, you'd need an attenuator for that. Cliff put it really well. One of the best things about the axe is the ability to easilty REMOVE that 'flubbiness' with the twist of a knob - that's something I struggled with for many years on various tube amps. The axe makes it SOOOOOO easy - I'm there in seconds! ;) I crank up the power section in the axe (MV) and leave the gain low - if you add some eq and some hi and low cut, you will reach sonic nirvana for crunch tones faster than you can say 'FAS RAWKS!'
 

jefferski

Fractal Fanatic
I crank up the power section in the axe (MV) and leave the gain low - if you add some eq and some hi and low cut, you will reach sonic nirvana for crunch tones faster than you can say 'FAS RAWKS!'
That sounds like the opposite of how Cliff does it...

I'm not sure if I want to say I'm confused, or that what I love about the Axe is how many different ways there are to get good sounds. l:)
 

jon

Fractal Fanatic
yea, that's about the opposite to cliff LOL but that's why I presented it in the context - for clean/crunch, where you almost always go for power tube saturation, but not much pre-gain fizz.

Different approaches tho - Cliff's method is actually what I use for higher gain styles - rock, metal etc.
My method is used for easy listening, country etc.

Different styles, so different approaches. If you're into searing leads, you'd want to use cliff's method. If you're into more southern 'twang' you might want to try mine.
 

immortal_soloist

Power User
Nice discussion going on here. Something for everyone to learn. And speaking of Guthrie,a huge influence of mine and his Creative Guitar Books are just pure platinum. Some of the best guitar related information and learning you could purchase. As for the discussion,I am glad we got some insight from Cliff. Great thread!
 

Tone Control

Inspired
The power amp = good OD, preamp = fizz advice is what I've followed for a long time
However, I just spent 2-3 years going mad on trying boutique and classic amps, and have learned a lot - my playing has improved, as have my ears (my hearing is probably damaged, but I'm more discriminating), and I've read around them a lot, and researched a lot

I'd say that the "power amp = good OD, preamp = fizz" advice is a good rule of thumb, but is not the full story, What additional info I can remember is:

Some Output valve OD is ugly, many find single ended amps sound bad overdriven, I think I usually agree. Most of them Overdrive asymmetrically, and it sounds rough, not smooth, some like this, I've owned a few SE amps, didn't like the OD on most of them. I first thought SE amps had an advantage for OD, but my ears plus the words of an amp manufacturer I know personally convinced me this is usually untrue, push-pull amps sound better to most people. Tell me any world beating SE amps, the old Champ is the most famous, but it is not a sophisticated amp by any means, the Princeton is not great for OD either


In many push-pull amps, most of the OD is actually the phase inverter, in these, post-phase inverter master volume controls leave more of the sound intact

Some phase inverters distort very nicely, some sound awful. Fixes exist for some, e.g. early 70s Princeton push/pull. Again, some sound quite bad overdriven, some sound great

Some amps have quite nice, smooth preamp OD. The Dumble amps' OD is almost all from the preamp extra distortion stage, and the rest most from the normal preamp OD. This amp can have extra TMB controls after the OD section as well as before (i.e. the HRM versions have preamp controls, plus controls after the extra preamp OD stage)


I run most real amps on master vol 75% or more, the clean tones and feel seem to improve. Getting a controlled OD that is actually the Output valves usually means some saturation and extra warmth, but AFAIK very few amps are really normally used so that most of the OD is from the power tubes.

Some amps can be changed in nature a lot just by using a lower-gain phase inverter - this can clean up a Tweed amp, adding clarity by losing early OD, and reducing mids

Cliff's amazing work-in-progress. although a great example of how software can be a creative force, works on Cliff's latest "conceptual model" of an amp, which from this manual is that an amp can OD in the preamp and/or in the power amp
Therefore at present, you will not get all the same combinations as in a physical amp, that is not say you cannot get good or sometimes better sounds


On Valve types:

Although you can change the output valves in real amps, and I've done this, the general result is "the amp was designed to run well with the stock tube, why expect that the rest of the circuit will suit a different one"
I changed 6L6s in a Twin to EL84s using yellow jackets, it sounded very poor to me.
Guys have spent 70 years playing with variations on valve amps, why would I be entitle to assume that me doing a pick and mix in one would come up with a world-beating sound, it's clearly hit and miss and needs a lot of trials
There are some amps designed to take diff valves, from the supported valves, the ones I've owned always seem to have a clear winner to me (e.g. Ceriatone Trainwreck Express sounds best with EL34), but I guess that's all they need - to allow me to tweak it a little to my liking
For now, I am happy taking an assumed valve type onboard with each amp sim. Although I love 6L6s, I would always want EL34s in a Marshall sim that expects them (well, KT66s in the JTM45 of course)
 

Steck

Member
There is a lot more involved in trying emulate an amp ..... I think more in terms of architecture when I am working on trying to replicate a tone. There are a lot of amps that share a similar architecture in the preamp and power amp, because they all share a common ancestry. It would be interesting if someone would take a stab at perhaps a family tree. That might be more useful.
I agree completely, that's even more important than the specific tubes. I like the 'family tree' idea.

And, as others have mentioned, general architecture and specifics of preamp sections and tone stacks have similar relevance.

Based on comments I've read elsewhere in the forum, I think Cliff & Co. have designed enough horsepower in the AXE FX II to consider adding significant flexibility to the amp and preamp and tonestack modeling, even if some aspects have to be addressed by outside software (I'm guessing it'll be called "amp construction language" or "amp compiler").
 

Smilzo

Power User
Would you agree that it's fair to approach tone on the AxeFx from the idea that "more fuzz=more Drive knob and smoother distortion=more master volume knob"?
No. Too generic. I disagree even with Cliff (sorry!). Preamp in reality can sound warm and smooth. It's all a matter of circuit.
 

Sebastian

Power User
No. Too generic. I disagree even with Cliff (sorry!). Preamp in reality can sound warm and smooth. It's all a matter of circuit.
Agreed. The Mesa/Boogie sims are a good example of that.
They get their trademark sound out of the preamp while too much master volume makes them sound washed out, undefined and/or flubby.
 
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