Axe-Fx III Firmware 25.00 Release

[Daveporter33765]

I finally had the time to build a preset based on the new Class-A 30W Brilliant Amp model using a 2X12 Class A 30W DynaCab. What a great sounding amp model this is. Thanks to everyone who participated in the development of this amazing firmware update, including all the beta testers. My Fractal Axe Fx III Turbo modeler is without a doubt the best music equipment related purchase I have ever made!

 

[yyz67]

@DJD100 I think we're in basic agreement.

Coolest (if possible) would be an added dial showing that ratio, tied to the current relative grid bias voltage dial... so users could use either as desired.

 

[IronSean]

People can also use their ears. Having the bias be off won't affect tube life so whatever sounds best is ultimately the best setting.

 

[DJD100]

People can also use their ears. Having the bias be off won't affect tube life so whatever sounds best is ultimately the best setting.

That's true, but it would be nice to have a reference if you don't have the specific amp to check with, or you're trying someone else's settings etc..

 

[rzjd]

That's true, but it would be nice to have a reference if you don't have the specific amp to check with, or you're trying someone else's settings etc..

Here's an explanation earlier from Cliff re: this
https://forum.fractalaudio.com/threads/grid-bias-value.178012/post-2168640

 

[yyz67]

Here's an explanation earlier from Cliff re: this
https://forum.fractalaudio.com/threads/grid-bias-value.178012/post-2168640

And this in the wiki:

This leads to the question of "what is the ideal bias point?" The pervasive school of thought is you adjust the bias so the idle dissipation is 60-70% of the tube's peak power rating. This is a safe approach and ensures that the tubes don't "red plate" and live fairly long and prosperous lives.

My opinion is that the ideal bias point is NOT a function of the tube's power rating. It's the point at which the power amp's transfer function is most linear. Unfortunately operating the tubes at that point can result in exceeding the tube's S.O.A. So the optimum bias point depends on the tube's power rating, the transformer primary impedance (matching) and the user's tolerance to tube replacement frequency.

 

[DJD100]

Here's an explanation earlier from Cliff re: this
https://forum.fractalaudio.com/threads/grid-bias-value.178012/post-2168640

Awesome, thanks for this @rzjd!

Cliff stated the below on Oct 24, 2021 for an older FW, so it's a ballpark start.

There is not a linear relationship between the grid bias value and quiescent dissipation as a percentage of maximum. 0.5 is roughly 66% IIRC but I'd have to run the numbers again to be sure.

More...

From Cliff's Why Your Amp Doesn't Sound Like Our Amp Tech Note:

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[…]
The transconductance (gain) of a power tube can vary greatly. This is why power tubes are color coded, sold in matched sets, etc.
Amps come in two flavors: fixed bias and cathode bias. Fixed bias amps apply a "fixed" voltage to the grid of the power tubes. Cathode bias amps use a resistor between the cathode and ground to self bias the tube.
Most, but not all, fixed bias amps allow the user to adjust the bias point of the amp. This allows the bias point to be set to an optimum value for the particular set of tubes installed (since the transconductance can vary greatly). Some fixed bias amps do not allow adjustment. Examples are Mesa/Boogies, 5150s, and several other brands/types. The drawback of this is that the bias can vary greatly depending upon the gain of the tubes installed. Due to this the manufacturers err on the safe side and the bias is usually much colder than the ideal value.
Most cathode biased amps are not adjustable. Again you are at the mercy of the tube's gain but these amps tend to be biased hot to begin with and have higher transformer matching which prevents excursion outside of the S.O.A. (safe operating area).
If the bias is adjustable where the manufacturer decides to bias their tubes is a matter of preference. Most manufacturers bias their tubes on the cold side to prevent premature failure and reduce warranty claims. Especially the larger manufacturers.
This leads to the question of "what is the ideal bias point?" The pervasive school of thought is you adjust the bias so the idle dissipation is 60-70% of the tube's peak power rating. This is a safe approach and ensures that the tubes don't "red plate" and live fairly long and prosperous lives.
My opinion is that the ideal bias point is NOT a function of the tube's power rating. It's the point at which the power amp's transfer function is most linear. Unfortunately operating the tubes at that point can result in exceeding the tube's S.O.A. So the optimum bias point depends on the tube's power rating, the transformer primary impedance (matching) and the user's tolerance to tube replacement frequency.
For example, if we bias an EL34 based power amp at 60% peak dissipation it's actually running fairly cold. If we know that the transformer is slightly overmatched we can bias the tubes hotter, 70% or even more. This will result in a warmer tone but the tubes will wear faster.
What does all this mean? Well, I bias the virtual tubes on the warm side. EL34s are biased at around 70% because we don't have to worry about them wearing out. 6L6s are biased a little colder, around 60% but this is actually as "warm" as the EL34s because of the higher plate dissipation of a 6L6.
In practice this means that the models in the Axe-Fx will biased warmer than a new amp straight out of the box as most amps are biased cold (too cold IMO). After you wear the tubes out and bring it to a tech the tech will replace those tubes and bias them hotter than factory. So if you're comparing your new, out-of-the box 5150 with the Axe-Fx model the amp will probably sound "colder". Some people like this, many do not. If you like a colder sounding power amp it's just a knob twist away.

FRACTAL AUDIO QUOTES

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[258]For as long as I can remember I've always biased my amps around 60% of maximum dissipation. But I just ran across a site that claims 70% is optimal for a Class-AB amp. So I tried it on my go-to Plexi patch and I do believe it sounds better.
Set Power Tube Grid Bias to 0.7 to do this.

[259][…] The early Cygnus betas were colder than the later betas. Originally I had set the bias equal to the reference amps but many of the reference amps are biased cold because most manufacturers bias amps cold from the factory for warranty reasons. In the later betas I put the bias points at where a technician would typically bias an amp after replacing the tubes, i.e. 60-70% of max. plate dissipation.

[260]To reduce/eliminate crossover distortion increase Power Tube Grid Bias. A value of 1.0 will have no crossover distortion.

[261]There is not a linear relationship between the grid bias value and quiescent dissipation as a percentage of maximum. 0.5 is roughly 66% IIRC but I'd have to run the numbers again to be sure.
With a real tube amp there is no hard-and-fast rule. Some say 60%, some say 70%. But that's a simplistic view of the problem. The optimum value is dependent on many things: B+ voltage, transformer turns ratio, etc. For example, if the transformer is overmatched you can run the bias hotter and this is indeed how some amps are designed (i.e. Trainwrecks).
The reason for the "rule" in tube amps is to reduce crossover distortion while also preventing premature wear to the power tubes. The hotter the bias the more linear the response. However too much bias and there's the danger of the tubes momentarily operating outside the SOA which shortens tube life. Even if you stay within the SOA if you operate near the limits you'll shorten tube life. If you overmatch the transformer you move the load line and increase the SOA margin so you can increase the bias.
The Axe-Fx is immune to tube wear so there is no danger in running the bias hot if you like that sound. However the hotter the bias the less dynamic the response becomes. Also some people like a bit of crossover distortion. Legend has it that EVH liked his amps biased cold. Whether that was because of the added dynamics or the extra grit due to crossover distortion is unknown.
As always the correct answer is what sounds best.

[262]The question is which amp? A 1968 100W Plexi with over 500V on the plates or a 50 watter with around 360V on the plates? The response of those two amps will be completely different at the same quiescent dissipation due to the different plate voltages. The 100W amp will require around 35 mA for 70% dissipation. The 50W amp will require 48 mA. Since the transconductance of a tube is fairly independent of the plate voltage the 50W amp will be operating in a much more linear region. However, it will clip the grids sooner. If you load up, say, a 1959SLP model in the Axe-Fx you'll notice the bias is lower than the 50W versions. This is why. This is also why a lot of people prefer the 50W Plexis.
Then we need to ask what type and brand of power tubes? NOS? The response of the tubes is dependent upon the exponent in the tube equation. The original Childs-Langmuir law says that the exponent is 1.5 but real tubes exhibit an exponent somewhere between 1.2 and 1.5. The greater the exponent the less linear the response.
Finally we need to ask what is the output transformer primary impedance. Is it 4K per pair? Is it 3.6K? Is it 3.2K? Primary impedance was all over the map in those days.
All these things interact. The hotter the bias the more linear the response, the higher the gain, the sooner the grids clip, etc. A good amp designer will balance all these things along with the transformer matching to get the desired response.
The correct answer is turn it up/down until it sounds how you want.

[263]So-called Class-A amps (like the AC30) don't actually exhibit much power supply sag because, well, they're Class-A (or nearly). The tubes are biased hot so when one tube is conducting more the other is conducting less and the net supply current doesn't change much. Contrast this with Class-AB where when one tube conducts the other goes into cutoff causing a net increase in supply current and concomitant supply sag.
What Class-A amps do exhibit is "cathode squish". The capacitor on the cathodes charges up and shifts the bias point. This reduces the gain and acts as a sort of compressor. It's a unique compression and the amp "opens up" at the same time and crossover distortion increases.

[264]Turn up Power Tube Grid Bias. In real life that is tough on tubes as it makes them run very hot.

[265]Turn Power Tube Grid Bias all the way up.

[266]Power Tube Grid Bias is quiescent bias current. This is NOT the same as percentage of maximum dissipation.

 

[DJD100]

Thanks all!

 

[yyz67]

To reduce/eliminate crossover distortion increase Power Tube Grid Bias. A value of 1.0 will have no crossover distortion.

I hadn't seen this before (never on TGP). Helpful to know!

So reading all of this info (thanks @rzjd @DJD100), each amp's bias is set by CC as a optimization of linearity and no crossover distortion, typically leaning on the warmer end compared to amps IRL. There could still be a range of colder/warmer PTGB that would also have no crossover distortion, but that would be unique to each amp/circuit and not directly derivable (via maths).

 

[utkanos]

this sounds so good, got the chance to try a bunch of the new stuff with a guitar this week and it's somehow even better each update. thanks FAS team 8)

 

[JRW824]

I have also noticed this,
I use my FRFR system with slight global EQ correction as monitors on my desk.

and experimented with the global Input 1 Gain and at 0.8 it is almost comparable to the 25b3.

However, if you reduce the output of your FRFR slightly without changing Input 1 Gain, i.e. 1.0, the effect is very similar.

Conversely, the volume is higher than usual
the amp usually has a more dominant effect on the sound image.
And here the 25b3 has a more homogeneous effect in the sound image, more sensitive and more mature.

FW25 final is great and if you recognize the above described everything is much better. I think so..

Try it out and compare it with 25b3.

Yup..I was getting a little worried initially until I figured this one out. The update sounded harsh and kind of oddly boxy until I made this very close to yours adjustment then it opened up wide. I am like many I think in that I tweek so many things that I sort of lost my center on what this gain should look like hahahaha!!