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B+ Time Constant...

shotgunn

Fractal Fanatic
Should I think of the like the Attack parameter on a compressor or is it like the release on a compressor?
 

shotgunn

Fractal Fanatic
Bump... Anybody? C'mon, I know somebody has a better understanding of this than I do. Oh wait... You're all jamming 5.0 still. I had to stop myself last night otherwise I would've never slept.
 

chris

Legend!
B+ TIME CONSTANT — Controls the rate of change in the power tube plate supply. Lower values give a bouncier feel, while higher values give a tighter feel.

Based on this from the wiki I'd say it's more like the attack parameter.... But not really.
 

iamkojak

New Member
B+ refers to the the anode voltage of the preamp tubes. it probably is intended to simulate the effect of worn out or low quality capacitors versus low-ESR high quality ones. the latter ones result in a more defined 'tighter' low-end, while 'vintage' worn out caps tend to be more muddy and bouncier feeling.
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
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.
 

farrakon

Inspired
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.
Jesus christ!!! What other company takes its time to personally explain stuff like this????? Thanks a lot for the knowledge! With this I can now change this value since I understand whats happening.
 

AdmiralB

Experienced
B+ sag also applies to preamp and PI voltages, which affects preamp headroom and tone - if that's modeled in the Axe (and I have no reason to think that it isn't).
 

shemihazazel

Power User
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.
 

acbarros

Inspired
Damn, Cliff! I graduated in Eletronic Engineering looooooong time ago (1989) and now you just made me remember stuff like Z-transform, Nyquist theorem...Matlab...argghhhhh lol
 

shotgunn

Fractal Fanatic
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.
This is one of MANY reasons why FAS is the best. First, the fact that they go this deep in the modeling process. Second, that he took the time to explain so well. I've built several tubes myself and have read quite a bit about them. I would never be able to explain it this well, but I can totally understand what he's saying.


Sent from my iPad using Tapatalk
 

Radley

Experienced
Excellent explanation Cliff. I can certainly hear the compressors at work (rather unnaturally) in a certain competitor's medium & hi-gain modeling algorithms! :)
 
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.
This is why this forum is so great. Until I read this post from Cliff, I never really got the sag I wanted out of certain models like the ODS. I simply didn't know about this relationship with the B+ Time Constant paramater. Now it sags like a skinny rapper's jeans.

And BTW, V5 is the freaking bomb. Other updates were good... this one is spectacular.

Karma
 

solo-act

Fractal Fanatic
Now somebody take Cliff's explanation, stick it in a forum called "Cliff Notes" and title it "B+ Time Constant".

Getting these distilled & archived into one place all by themselves (I love the Wiki, but Cliff's notes are all spread out!) would be extremely handy to everyone. Does anyone agree? If so, please click "like" :)
 

acbarros

Inspired
Now somebody take Cliff's explanation, stick it in a forum called "Cliff Notes" and title it "B+ Time Constant".

Getting these distilled & archived into one place all by themselves (I love the Wiki, but Cliff's notes are all spread out!) would be extremely handy to everyone. Does anyone agree? If so, please click "like" :)
[+1] Yeah!
 
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