B+ Time Constant

[lscottk]

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 on your suggestion solo-act, as long as the trolls don't attempt to highjack the thread.

Are you volunteering to start the thread?

Also want to send a big thank you to Cliff for continuing to share this information. Thanks Cliff! :thumbup:

Sent from my ADR6350 using Tapatalk

 

[cobbler]

Now it sags like a skinny rapper's jeans.

Too funny!

 

[xrist04]

No reason why there can't be a Wiki page that collects all the references to FAS wisdom with link pointers. :geek

 

[solo-act]

+1 on your suggestion solo-act

[+1] Yeah!

You guys forgot to hit "like" -- I wanna' feel popular!!!!

Kidding aside, having "CLIFF NOTES" on-site as a separate forum would be awesome. I had a slew of un-sent ideas for Fractal communication, and this was one of them. Wiki links to posts is nice as well. I favor on-site because we're already here.

 

[nikki-k]

You guys forgot to hit "like" -- I wanna' feel popular!!!!

Kidding aside, having "CLIFF NOTES" on-site as a separate forum would be awesome. I had a slew of un-sent ideas for Fractal communication, and this was one of them. Wiki links to posts is nice as well. I favor on-site because we're already here.

YES!!!

I created a Wish for this in some form awhile back. Died a quick death, so I left it alone. Cliff's reply here, and the subsequent applause, seem to suggest a broader desire for it, in some form, after all.

 

[Patzag]

I'd prefer to see this as a page in the wiki. No chance for added inapplicable comments and other undesirable posts which would again dilute the content.

The page would have to include the following to be sensible: Original question or whatever triggered the Cliffnote and, if possible, link to the thread.

For my own use, I made a bookmark on my browser called Cliff Posts. I click it at least once a day to see what Master Cliff has to say and keep abreast of current things.

 

[solo-act]

I'd prefer to see this as a page in the wiki. No chance for added inapplicable comments and other undesirable posts which would again dilute the content.

The page would have to include the following to be sensible: Original question or whatever triggered the Cliffnote and, if possible, link to the thread.

I'm with you on distilled, undiluted content. Can be done with a locked thread containing what you described.

I see CLIFF NOTES as on-site archive vs living threads. Chances are more will read/learn if it's both here and on wiki.

Face it, many of us are archiving his tips already, why not a locked public archive on-site?
Anyway...I'll shut up now

 

[gdgross]

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.

Jeez, took me hours to do this in college...

 

[Dancing Frog]

Jeez, took me hours to do this in college...

Ahhhh! The wonders of the digital age.

 

[cobbler]

I'd prefer to see this as a page in the wiki. No chance for added inapplicable comments and other undesirable posts which would again dilute the content.

The page would have to include the following to be sensible: Original question or whatever triggered the Cliffnote and, if possible, link to the thread.

For my own use, I made a bookmark on my browser called Cliff Posts. I click it at least once a day to see what Master Cliff has to say and keep abreast of current things.

You could also have a firmware update index:

Section one: The official firmware release notes
Section two: Cliffs or FAS reps comments with respect to said firmware upgrade.
Section three: User input with research and references.

 

[nikki-k]

re: Cliff Notes/ Cliff's Notes: While a wiki or similar point of origin, and thus storage/access, would be great, I would love to see a linked master thread here as well. This could afford additional commentary, tips and tricks, etc. to be added without *contamination* of the original (pure) content. Elaboration on content, as well as "interpretation," application, etc. would also be possible, once again without *contamination* of the original (pure) content. I believe the B+ thread and subsequent "Cliff Note" could be an appropriate example: Imagine Cliff's post as being in the wiki, then linked out to a thread with it as the first post (and cross-linked to the wiki as well). Next, fill in resulting conversation; discussion of reference material, application of this information, examples (possibly with patches, audio/video examples), and so on.

As an "information and experience sponge," stuff like that/this is The One Ring (or whatever "preciousness" you feel like citing ).

 

[shasha]

I think that we should have a website where we can discuss things, ask questions, share presets, etc... and then occasionally Cliff can chime in and comment on things when we're way off base or need clarification. That would frickin' rule.

We could call it Shasha's AxeFXII Discussion Forum or something awesome like that. You know, since it's my own original idea and all.

On a serious note a lot of Cliff's comments and explanations (as well as other's) are linked in the Wiki as sources. Lots of good ideas and stuff here, but I'm not sure if we really need to abuse Yek to the point that he breaks.

 

[DonPetersen]

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.

I might not have understood (but appreciated) all the facts mentioned before, but this I understand!
outstanding!

 

[Stratoblaster]

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.

Cool, thanks for the peek under the hood....

 

[wezx]

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.

Thanks Cliff for these technical yet understandable explanations! I'm a technical guy, but not very informed on tube amp physics. I find these explanations helpful and interesting...I know others here do as well.

 

[wezx]

So am I understanding this correctly? If I lower the B+ in conjunction with higher Sag levels I'll get a "bouncier" Sag in the amp and the opposite will yield a "tighter" Sag?

 

[orpeus]

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.

IANAEE. I don't know how much of that description I interpreted correctly, but insofar as my understanding is correct, it would seem to have the following implications regarding the impact of the parameters on tone:

1. If Sag is set to near 0 (I notice in the documentation that setting it to 0 defeats the power amp modeling altogether), your tone is basically unaffected by the described aspect of the circuit simulation and B+ does nothing. If you want to make the effect of varying the B+ time constant more obvious to test it out, turn Sag higher than you normally would.
2. At higher Sag levels, high current (combination of input level to the power amp and the master volume setting?) has the effect of reducing headroom (meaning compression and distortion?).
3. Increasing the power tube bias has the effect of lowering the effective current level required to trigger sag.
4. My understanding is that generally, bass frequencies take significantly more energy to produce than treble frequencies. If that is applicable here, that means that lower notes would cause the power amp to draw more from the power supply, thus inducing sag more easily than the upper registers.
5. The Sag effect doesn't happen instantaneously. Instead, there is a delay as the power supply capacitance is drained. So, when you play a chord loudly, the initial attack is not affected by the Sag effect. With a shorter B+ time constant, the sag will kick in quickly and bite off the initial attack a bit. With a longer time constant, more of the initial attack would go through before the sag gradually clamps it down.
6. Whereas the volume spike and transients of the initial attack change quickly, the tone when sustaining varies very slowly compared to a 10ms time constant. So, the effect of varying the B+ time constant should be primarily on the attack and should not really be noticeable as the signal sustains and decays.
7. During Sag, the screen voltage also drops, which affects the tone in some way (but I don't know how).
8. In amps with a filter choke, some of the screen voltage's effect on the tone happens when the current level is changing, rather than specifically when it is high. So, whatever the effect is, you would hear the effect come in and then possibly even reverse itself according to whatever its time constant is as you go over the amplitude peak of the initial attack.

If anyone with a circuits background wants to chime in, please correct me on anything that I got wrong. Also, I would be curious as to whether or not the compression and/or distortion of the sag effect is frequency-dependent (does it compress bass frequencies more due to the fact that they take more current to move?).

 

[Smilzo]

3. Sag is the result of change in current in power supply unit(s)... it depends on starting and endpoint, yes, but in many different ways. So I doubt "Increasing the power tube bias has the effect of lowering the effective current level required to trigger sag" is always true.

I would be curious as to whether or not the compression and/or distortion of the sag effect is frequency-dependent

No, but bass signal is a sort of "carrier" overimposed over other freq. So we can assume the more bass, the more pronunced the effect. Note: this is not exact tech explanation, but guitar speaking... that's it!

 

[Billguitarvin]

Now it sags like a skinny rappers jeans.

Karma

Sorry, I know this is an 8 year old post, but just came across this reading about the B+ Time Constant and thought this was an absolutely hilarious way to talk about sag. Thanks Karma for making my day!

 

[Decimator]

I have to admit I don't fully understand what is going on here even after reading the fractal reply lol but I do get it and I know how it works in practical application and this is a very important parameter in my patches. Even my vocals patches need this tweaked. It is very useful when going from playing fast choppy metal notes to playing a more melodic or cleaner part with longer notes. It makes the notes sound and behaves naturally instead of just compressing/ releasing and sounding a mess. It also changes the timbre of the note I believe but don't quote me on that one.