Power sagging

    I have read the manual several times in this section but don't understand. It reads:“Power Sagging models the interaction between the guitar signal and the distortion stage. Increase the amount of “Power Sagging” to emphasize the velocity and energy of crunch sounds." it also reads: "Power sagging is a phenomenon that occurs when the tubes draw a lot of electric power at high distortion rates, thereby weakening the supply voltage. Under such load, the tubes will change their distortion characteristics and sort of shut down; then as soon as the load lessens, the tubes catch their breath again" That part makes sense but seems redundant with the first statement. In a real amp increasing the amount of sag makes things kind of fold in and then open up. It doesn't create more velocity and energy. Ive noticed the manual and the videos give very different angles on controls. Read the manual section on "Clarity" then watch the video for the amp section. I have read the manual pretty well but realize I should watch the videos as I think they explain things better.


    EDIT: watched the amp section video and understood differently than was written in the manual. Power sagging makes sense the way it is described in the video. I don't get why the manual states it increases the velocity and energy. As you turn the control up it decreases the dynamic gap between soft and hard playing not like a real amp does but sort of with more control.

    I think the problem of communicating power sag and compression(in the amp section) is difficult to describe with what I'll call 'pedestrian' words. I believe that some technical background is necessary in order to effectively communicate what is happening to the signal. IMO, one needs to know common engineering concepts such as Fourier transforms, transient analysis, tube biasing and saturation, etc... in order to have any significant conversation. Then there are the additional DSP inventions that Kemper has developed and applied to these behaviors that are outside of the standard engineering terminology. Kemper does not seem interested providing technical details regarding quite a few of their algorithms(and probably for good reason) so it seems we are left with using our ears to understand what is happening.

    Quote from lbieber

    I think the problem of communicating power sag and compression(in the amp section) is difficult to describe with what I'll call 'pedestrian' words. I believe that some technical background is necessary in order to effectively communicate what is happening to the signal. IMO, one needs to know common engineering concepts such as Fourier transforms, transient analysis, tube biasing and saturation, etc... in order to have any significant conversation. Then there are the additional DSP inventions that Kemper has developed and applied to these behaviors that are outside of the standard engineering terminology. Kemper does not seem interested providing technical details regarding quite a few of their algorithms(and probably for good reason) so it seems we are left with using our ears to understand what is happening.

    I understand tube amp sag just fine. At first I didn't understand how the manual was describing the control. I realize it doesn't work in the same way. The amp section video helped me understand.

    Fair enough. Perhaps you can explain to me how power sagging works? :) If it is modelling the interaction between the guitar and the distortion stage, which distortion stage(in the traditional sense) is being referred to? Is it modelling sag in the preamp stage(s) and/or the output stage? Do multiple gain stages even exist in the algorithm? The type of sag is very different in many classic amps and results in different qualities and feeling. Is it modelling basic resistive sag or is it modelling sag due to a complex impedance model? When we speak about compression in a tube amp, it is generally accepted that it is partially due to power sag. How is the compression modelled and how does it interact with sag, if at all? Both sag and compression are non-linear and the order in which they are applied matter. What is the order? There are many more questions that could be asked and answered. Hence, why I say we are left to using our ears since I have not seen any specific technical explanations regarding the algorithm.

    Quote from lbieber

    Hence, why I say we are left to using our ears since I have not seen any specific technical explanations regarding the algorithm.

    After reading all of this, I played around with sag a little. And to me it makes gainy profiles sound like the "amp is on 10". It gives a very tight and thumpy compression.


    I never tried it too much. To me it seems like your amp runs on a DC power supply. DC supplies use capacitors to store and filter energy. These caps will give you large instantaneous currents (power). Once you start pulling a lot of current the power supply voltage will drop/sag. This is why you always see cheap amps stating power in peak or instantaneous power. The true rating is in RMS, meaning power it can sustain forever, which is much less than peak.


    To me this will act like a compressor. Letting the attack of notes thru (peak power) and then reduce volume as the caps stored power begins to get drawn out (RMS).


    I dismissed the setting as just another compression option. I was not aware of the effects on the tubes etc.


    But after playing with it on high gain stuff I finally get it. And it sounds very good if you are trying to make the amp sound like it is on full tilt. Good for NuMetal kind of chunky thumps, metal solos where the low strings sound really beefy like the amp is about to blow during your runs, etc.


    Dyno, thanks for bringing this topic up. Forced me to try something new.

    It’s another one of those controls that I’ve never played around with as I always assumed it is already captured in the profile itself. Probably should investigate it further though as I always loved tube sag. I ran my Dual Rectifier in low voltage and tube rectification modes for 20+ years so this should be right up my street 😁

    I mainly had a problem with the way the manual described the control. I understand tube sag, what it does and how it feels when playing an amp with sag happening. Before I fiddle around with knobs and use my ears, I like to have an idea what the control is supposed to do. The manual states sag "emphasizes the velocity and energy of crunch sounds". But having played countless amps with sag, I don't believe sag emphasizes the velocity and energy of crunch sounds. To my ears, when my tube amps "sag" it does the opposite, drops out a bit until it catches up. I haven't really messed with it yet because I still don't understand why it is called "sag". If you watch the video, the control seems to affect clean and dirty sounds differently, more like a type of compressor, and a bit like "clean sense" where it changes the balance between clean and dirty sounds to even them out or make them different. I'm going to watch the vid again then try it and see what I hear happening.

    Quote from Wheresthedug

    It’s another one of those controls that I’ve never played around with as I always assumed it is already captured in the profile itself. Probably should investigate it further though as I always loved tube sag. I ran my Dual Rectifier in low voltage and tube rectification modes for 20+ years so this should be right up my street 😁

    For quite a while I never touched any amp controls like "sag" "clarity" "tube shape" etc. If I didn't like something I went to a different profile. I was scared only because I didn't fully understand the controls and stayed away figuring I would screw something up. Then I discovered some of the differences between ones I liked and ones I didn't were sometimes because of how the amp controls were set and started finding out how the amp controls can give me more or less of what I want and change a profile I didn't like into one I loved. Some profiles sound great with definition cranked, and sometimes cranking it makes it sound like a strat pickup through a treble booster. All the controls seem to interact and you don't need extreme adjustments to make a difference in tone and feel. As I have wrote, I suggest watching the vids on the amp section controls rather than the manual. I think it explains them pretty well and lets you hear it for yourself.