rectifier bias excursion time

mixermang

Inspired
In real life is bias excursion time in a rectifier amp affected by sag or B+ Time or variac?

rectifiers are all default modeled on silicon power (sag 2.00), i would assume other measurements taken for defaults would have been made in silicon power mode as well, such as bias excursion time

reason one would ask is when you bring sag from 2.00 to 7.00 (~ tube rectification speed), you can hear the low end bass response slow down, but the breakup (bias excursion) stays the same speed, it's quick and snappy even though the low end is loose, feels weird. if you go to increase bias excursion time from its default of 2.70 up towards 12.9, you can hear the breakup slow down separately from the low end response, so then the low end and the bias excursion are lined up together....that's a characteristic i'm used to hearing connected to the bold/spongy switch or changing from silicon to tube rectification on a rectifier.

bias excursion time is what i've been looking for to make the rectifier breakup less stiff under the pick, in that it takes longer to breakup....sag (power speed) and variac (power amount) and grid bias (bias excursion start point) weren't doing it because they tug on each other but they don't tug on bias excursion time. probably a good thing so you can adjust it yourself and experiment. if bias excursion time ever got connected to the other power section stuff so it slows down when you change variac or sag settings this user would not be mad, that way bias excursion time would slow down when sag or variac get slowed down
 
Lol. This man is exactly who the Axe-fx’s advanced parameters were made for.

I’m assuming an electrical engineering background of some sort?
 
Bias Excursion is unrelated to the power supply stuff (sag, Variac, etc.).

Bias Excursion is grid bias shift caused by grid conduction. When a grid goes into forward conduction, charge "leaks" off the coupling capacitor. When the grid comes out of forward conduction the capacitor now has a net negative charge. This causes the bias point to shift negative.

Excessive bias excursion causes "blocking distortion".

Old amp designs, i.e. Fender 5E3 Tweed, exhibit copious bias excursion in the preamp and, hence, lots of blocking distortion. Part of the charm I suppose.

Power tubes also exhibit bias excursion. Some bias excursion in the power amp can be desirable. If you design the power amp in such a fashion so as the tubes go into forward conduction as the plates are clipping this will effectively reduce the bias and lower the gain causing the amp to "open up". Too much bias excursion can cause excessive blocking distortion AND crossover distortion.

Designers can tailor the amount of bias excursion by adjusting the grid stopper resistor value.
 
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maybe it's a parasitic that tugs on bias excursion time in the amp, when you change rectification type or power settings, in helix the breakup under the pick gets stiffer or softer along with sag adjustments just like the amp does. i'm sure glad it's in there to adjust in advanced, it's like sag but for the distortion
 
Bias Excursion is unrelated to the power supply stuff (sag, Variac, etc.).

Bias Excursion is grid bias shift caused by grid conduction. When a grid goes into forward conduction, charge "leaks" off the coupling capacitor. When the grid comes out of forward conduction the capacitor now has a net negative charge. This causes the bias point to shift negative.

Excessive bias excursion causes "blocking distortion".
So what would this sound like? I don't even know how to ask this... Is there a real world example you can give, where turning the value from X to +X, or -X would give you a different sound, or response of some type, that you'd notice, or find desirable, in certain situations?
 
So what would this sound like? I don't even know how to ask this... Is there a real world example you can give, where turning the value from X to +X, or -X would give you a different sound, or response of some type, that you'd notice, or find desirable, in certain situations?
+1 - I don't know how to listen for these type differences and am often lost in term of how to learn to listen for them which I think is required to use some advanced controls effectively.
 
I don't know how to listen for these type differences and am often lost in term of how to learn to listen for them

it's subtle, and if you're not missing the characteristic in use of a very specific playing style (intricate light gain palm muted flat picking) it might be hard to ID at all, it's legit the difference between bias excursion taking 2 ms to expand fully vs taking 12 ms to expand fully.

Recto 2 Red Modern at default load settings, master at 1 and sag at 7. Palm muting a single note, as bias excursion time comes up from 2 ms towards 12 ms, you can hear the distortion 'envelope' take longer to bloom out fully, the breakup starts to feel softer under the pick because the bias excursion time takes longer to expand, it's not immediately snappy. To get that slower breakup characteristic out of the amp it's just a flip of the rectification mode to tube and a flip of power mode to spongy, but in the models sag and variac don't directly effect that particular preamp breakup characteristic like flipping the amp switches do, and i could never get the slow/soft breakup characteristic to show up with any of the other advanced parameters or by adjusting the preamp advanced settings, because it's power amp bias excursion time but it shows up directly in the preamp breakup characteristic
 
it's subtle, and if you're not missing the characteristic in use of a very specific playing style (intricate light gain palm muted flat picking) it might be hard to ID at all, it's legit the difference between bias excursion taking 2 ms to expand fully vs taking 12 ms to expand fully.

Recto 2 Red Modern at default load settings, master at 1 and sag at 7. Palm muting a single note, as bias excursion time comes up from 2 ms towards 12 ms, you can hear the distortion 'envelope' take longer to bloom out fully, the breakup starts to feel softer under the pick because the bias excursion time takes longer to expand, it's not immediately snappy. To get that slower breakup characteristic out of the amp it's just a flip of the rectification mode to tube and a flip of power mode to spongy, but in the models sag and variac don't directly effect that particular preamp breakup characteristic like flipping the amp switches do, and i could never get the slow/soft breakup characteristic to show up with any of the other advanced parameters or by adjusting the preamp advanced settings, because it's power amp bias excursion time but it shows up directly in the preamp breakup characteristic
Thanks - your post got me listening in the right direction. With master at 1 on rectoredmodern though I still struggle to hear any difference - with drive low and master max'd though, I definitely can hear it (hits me over the head which is what I need!) and I am getting a sense of the excursion time/recovery time impact - makes sense I guess as with master at 1 all the power tube settings would have much less of an overall effect. The 15Watt ClassA TB seems great for listening to how the advanced power amp section parms take effect.
 
With master at 1 on rectoredmodern though I still struggle to hear any difference

if preamp gain is up past halfway it's easier to hear what it's doing, how long the preamp breakup is taking to bloom/unfold relative to bias excursion time
 
Maybe a stupid question, is this related somehow to what seems to happen when a tube amp gets kind of "heated" after half an hour or more and seems to become more "souple" and "spongier" (at least for what I remember when I still played tube amps).
 
Maybe a stupid question, is this related somehow to what seems to happen when a tube amp gets kind of "heated" after half an hour or more and seems to become more "souple" and "spongier" (at least for what I remember when I still played tube amps).
No.

When a tube amp fully warms the the power tube grid bias typically increases.
 
Way back in the day I had a coach who said you don't replace the QB with a cannon firing rockets, you replace the receivers who can't catch 'em.

I realize I'm the butterfingers in this situation, but I really wish there were a "for dummies" available on this kind of stuff.
 
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