• We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.

PI Bias Excursion

neale dunham

Inspired
Can anybody explain what the PI bias excursion control in the amp block P.A section is controlling as when I turn it, it makes no difference to my sound whatsoever?

I am using a 59 Bassguy sim with Power amp sims enable with no cabinet sims if that helps?

Would really love to know what this is supposed to be doing??

Thanks.
 

yek

Moderator
Moderator
“Sets the time constant of the virtual RC cathode network for amp types that are cathode-biased”

Bassmans aren’t cathode-biased.
 
Last edited:

neale dunham

Inspired
Ive tried a Fender Deluxe a Princeton and still cant hear any difference?

These amps I thought were cathode biased?

If not can you point me in the direction of an amp that is please?
 

yek

Moderator
Moderator
Hmm, my quote was based on an error in the wiki.

"PI Bias Excursion" is the same as "PI Bias Shift" in the Axe-Fx II. It controls how much the virtual phase inverter (PI) bias shifts when overdriven. it's key to the Trainwreck sound.
 

yek

Moderator
Moderator
Seems to be relevant to the Trainwrecks primarily, based on the published comments.
 

neale dunham

Inspired
Ok, thats interesting...

Going back to what you mentioned above, how would I "virtually" add a Cathode Follower to an amp that doesn't have one?
 

FractalAudio

Administrator
Fractal Audio Systems
Moderator
PI Bias Excursion controls the virtual Phase Inverter bias excursion. It's a subtle effect on most amps but some amps display significant bias excursion, i.e. Trainwreck Express. It's a form of blocking distortion.
 

yyz67

Inspired
I think one lesson is that the relative impact of some amp controls can (strongly) depend on amp topology.

For example, tone controls (circuits and location) are not universal and affect tone differently in different amps (and can depend on MV etc).
 
Top Bottom