PI Bias Excursion

neale dunham

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

Bassmans aren’t cathode-biased.
 
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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?
 
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.
 
Seems to be relevant to the Trainwrecks primarily, based on the published comments.
 
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?
 
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.
 
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).
 
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