When you connect an external cab to a combo using the external speaker output, is the output of the internal speaker reduced, or does it remain the same?

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Current Rig:
-Guitar: 72 Tele Custom Reissue
-Amp: Peavey Prowler 45w Tube Combo (6L6's)
-Effects: Boss TU-2 > Boss Blues Driver > Boss OD-3 > FX loop: Guyatone Slow Volume > Boss GE-7 > Boss BF-2 Japan > Ernie Ball Jr. VP > Ibanez DE7 > Boss DD-5 > Digiverb

Some (most?) amps that can drive an external speaker have an impedence switch.

My Laney has a switch from 8ohm to 4ohm. 8ohm is used for the internal speaker. I switch it to 4ohm when I connect an external cab. The volume is not noticably reduced.

I'm sure it goes down some though, since the amp is now powering more speakers.

Which amp are you talking about?

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g&l asat classic (tobacco burst) -> technophobia tb loop -> digitech wh-4 - dunlop 535q wah - boss dd-3 (mij) - boss tu-2 - ehx small stone usa - danelectro daddyo - danelectro fab tone - danelectro danecho -> tb loop -> laney lc30-ii (+ laney gs410 cab sometimes)

Peavey Prowler 1x12 combo.
http://www.peavey.com/media/pdf/manuals/80304549.pdf

The external speaker output impedance isn't adjustable, it's fixed at 8 ohms. The internal speaker is also 8 ohms, so the total impedance would be 4 ohms with an external cab.

But the thing I don't understand is whether the external output just puts a cab in parallel with the internal speaker, or if it's somehow some sort of isolated output that's only active when connected. Does that make any sense?

I'm kind of wondering if the internal speaker recieves a signal at 4 ohms until another cab is placed in parallel with it, or if the speaker out jack is a switching jack, that switches the overall impedance to 4 ohms when something is plugged into it...

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Current Rig:
-Guitar: 72 Tele Custom Reissue
-Amp: Peavey Prowler 45w Tube Combo (6L6's)
-Effects: Boss TU-2 > Boss Blues Driver > Boss OD-3 > FX loop: Guyatone Slow Volume > Boss GE-7 > Boss BF-2 Japan > Ernie Ball Jr. VP > Ibanez DE7 > Boss DD-5 > Digiverb

I have a Blues Deluxe that has an internal 8ohm jack and an external 8ohm jack. The internal speaker jack must be 8ohms, as well as the ext. On mine if you put just a plug without ant cable on it into the internal jack, you then can hook upa 4ohm load to the ext. hookup. Not knowledgable with your amp, but I would gander a guess that it is similiar.

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SeNzU, Why not Epiphone...

My Rig
Epiphone Dot VS
Epiphone Les Paul Custom VS
Fender Blues Deluxe
Epiphone 1X12 ext. cab with a Greenback