#1
I have a peavey classic 30 combo and want to bring in a 2x12 cab. since there is already a speaker in it, there is only one speaker output and i dont know how many ohms it can handle. if somebody knew it would be great!
#2
ethanball888
Here's what the manual says:

"EXTERNAL SPEAKER JACK
This is provided for connection of an external speaker cabinet. Minimum total impedance is 8 ohms."

The amp has 16 Ohm stamped on it. To me it seems like this external jack will run the internal speaker and the external cabinet in parallel. I would check that your internal speaker is indeed 16 ohms. If so, this would mean that the amp wants a 16 ohm external cabinet. The 16 ohm internal and the 16 ohm external is a total of 8 ohm when connected in parallel.

This was the way my 5150 212 combo worked as well. It had 2x 8 ohm internal speakers in series for 16 total ohms.
Guitars:
Ibanez RG1570 Prestige
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Amps:
Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .
#3
ethanball888
Farther down in the manual it lists this info as well:

Rated Power & Load:
30 W RMS into 16 or 8 ohms

Speaker (Combo Only):
One 12" Blue Marvel, 16 ohms
Guitars:
Ibanez RG1570 Prestige
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Amps:
Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .
#4
I also have a classic 30. To my understanding the external jack is for a 16 ohm cabinet. However I have run an 8 ohm cabinet on mine with the internal speaker unhooked and have experience no problems. Ive also heard of other people online replacing the 16 with an 8 ohm speaker with no ill effects. To be safe though I would just use a 16 ohm cab if I were you.