Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Can I power an Ampeg SVT 4x10hlf with ext. speaker of my JC 120. Its a 79' JC.  I play synth and bass guitar. I just sold my other bass heads.

Thanks

That cab is 4 ohms.
The JC120 puts the ext. spkr in series so it will be a safe load.  However, it will not share power equally with the internal 8 ohm spkr arrangement.  The internal speaker will get more power than the ext. spkr in this case.
Also, this is a stereo amp, so if you have the chorus or vib. turned on, one ext.spkr jack will give the dry signal, the other ext.spkr jack will give only the chorus/vib signal.

Interesting question, following...
Looking at the schematic I don't see why the external speaker would be in series (pic attached). It looks like it's parallel, or am I wrong? It also says 8 ohm minimum next to the ext.spk. outs. I also have a late 70s version and this is from its schematic. Maybe Roland changed the ext speaker wiring over time?

Follow-up: would it be possible to hook up 2x 8 ohm (bass) cabinets to this amp? I wonder if the internal speakers are fine with the bass frequencies, as some guitar speakers aren't.

Thanks Rens.  Looking further it seems they changed from parallel ext. spkr to series somewhere around '79 or '80.
It would be wise to assume parallel unless proven otherwise.

So Zakky, no you can't do that, the minimum for the ext.spkr is 8ohms, that ampeg cab is 4ohms so you could blow the amp.
If you really wanted to do it, you would have to disconnect the JC internal speaker.

Thank you all. I need to figure out if my JC is 79' or before. I now have the tools to do the research.