Using 2 cabs with one amp

[bluesbear]

Here's another concept I've never had to deal with! My son has a Plexi clone (Doug's). He's just added a second 4 ohm cab. He's always used 1 cab with the amp set to 4 ohms (of course). If he plugs one cab into the main speaker jack and the other into the extra speaker jack, does that come to 8 ohms... or does it come to 2 ohms?
Thanks,
Dave

[Geezer]

2 ohms.......the 2x 4Ω cabs are parallel & so = 2 ohms

[bluesbear]

Argh! The amp just has 4 and 8 ohm outputs. Is there some way to:
1. wire the individual cabs for 4 ohms, run a wire between them, then off to the amp at 8 ohms? or,
2. wire each cab with a 4 ohm out and a 16 ohm out?
Thanks,
Dave

[jhadhar65]

What is the impedance of the speakers themselves and how many speakers are in each cab?  You said 'plexi' so I'll guess we're talking about 4x12 cabs.  Here's a general guide for 4x12's with speakers of common impedance and a single input jack:

*  A 4ohm cab must contain:  a) 4ohm speakers wired in series/parallel or parallel/series, or;
                                           b) 16ohm speakers wired in parallel

*  An 8ohm cab must contain:  a) 2ohm speakers wired in series (not sure if these exist, but they might)
                                             b) 8ohm speakers wired in series/parallel or parallel/series, or;
                                             c) 32ohm speakers wired in parallel (not a common speaker, but they're out there)

*  A 16ohm cab must contain:  a) 16ohm speakers wired in series/parallel or parallel/series, or;
                                             b) 4ohm speakers wired in series

The output jacks on his head will be in parallel, so this will halve the impedance of a single cab when two are plugged in, as Geezer said.

[bluesbear]

I guess I didn't make that clear. There are 2 cabs. Each has 2 - 8 ohm speakers wired for 4 ohms. I need to figure out a way to run a wire from one cab to the next, then run a wire out to the amp at 8 ohms. Is this possible? The amp only has 2 impedence settings, 4 ohm and 8 ohm.
Thanks,
Dave

[bluesbear]

Anyone have any ideas?
Thanks

[G._Hoffman]

Wire the cabinets so the speakers are in series (the negative from the jack goes to the negative of one speaker, then the positive from that speaker goes to the negative of the other speaker, and the positive of speaker 2 goes to the positive of the jack).  The two two cabinets are now 16ohms.  If your speaker output jack are in parallel, then if you plug in both cabinets you have 8 ohms. 

I knew I remembered Doug having a page about this.  HERE.


Gabriel

[bluesbear]

"If your speaker output jack are in parallel, then if you plug in both cabinets you have 8 ohms."

Is it possible to wire the speaker jacks in series? Thst way my son wouldn't have to:
a) replace his speakers with 16 ohms (for 8 ohm cabs)
b) replace the OT (my problem) or
c) not be able to use only one of his cabinets?
Thanks,
Dave

[Structo]

Two cabs, 4 ohms each.

If both are plugged into the amp it sees 2 ohms.

If you were to wire the two 4 ohm cabs in series to each other, the amp would see 8 ohms.

You probably need to make a little break out box to easily wire it up.

You would wire one cab positive to the other cab negative.
Then you have one positive and one negative wire from both to a plug that plugs into the amp.

Is that clear?

Personally, I would just run one cab but I know how youngsters are, they like a lot of speakers.

[sluckey]

Try this...

[bluesbear]

Structo, I believe you're saying what Sluckey drew? Anyway, this does it. Sluckey's drawing needs to be in the archives, I believe. Useful!
Thanks to all!
Dave

[sluckey]

Dave, you may want to put a caution tag on that Y cable. Warn young innovative musicians about the dangers of connecting two amps to one speaker cab with this cable.

[bluesbear]

Thanks, I will.
Dave