Ampeg cab with ohm switch help

[Oren Hudson]

I have one of the B25B Ampeg heads with the B25B 2-15 cab. The cab has a selectable ohm switch on the back that slides from 4 to 16 ohms. How do I check the speaker jack (or can I check the jack) for the ohm output there? I'm wanting to see if the switch actually works, and if it doesn't, is it doing 4 or 16 as it sits, but I'm not sure how I'd do that. I have the speakers wired to 4 ohms. Thanks.

 

[sickest beast]

I have the same cabinet, same switch, same doubts.

I looked inside at the switch, it's really a simple little switch, I don't understand what it does, or how it could change the impedance from 4 to 16. The cabinet is normally rated 4ohms, which means the speakers should be 8ohms each, how does a little switch turn that into 16ohms?

Someone help!

 

[coreyfyfe]

Probably switches between series and parallel. Two 8 ohm speakers in parallel will give you 4 ohms, and the same two 8 ohm speakers in series will give you 16 ohms.

Best was to test would be to get a multimeter, plug a 1/4" cable into the cab and put one end of the meter on the tip, one on the sleeve of the exposed end of the cable. Should read slightly below the rated impedance.

 

[coreyfyfe]

Like this...

(http://colomar.com/Shavano/spkr_switch.html)

 

[sickest beast]

Ahhh, why didn't I think of series/parallel...

that makes perfect sense. thanks coretfyfe

So what kind of difference would the speakers being series or parallel make? Either way the speakers don't change, it seems the route to and from the speakers changes, does it just make one louder than the other?

http://www.termpro.com/articles/spkrz.html

nevermind, i seem to have found an article that explains all this.

 

[coreyfyfe]

It's probably simpler than that. The output tap of the B25b is rated for 16 ohms, but you may want to use the speaker cab with a different head (say a V4 or SVT) which don't do 16 ohms, but can do 4 ohms. So, wire in a switch, and bingo, bango, you have an quick way to increase sales potential.

 

[sickest beast]

I don't have the original Ampeg head that was paired with this cab.

If I could find another one of these cabinets I could run them both at 8ohms or 2ohms. Pretty cool.

 

[Oren Hudson]

I checked the ohms, both with the speaker jack and the cord plug, and they both read less than 4 ohms, no matter if the switch is on the 4 ohm side, or the 16 ohm side. So, I guess the switch has either been disabled, or is broken. In any event, it sounds really good with an Altec 421A and a JBL D140F loaded in the cab. The other Altec (421As being the correct optional speakers) is off getting rebuilt. Will probably sound even a bit better when it's back and in the cab.