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OHM Rating on my new cab?

tombba

Member
Messages
121
I just picked up an 2x12 cab loaded with 2x 15 ohm celestion g12h65"s out of an old Marshall cab.Don"t know what that makes my cab rated and want to make sure I match it correctly to my Jet City JCA50 amp "2x8ohm"or 16 ohm jack selection.I don"t know if the cab is wired in series or parallel and don"t know if it matters.I think it makes the cab close to an 8ohm.Also,I beleive I"ve heard you can be safe and always run an 8 ohm head into a larger ohm cab but not the other way around.Thanks for the help!
 

michael patrick

Silver Supporting Member
Messages
10,813
Hold on there... There's two pieces of information you need to figure out the ohms rating:

1) The ohms rating of each speaker, and
2) Whether they are wired in series or parallel.

A guitar cab is most likely wired at either 8 or 16 ohms. But if you want to know for sure, you'll need to figure out the two things above.

<edit>

When I first read it, I thought you were saying they were 15" speakers. But now I see they are 15 ohm speakers. Which means your cab is most likely rated 8 ohms (2 15/16 ohm speakers wired in parallel). I don't think I've ever seen a 32 ohm cab, which is what you'd have if they were wired in series.

Probably still a good idea to open it up and confirm it is what you think it is.
 

VaughnC

Silver Supporting Member
Messages
17,914
Most likely the speakers are wired in parallel for a total of 8 ohms...but you should check the cab with a meter to be sure. It should read around around 6 ohms resistance on a meter for 8 ohms impedance. For an 8 ohm cab, given the choice, I'd use the amp's 16 ohm jack as there's less chance for flyback voltage than with the amp's "2x8 ohm" (4 ohm) jack. But that's only one impedance step so either jack should be ok.

With a tube amp, its less risky to have the cab impedance lower than the amp's impedance than vice versa. Flyback voltage risk increases as the cab impedance increases in relation to the amp....and flyback voltage is usually what kills output transformers. For example, never run a 4 ohm amp into a 16 ohm cab...this is very risky to your amp. Conversely, a 16 ohm amp into a 4 ohm cab will stress the tubes but that's unlikely to harm the amp itself. An 8 ohm amp into a 16 ohm or 4 ohm cab is usually safe as most amps will tolerate a one step mismatch...but the 16 ohm cab would be slightly more risky than the 4 ohm cab.
 

dragonbat13

Member
Messages
87
Most likely the speakers are wired in parallel for a total of 8 ohms...but you should check the cab with a meter to be sure. It should read around around 6 ohms resistance on a meter for 8 ohms impedance. For an 8 ohm cab, given the choice, I'd use the amp's 16 ohm jack as there's less chance for flyback voltage than with the amp's "2x8 ohm" (4 ohm) jack. But that's only one impedance step so either jack should be ok.

With a tube amp, its less risky to have the cab impedance lower than the amp's impedance than vice versa. Flyback voltage risk increases as the cab impedance increases in relation to the amp....and flyback voltage is usually what kills output transformers. For example, never run a 4 ohm amp into a 16 ohm cab...this is very risky to your amp. Conversely, a 16 ohm amp into a 4 ohm cab will stress the tubes but that's unlikely to harm the amp itself. An 8 ohm amp into a 16 ohm or 4 ohm cab is usually safe as most amps will tolerate a one step mismatch...but the 16 ohm cab would be slightly more risky than the 4 ohm cab.
+1

To answer your speaker wiring question. If you have two 15 ohm speakers and the positive side of each speaker is connected before going to the tip of the speaker jack, then they are parallel. If one speakers positive is hooked to one speakers negative, and you have one wire from each speaker going to the jack they are series. Parallel= close to 8 ohms, Series=close to 4 ohms.

COOL TIP:
So what speaker connection is positive you may ask? Well if they are somehow not marked or marked in a confusing way you can find out.

Take a AA or AAA battery and hook it to the speaker terminals for a second. Just touch one side of the battery, while holding the other. You will hear some scratching and see some movement from the speaker. If the cone of the speaker moves OUT while touching the battery, then the wires and terminals are hooked up "correctly" to the battery. If the cone goes in, the battery is backwards. I have been doing this for as long as I can remember, and have never messed up one speaker. But I wouldnt go around doing it with car batteries, just use a small AA or AAA or maybe a worn out nine volt at the most.
 

RussB

low rent hobbyist
Messages
11,165
tomba, do you have a volt/ohm meter?

If not, find someone who does and check the resistance of the cab by inserting a speaker cord, and placing the probes on the center and sleeve of the plug.

The cab is most likely wired for 8 ohms (actually, 7.5 ohms)
 

tombba

Member
Messages
121
Thanks for all the detailed answers guys.That helps a bunch.I actually had it backwards:messedup.It"s good to know the ohms on the amp always wants to "see "an equal or lower ohm rating of the cab.
 




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