Speaker impedance?

[Ravenswerld]

Hello all,
    Maybe someone can help. I have a fishman loudbox artist that I want to be able to run an extension cab on. So, I will have to go in and do the mod myself as there is no ext cab capabilities. My question is if I tap off the wire inside the cab do I have to worry about impedance? Or is it better to go from the speaker itself and wire it accordingly so the impedance is correct with the amp? Or maybe someone has an suggestion? Anyways, thnx all for the kindness, have a great day!!!

[Neil mcNasty]

Speaker impedance is very important to pay attention to!
If the speaker has lower Ohm than the amp... You destroy the amp!
It burns the output transformer, since there is not enough resistance and the coils overheat, burn the insulation and then short-circuit.
But if the speaker has a higher Ohm than the amp... You are OK! No problem!

My recommendation/how I would do it:
Check the Ohm output of your amp (check the speaker wiring and Ohm ratings of the speakers and calculate the Ohm rating if it is not already labeled on the amp).
Then add a switched jack between the amp's speaker output and the speakers, that breaks the connection to the built in speakers when a speaker jack is inserted, and then sends the signal to the external cabinet.

If you have a dedicated external cab that is not being used for anything else, I would probably rewire it to match the amp...
If you do not have a dedicated cab to use for this, the rules are as following:
If the external cab has a higher Ohm rating than the amp (example: amp is 8-Ohm, cab is 16-Ohm), then you can safely use it!
BUT! If the cab has a lower rating than the amp (example: amp is 8-Ohm, cab is 4-Ohm) you should not use it as it is. As it might damage the amp!
(sorry for the repetition, but it is so important that it is worth hammering in!)

I have done this mod to several of my small vintage amps (that has very poor and small speakers) and it works like a charm!
I sometimes run a 16-Ohm cab on my Silvertone's 8-Ohm output, with no problems, except that you might loose a tiny bit of top end and get slightly lower volume than usual...

Hope this helps you to accomplish your task!
Cheers and Good Luck!

[ibanez48]

I bought a brand new crate g160 combo back in 1990. Well I started my first hard rock band back then my new crate was working great. With only one 12 inch speaker in it I wanted a 4x12 cab. It has a ext cab out on it. I found a used Jackson 4x12 cab at guitar center I bought it got home and plug it in I unplugged the 12 in my amp and used the cab only sounded great. I didn't know anything about ohms back then and I couldn't figure why my volume level dropped so much. My amp was 4 ohms and the cab was 16 ohms. Well that was the problem. I found a guy at this little guitar store and told him about was going on  and he said running 4 ohms into 16 is no good that's why I lost volume he ask me if I new if the speakers in the cab were 16 ohms I didn't know so I checked them and they were. I took the cab to him and he rewired the speakers down to 4 ohms got it back home plugged it in holy crap couldn't  believe the difference in volume. So ohms is really important. Sorry for the long story. And if I remember right I burnt the transformer out 2 times using 4 into 16 and he said that's what burned the transformer out after rewiring the speakers never had a problem again.

[Travis]

So the tech replaced two output transformers before he informed you about the impedance mismatch? That sucks!

IMO Neil's advice (while appreciated) is sort of only partially correct and to be honest I wouldn't recommend following that advice.

A couple huge flaws in that logic..

He doesn't seem to address the difference between doing this with a solid state or tube amp

He seems to suggest that this is a generally reliable solution, but it completely depends on the amp. There is definitely potential to damage your amp by following that advice

Many amps can handle an impedance mismatch without any issues. For example many Fender combos have an extension speaker jack which puts the extension cab in parallel with the internal speaker. In the case of a 5e3 for example, the internal speaker is 8 ohm and when connected to the recommended 8 ohm extension cab, the total impedance is 4 ohms. Note that in this case you are using a smaller speaker load than recommended which, contrary to Neil's advice, does not damage the amp. So what I'm saying here is basically just connect the correct load or do your homework to figure out what you can get away with, but do not just blindly follow advice about mismatching the speaker load if you care about your amp

[ibanez48]

I burnt the transformer out before the guy rewired the cab. I took it to a crate repair dealer and they fixed both times I was lucky it was still in warranty. And my crate is a solid state. And I agree with you on the ohms it's not good to mismatch ohms. If u do mismatch it before to long something is going to give I learned the hard way burning out mine. It's best to run it as close as possible. What's crazy is on a solid state amp the lower the ohms the better and louder it is. I don't think I have ever seen a solid state amp with 16 ohms. Back 25 years ago I never seen a tube amp with 4 ohms all the ones I seen only had 8/16 ohms. The techs I talked to said running a tube amp at 4 ohms wasn't good but now pretty much all tube amps have 4/8/16 ohms output.  

[Ravenswerld]

Thanks for the replies!
    What I was trying to do was be able to run the internal stock speaker. I don't think that's going to happen. Since I'm going to be building the cab. I can wire it to the impedance of the transformer and either put a switch or one of those jacks that cuts the other speaker in while plugged into the jack.

[Travis]

Ibanez48 you're right that many modern amps have multiple speaker outputs with different impedance. The way that usually works is that the output transformer has extra secondary windings which are compensated to give you the different impedance options. The important thing to remember with these is that only one of the secondaries can be connected to the speaker load at a time.

Ravenswerld your best option is probably the switched jack as Neil recommended. Having a toggle switch works the same but makes it easier to accidentally power up the amp without a speaker load

[induction]

ibanez48 wrote

I burnt the transformer out before the guy rewired the cab.
...
And my crate is a solid state.

Do you mean you burnt the output transistors? Solid state amps generally do not have output transformers.

The detail that your amp is solid state is fairly important in this case. For solid state amps, Neil is partly right, you can use a higher impedance cab with no worries (but less volume). You can even run it just fine without a speaker at all.* But if the cab's impedance is too low, you risk running the output transistors into thermal overload. In the extreme case, a short circuit in the speaker cable can kill the output transistors very quickly.

For tube amps, it's the other way around and Travis is right. Lower impedance cabs are fine, though they'll burn your output tubes faster. Tube amps can usually even handle short circuits fine.* But too high cab impedance causes flyback voltage that can instantly kill the output transformer.

This is actual reason you can't use a guitar cable as a speaker cable. (For some reason, nearly everyone I've ever overheard at GC says 'you can't use shielded cable for speakers'. I don't know where this idea came from, but there's no problem using shielded cable for speakers, though it's mostly pointless from a noise perspective and the increased capacitance could cause treble bleed.) The real problem with guitar cables is that the conductor in the core is tiny and therefore easily breakable/shortable. A short can kill most SS amps, and a break can kill some tube amps.

* Neither of these are entirely risk-free, so please don't.

[ibanez48]

Sorry meant the power transformer.