I've got a Fender Bassman 250watt 2x10 and am thinking about attaching a 1/15 cab to it for more umph (my guitarist just bought his new 4/12 stack and i don't want to be drowned out on stage or in our rehearsal space). The head puts out a minimum 4ohms at 250watts. So what specs should I be looking for in a speaker cab to ensure i don't blow something up? does it just need to have a minimum 4ohm capacity?
I think you mean "250watts at a minimum 4ohms". As long as you keep the total impedance at 4 ohm or greater, you should be OK. Just remember, the higher the impedance, the less power your amp will output.
Right, so check and double-check to be sure what the ohm rating is of the speakers already in your combo. If it's a 210, you're hoping each speaker is rated 16 ohms. That would mean the two of them together, in parallel, total 8 ohms. Then you can buy an 8 ohm external cab, and it will combine in parallel with your combo to equal 4 ohms. That will get you the maximum power at safe operating parameters. If each speaker in the combo is 8 ohms, then it's already a 4 ohm total load and you can't add an external speaker. But actually that's uncommon, especially if there's an "external speaker" jack on the amp, which would mean they expect you to be able to add another cab. Short answer: make sure your amp's total load is 8 ohms, then buy an 8 ohm cab to go with it.
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