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SurfGuitar101 Forums » Gear »

Using Extension Cabinets

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JakeDobner Joined: Feb 26, 2006 Posts: 12159 Seattle	Posted on May 21 2006 06:44 PM Why does adding an extension cabinet do to the ohm rating? For example, a 8 ohm cab and a 4 ohm cab. Top
unlunf Joined: Feb 28, 2006 Posts: 106 The Vatican	Posted on May 21 2006 08:52 PM Jacob, Short Answer It reduces the overall ohmage presented to the amp. Given your example, the combination of the two cabinets will yield 2.666 ohms. (This is because all Fender amps are wired to put both speaker jacks in parallel.) Your next question is, "is this safe for my amp?" If your rig has a label stating "Minimum 4 ohms", then yes, 2.666 is OK. Not the greatest thing in the world for it, but the chances of blowing things up are very slim. Longer Answer The usual accepted tolerance figure is anywhere between 50% of the labeled minimum ohms, up to double the minimum ohms. If your amp says "Minimum 4 Ohms", then you shouldn't hook up anything less than 2 ohms to it, nor greater than 8 ohms. Doing otherwise puts the output tubes and both of the transformers at risk. All this sloppiness is due to component tolerances...... from a design standpoint, I'm sure most engineers would cringe to hear me say these things. The one quick rule to remember when playing with ohms and impedances is: when you put them in parallel, the total value will be less than the smallest individual value. If you keep this in mind, you should be able to do a 'quick reality check' whenever your have to solve an emergency speaker problem. A very quick guide: 2 x 8 ohm cabs = 4 ohms total 1 x 8 and 1 x 4 = 2.666 ohms total 2 x 4 ohm cabs = 2 ohms total Two or more cabs of the same impedance - divide one cab's rating by the number of cabs For non-equal cabs, approximate between the lesser and the greater cabs, then cut that in half (4 and 8 becomes 6, half that is 3) You can hook up as many cabs as you want, provided you stay within the impedance tolerance region, and provided further that you supply enough amplifier power (watts). HTH unlunf — MY RIGHT HAND IS FASTER THAN YOURS! (copyright 2003, Bruce Welch) Top
WR Joined: Feb 27, 2006 Posts: 3832 netherlands	Posted on May 22 2006 02:41 AM small added note: most Fender amps have the speaker jack and ext. speaker jack wired in parrallel, other (later SF I presume) have a weird kind of switching jack, which actually hooks up to another OT tap, with a different impedance. so that you can hook up an ext cab without any trouble. I've only seen this in UL twins, I suspect it only is available on the UL amps, but maybe Fender uses a same sort of thing too on new amps. my suggestion is to open up the amp and look how it's wired. Or maybe someone else knows exactly what the deal is. WR — Rules to live by #314: "When in Italy, if the menu says something's grilled, don't assume it is." https://www.facebook.com/The-Malbehavers-286429584796173/ Top
D22 Joined: Apr 20, 2006 Posts: 2054	Posted on May 22 2006 04:23 PM ....on the market is the Weber Z-Matcher. I mentioned this to Armin Brown for use with his Showman-dual cab rig. I use one and like it a lot. He bought one and tried it out on Sunday at the HBISM and it made the newly rebuilt Showman rock through a 2x15 cab and a 1x15 cab. Well-balanced and lush. The crowd was mucho happy. https://weberspeakerscom.secure.powweb.com/store/zmatch.htm — Top
unlunf Joined: Feb 28, 2006 Posts: 106 The Vatican	Posted on May 22 2006 11:48 PM Wannes, UltraLinear models only. The cost of that output transformer is exhorbitant! DGG, There's a reason I didn't mention this device in my answer, can you guess what it is? unlunf — MY RIGHT HAND IS FASTER THAN YOURS! (copyright 2003, Bruce Welch) Top

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