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Offline kurt_m

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ohm math
« on: March 11, 2003, 09:45:12 am »
i'd first like to say that i just picked up a model t reissue, and it rocks! my band's other guitar player picked up a 5150 (blah) and i just couldnt compete with my old ampeg vl-1002 as far as being loud, i'd have it cranked, and his amp would be on 3...so i picked up the model t and anywhere past 1 my house shakes... second, i have a old ampeg cab that is 16 ohms.  if i were to replace one speaker with a 8 ohm (it was a free celestion from a friend), would that make my entire cab 14 ohm instead of 16?  is this ok if i set my amp at 16 ohms?  it is a mono cab with a single input jack on it...thanks for any help!- kurt
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Offline JoeArthur

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ohm math
« Reply #1 on: March 11, 2003, 03:35:33 pm »
The total system impedance of your speakers depends on the impedance of all of them and how they are wired.  

How many speakers and how are they wired related to their individual impedances?
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Offline kurt_m

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ohm math
« Reply #2 on: March 11, 2003, 10:34:16 pm »
it's a 4x12 cab with all 16 ohm 50 watt celestion speakers, making it a 16 ohm cab.  i wired them according to celestion's diagram:

http://www.celestion.com/pro/wiring.htm

the one that says 4x12" series/parallel is the diagram i followed.  If i replaced one of the speakers with an 8 ohm,   what resistance does it make it? is it safe? the extra speaker was given to me after a van unloading mishap put a hole through one of the speakers in the cab.

an interesting note- when i bought the cab and pulled the original blown celestions (75 watters i think) out of it, they had 15 ohm stamped on the back of them. i thought that was really strange, i've never seen any speakers with a 15 ohm rating before...
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Anonymous

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ohm math
« Reply #3 on: March 12, 2003, 05:27:57 am »
In a series-parallel combination with 4 speakers...

One 16 ohm and one 8 ohm in parallel would give 5.34 ohms.  The other parallel 2-16 ohm speakers would provide 8 ohms.  So for the series connection of these parallel pairs... 8 + 5.34 = 13.34.

Speaker impedance ratings are "nominal"... because the exact impedance will vary with frequency.  The 15 ohm stamp... well, my first thought was that might be the voice coil resistance which is usually somewhat lower than the impedance rating, but 15 ohms seems too high for a 16 ohm impedance.  Maybe they are 15 ohm nominal impedance... Don't know.

The 8 ohm speaker would receive about 1.4 times more wattage than it's 16 ohm brother.  It may also be louder depending on it's efficiency.
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