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impedance

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6/11/1998 12:47 PM

D

Probably a stupid question, but an important one, nonetheless...

I have a 50 watt Marshall master volume head which allows me to select the impedance for the type of cab I'll be running. Usually this is a no brainer, as all of my cabs -- until now -- tell me how many ohlms they are. But now I have this vintage Vox 2x12 that has been modded by having its original Bulldogs taken out and new Celestion 75 watters put in. The Celestions are rated at 16 ohlms, and the cab is mono. Does this mean I should flip the switch on the back of my head to 16? Or would it be 8 for some reason?

Pls. e-mail me -- I'd greatly appreciate it!

Thanks!

6/11/1998 7:32 PM

Benjamin Fargen

D, This is not a stupid question and yes it is a very important one! The correct setting will be determined by how the speakers are wired...this will be either series or parallel.
Series is when your speakers are running one into the other and then back out. For this type of wiring setup your ohm rating would be 32(16+16=32). Not very likely for a guitar cab which is usually 4,8 or 16 ohms.
Parallel is when both positive are run together and both negatives are run together.This cuts your ohm rating in half... 16 & 16 = 8 ohms.(so leave the amp on an 8 ohm setting) This is most likely the way it is wired. Still double check because running in series with 32 ohms will give you 1/4 the output volume you should have and could cause your amp to work to hard and overheat!
Hope this helps, Benjamin.

6/12/1998 7:47 AM

A rough estimate of a speaker voice coil's rated impedance can be gotten by measuring the DC resistance across the terminals. The DC resistance will be less than the rated AC impedance, usually around 75%.

Plug a speaker cord into your cabinet, and with your ohmmeter measure the resistance between the barrel & the tip of the cable's plug at the other end. If you read somewhere between 5-7 ohms, you can assume the rated impedance is 8 ohms. Speakers that are supposed to be 4 ohms will have a DCR of around 3 ohms; those reading 10-14 ohms will be rated at 16 ohms.

These readings can differ from speaker to speaker, due to differences in voice coil construction. But the DCR will always be less than the rated AC impedance, but greater than a speaker with the next lower incremental impedance rating. Ex: DCR reads 5.3 ohms.....you can bet it's not a 4 ohm speaker, but an 8-ohm. Note that some 6-ohm impedance woofers do exist (Pioneer), but you'd find them in stereos, not guitar amps.

6/12/1998 8:55 AM
D	Many thanks to you both (Benjamin and Doc) -- your help is greatly appreciated!!!

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