Don't know the resistance on my speaker

Discussion in 'Amps and Cabs [BG]' started by A. D. Fairhurst, Jan 17, 2018.

  1. My current amp is a Maine Bassman 100B, a weird 100 watt combo amp from the early 80's with a rear-firing 15 inch speaker. It sounds pretty good, I but I am planning on buying a 100 watt Blackstar valve head and running it into the speaker instead of the built-in amp. Only problem is that whilst the back panel does have an input for an external head to the speaker, it doesn't say anything about the resistance on the speaker. Does anyone here have any idea on what it might be?
     
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The input is likely not for powering the speaker, it's much more likely to be a connection for using an external speaker with the amp, either in addition to or in place of the internal speaker.

    If you need to know the impedance of the speaker, open up the cab and read the resistance across the terminals with a meter. The DC resistance read will usually be ~70% of the nominal impedance - for a 4 ohm speaker you should read around 3 ohms, for an 8 ohm speaker around 6 ohms, etc.
     
  3. Raf Seibert

    Raf Seibert

    Dec 16, 2013
    Texas
    You will need to disconnect one or both leads to the speaker temporarily. Test the passive resistance (dc) of the speaker with a multimeter. If it’s an 8 ohm speaker, you should see a little over 6 ohms on the meter. If the meter shows just over 3 ohms, it will be a 4 ohm speaker.
     
  4. Here's what it shows.
    IMG_0018.JPG
     
  5. Rick James

    Rick James Inactive

    Feb 24, 2007
    New Jersey
    By his picture it looks like he can plug in an external amp, so there's no need to open the cab. Just plug in a speaker cable and read the DCR across the plug at the other end.
     
  6. How would I do that?
     
  7. Rick James

    Rick James Inactive

    Feb 24, 2007
    New Jersey
    With a DMM or ohm meter.