Speaker ohm testing

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Bruceman, Dec 19, 2009.

  1. Bruceman

    Bruceman Member

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    For the first time tonight I got out the multimeter and tested all my speakers. On my old Silvertone 2 x 12 model I got 7.4 on one and 1.5 on the other. Even tho I think I know the answer, being a fledging electro-maniac I wanted to ask the experts. (or someone that knew) !

    What's with the 1.5 reading? And I'm guessing the 7.4 would be called an 8 ohm speaker?


    thanks
     
  2. Marble

    Marble Member

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    were they still wired together when you tested them?
     
  3. Patrick620

    Patrick620 Member

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    and disconnected from the amp?
     
  4. Bruceman

    Bruceman Member

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    Yes, they were still wired to the amp.
     
  5. Fader

    Fader Member

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    Isolate the speakers from any wiring and try it again.
    Depending on the wattage, you should get a reading of around 5 ohms if it's an 8 ohm speaker and around 12 ohms for a 16 ohm speaker.

    The difference is because you are measuring only the resistance of the coils and not the impedance. Impedance is the combined resistive and reactive components of the speaker load.
     
  6. Bruceman

    Bruceman Member

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    So.....without th speaker being loaded, is that why an 8 ohm will read approx 5 ?
     
  7. SatelliteAmps

    SatelliteAmps Member

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    It has nothing to do with wattage. A three watt speaker and a hundred watt speaker will both measure roughly 20 percent under when measuring impedance with a resistance checker.
     
  8. Bruceman

    Bruceman Member

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    Thanks for the info guys. I gotta pull the Silvertone apart a bit to isolate the speaker wiring. Then I'll measure them again. I hope the speakers are ok cause the way the chassis is made the tubes are in the way of installing speakers with larger magnets. These magnets are really small. Still trying to date the amp. No doubt it's a 50's something cause all of the large 1960 Silvertones I see are grey two piece units. This one is 2 tone brown and the speaker face is slightly angled.
     
  9. SatelliteAmps

    SatelliteAmps Member

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    Get some EIA codes off either the speakers, transformers, or potentiometers, and you can find the date really quickly.
     
  10. Bruceman

    Bruceman Member

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    ok - will do.
    thanks.
     
  11. Bruceman

    Bruceman Member

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    Here goes:
    pots = bb2917 / 137707 1 meg

    speakers = V12L7 / 328547

    Trans - Freed 24246

    Speaker wires go into chassis and are hard wired to a coil or little transformers...remember, I'm a fledging electro-rookie - the number on those things are 328609

    Appreciate the help!

    Bruce
     
  12. Bruceman

    Bruceman Member

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  13. SatelliteAmps

    SatelliteAmps Member

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    Pot code indicates CTS (137) 1957 (7) week 7 (07). Can't be earlier than that. Usually parts were made a few weeks or months before a product was released. That's a pretty safe bet, about mid 1957. The 7 indicating year could indicate 1947 or 1967, but that amp wasn't made in that covering in either of those years.
     
  14. Bruceman

    Bruceman Member

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    Cool. Thanks! It has four 6L6's and a fairly big tranny. I can hardly wait to fire ''er up. Hope I can find some speakers with small diameter magnets to fit. I just imagine it will sound better with better speakers than 1957 Silvertones. Any info on those speakers numbers? Thanks for your help. Really appreciate it.
     
  15. SatelliteAmps

    SatelliteAmps Member

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    328 is the code for Utah. Same code system for year and month. Those speakers are actually pretty good sounding in that amp, if they are the stock ones. If it's the amp I think, it is actually a stereo amp, running two power sections through two output transformers, one into each speaker. Great tremolo when it is working correctly.
     
  16. Bruceman

    Bruceman Member

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    Stereo 1957.....?
    There's only one transformer.
    ?
    One of the speakers is evidently blown. I'm going to try the 9v batt test this afternoon. But one measures 6.8, the other 1.4
     
  17. SatelliteAmps

    SatelliteAmps Member

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  18. Bruceman

    Bruceman Member

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    Yes! That's it. Model 1336 - How cool. With those friggin tilted speakers which looks like it will be a pickle using any other speaker, unless I can find a good speaker with a very small diameter magnet. I see Fender makes one that should fit. One of mine is blown.

    I see what you mean about stereo. You could essentially split your cord, one into each channel. I have a DeArmond pedal that looks like a volume pedal but it's just a channel switcher with a pot, so you can roll from one channel to another. THAT'd be cool....with different effects on each channel.

    Hmmmmm. Got me thinking now. Maybe wire one output with a speaker jack & run another cabinet. Eh?

    50 watts! That was bad ass for 1954....!
     
  19. SatelliteAmps

    SatelliteAmps Member

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    I own one, and had a customer bring one in. Got to run both at the same time, it was one of the more awesome dual amp setups I've ever run. Those things are great amps, very undervalued compared to some other brands of the same style.
     
  20. Bruceman

    Bruceman Member

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    What kind of speaker at what rating do you think would be good for a honkin' blues sound?
     

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