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Little help, dumb questions, please bear with me...

Discussion in 'Amps and Cabs [BG]' started by JoeDaddio, Jan 18, 2012.


  1. So I'm back to playing bass as I seem to have joined a band on accident.

    It's been *years* since I've played, and in that time my equipment has just been sitting around. But the opprotunity arose for me to jam with some people, and it looks like I might be their bass player now.

    Anyways, on to the meat of the issue: I have no idea of the impedance of my cabinet.

    I am assuming that I bought an 8 ohm cab, but there are no markings on it to tell me for sure. Normally I wouldn't care too much, but because I have a tube head (400+) rather than an SS head, I think it might become an issue.

    The cab is a Goliath III, made before SWR was sold off to Fender. I emailed SWR with the serial number but honestly I don't expect much.

    I went to practice for the first time last week, had the cab plugged in to the 8ohm plug on the 400+ and nothing odd happened, and it was cranked up pretty well. The pair sounded fantastic, there was no overheating, no farting noises, no funny smells, or anything like that. It wasn't till afterwards that I started to question whether or not I had the impedance right.

    I don't know if this would cause an immediate issue (running 8 ohms in to 4, possibly) or if it's something that over time is going to ruin my amp. So should the damage already have been done, or if it is wrong, will the issue pop up later on?

    To be honest I was never all that good at the electronics aspect of it all, but now, especially after so much time has passed, I'm *really* miserable at it.

    Also, would it be safer to run 8 ohms in to 4, or 4 in to 8?

    My (totally flawed, based purely on unknown assumptions) understanding is that 8 into 4 would be better, as you're going from more resistance to less resistance, but this could be totally wrong and misguided.

    The more I think about this stuff the more it seems to confuse me, so any advice is appreciated.

    Thanks in advance for not mocking me too badly :help:


    joe
     
  2. Find someone with a DVOM, DMM, VOM any test meter that will measure resistance (even a $10 one from sears or an auto parts store). Measure the resistance (DCR in this case) at the amp side plug from the cab. DCR that you will measure will read lower than the impedance so if you read around 6.5 ohms it is a 8 ohm cab, around 3.2 ohms it is a 4 ohm cab. No more worries. :)
     
  3. will33

    will33

    May 22, 2006
    austin,tx
    Find some one with a little multi-meter if you don't have one and test between the tip and sleeve of the end of your speaker cable. An 8ohm cab will read between 5 and 6 ohms. A 4 ohm cab will read 3.something.

    Tube amps work the opposite of SS in that it's safer for them to run below the rated impedance rather than above. If you just don't know, plug into the 8ohm tap like you did. If it turns out to be a 4 ohm cab it won't hurt it. Although it is best to get your cab figured out and then plug it into the matching output on the amp.
     

  4. I had been thinking of dropping by Home Depot and getting one as it'd be a good thing to have anyways, but when looking online I found a bunch of stuff about how ohm meters on voltmeters were totally unreliable at finding ohms, and a true impedance meter was like $300.

    But if it's as simple as just rounding up, then there shouldn't be any problem at all. Thanks!

    Just to be sure, (I'm a bit of a moron at this stuff), I'll plug the cable in to the cab, then take the other end of the cable and place the leads from the meter on the other end of the cable (the top and side of the cable, separated by the black line). Is this correct?

    !BoC3JhgBmk~$(KGrHqIH-E!Etr2s)ytZBLl-BE6MDQ~~_35.

    Thanks again! If so you just saved me a ton of worry, and I'll head to the Depot on my way home from work tonight. If you're ever in Orange County I'll buy the first round!


    joe
     
  5. will33

    will33

    May 22, 2006
    austin,tx
    B-string types faster than me^:p

    With the cheap little meters like a lot of us have who don't make their living with them, you have to factor in the resistance in the meter leads, there will be a little in your speaker cable as well so I'll say your 8 ohm cab would read somewhere between 5 and 7...the 4 ohm between 3 and 4.
     
  6. Broadstbully22

    Broadstbully22

    Dec 5, 2011
    Do you have a friend who is a electrician? Save some loot and have him do it. I wish you were close I'd let you use mine. I'm not a electrician,i just have a ton of tools. lol.
     
  7. will33

    will33

    May 22, 2006
    austin,tx
    Yes, that's correct. On the 1/4" plug, Tip = top or end. Sleeve = side.

    The cheap little $10 meters read resistance, not impedance, but they're all you need to see which cab you have, test cables, etc.
     
  8. Thanks so much guys. I wanted to have the issue solved before practice tonight, and it looks like I will. I really appreciate all the help!


    joe
     
  9. N.F.A.

    N.F.A.

    Jun 25, 2009
    In a blue funk
    His Mesa 400+ can switch to 2 ohms. Couldn't he just switch it to 2 or even 4 ohms and forget it?
     
  10. Yes the tip above the black line and the silver part below the line. The meter doesn't have to be real accurate as you are reading the DCR (Direct Current Resistance) instead of AC Impedance which varies with pitch and can not be measured directlywith a hand held meter only. :) OC old stomping grounds. :)
     

  11. The cab is either 4 or 8 ohms (that's all they offered), so that's why I only mentioned those numbers.


    joe
     
  12. tom-g

    tom-g

    Oct 2, 2007
    Not a good idea. Always match impendances with tubes.
     
  13. Because it is always better to match the plate load (in the amp) to the speaker load. Usually no harm will come of mis-matches but why take the chance of shortened tube life? That is not a cheap amp to re-tube. ;)
     
  14. will33

    will33

    May 22, 2006
    austin,tx
    Tubes work different than SS when it comes to matching impedances. He should figure out what he has and use the proper tap. When in doubt use the highest impedance setting, not the lowest. Most of the old Fenders were extremely sturdy when it came to mismatched loads. Other amps could fry the output transformers when running a higher impedance.
     
  15. Crater

    Crater

    Oct 12, 2011
    Dallas, TX area
    I know nothing about these cabs, but a Google image search for "SWR Goliath III" showed different cabs with the ohms marked on the front speaker grille.

    4410500800v8_hi-8eb9adbaec004dc937627e9c54654bfe.
     
  16. N.F.A.

    N.F.A.

    Jun 25, 2009
    In a blue funk
    Wow, ignorance is bliss in my case. I have done this for years with cabs and never had problems. My Marshall SLP switches. I always just switched it to 2 ohms and didn't worry anymore about it.
     
  17. Yeah, it would be super cool if mine had that.

    (super crappy phone pic from a while back)

    [​IMG]


    joe
     
  18. The meter is reading around 4.8 to 5.1 (jumping around a bit) so I'm hoing to assume it's an 8 ohm.

    Thanks guys.


    Joe
     
  19. I'd say that is a safe bet.:)
     

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