impedence of a blackface twin reverb

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mlynn02, Jun 27, 2005.


  1. mlynn02

    mlynn02 Member

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    i've got on old blackface twin that's about to get some new speakers. but, i don't know what the impedence of the amp is. can someone help? it's a 2x12, of course, so what ohm speakers should i buy? also, how many ohms should the ext. out be?

    thanks,
    matt
     
  2. hasserl

    hasserl Member

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    Speakers/Load: 2 x 12"/4 ohms (8 ohms each in parallel)
     
  3. mlynn02

    mlynn02 Member

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    so each speaker should be 8 ohm, because the line out from the amp and the ext. out are each 4 ohm?

    did i understand that right?

    thanks
     
  4. KCblues

    KCblues Member

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    That's correct, 8 ohms each in parallel for 4 ohms total is standard for a Twin Reverb.
     
  5. hasserl

    hasserl Member

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    Not quite, each of the internal speakers are 8 ohms each for a 4 ohm load at the internal speaker jack.

    But if you want to plug into the ext jack you need to juggle the speakers so there is still a 4 ohm oad on the amp. The output transformer only has one tap, and it wants to see a 4 ohm oad. So whatever combination of speakers you use should add up to 4 ohms.

    If you put a 4 ohm load into both the internal speaker jack and the ext speaker jack at the same time that would make for a 2 ohm load total, not the best arrangement, but you can probably get away with it.
     
  6. John Phillips

    John Phillips Member

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    You want two 8-ohm speakers for the amp, wired in parallel for 4 ohms and connected to the Main jack.

    You can then run any 4, 8 or 16-ohm cabinet from the Ext jack, at the same time.

    All those Fender amps are designed to handle a low mismatch down to half the matching impedance with no problem at all.

    The only thing not to do is run a 16-ohm cab as the only load (ie with the internal speakers disconnected). Even then you probably won't kill it, but it's a bad idea.


    One major reason old Fenders are so reliable is because Leo understood that musicians didn't pay too much attention to correct matching, so he did it for them by making the internal speakers (how the amp was normally expected to be used) the right load, and then making sure the amp would handle any normal mismatch too. There's even a shorting switch in the Main jack so if you forget to plug the speakers back in, the amp doesn't run with no load.
     
  7. mlynn02

    mlynn02 Member

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    here's to leo! glad to learn it's so easy afterall. thanks for the info guys!

    -matt
     
  8. AdamGian

    AdamGian Member

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    I wish I had a nickle for every time John has provided this answer to this question - I'd have two bucks :D
     
  9. TheAmpNerd

    TheAmpNerd Member

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    Adam,

    Good thing you aren't greedy 'cause a C-note will yield ya
    four grand!
     

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