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Thread: late 70's UL super reverb speaker impedance

  1. #1
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    late 70's UL super reverb speaker impedance

    Does anyone have this amp? I'd like to know the individual speaker impedance for this amp, schematic says 16 Ohms: http://schematicheaven.net/fenderamp...-bmstr_rev.pdf

    But I have four 8 Ohm speakers (they look stock, blue fender label, CTS I guess) in my amp, paralleled total load measured is 2.3 Ohms.
    The back of the amp says 4 Ohm minimum load. My best guess is that the previous owner replaced the speakers with incorrect impedance. Or Fender screwed up...

  2. #2
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    So it looks like it should be a 4 ohm load the amp is expecting so the speakers should be 16 ohms each.
    Do the speakers have an ohm spec on them? Do they say 8 ohms or are you assuming?

    I would start by un-soldering all 4 speakers and measuring them individually at the speaker with no wiring attached. They all measure about the same.
    If they were 8 ohms, the reading should be less then 2 ohms (like 1.x).
    2.3 does seem on the low side; what type of meter are you using? is it set to the lowest setting for ohms?
    pdf64 likes this.

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    Yeah, the schematic shows (4) 16 ohms in parallel for a total of 4 ohms.

    I wouldn't put the blame on Fender.

    Aside: the output transformers used back then are pretty tolerant of slight mismatches.
    2.3 compared to 4?

    Meh.

  4. #4
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    Well, the speaker resistance should be a bit lower than the nominal impedance; but then most meters will read high on such low resistances. So best to do as per post #2.
    The VHT on these amps is rather high, so if it's going to be pushed hard, to give the tubes a fighting chance it may be a good idea to ensure that the intended load is connected.

  5. #5
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    I think the extra .3 or so ohms can probably be accounted for with meter lead resistance, along with that mile of speaker wire back there. I'm with Jazz on this one, especially if that OT is original. Because even if they WERE all 16 running in parallel, add an extension cab, bang, 2 ohms.

    My Bassman has run for many years with the wrong load, and not quietly, either. And the only tube death came when the bias caps dried out & failed.

    Justin
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