What's going on with these Altec 421A's?

Discussion in 'Amps [BG]' started by portaflexer, Feb 9, 2013.


  1. portaflexer

    portaflexer

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    So... I recently bought an old Ampeg 215 cab. It was loaded with Altec 421A's. The previous owner says he bought it new in 1970 or 71, and it is 100% original. I pulled the speakers out to take a look at them and found some odd things. Check this out.

    1. Both speakers have similar date codes from 1969, so they're probably original... nothing weird yet
    2. Speaker 1 meters at 7.8 ohms and speaker 2 meters at 4.0 ohms, but both are marked as 8 ohm speakers
    3. Speaker 1 appears to have a non-original cone and a larger dust cap
    4. Speaker 2 (the one that meters at 4 ohms) appears to have the original cone, dust cap, and cork gasket

    So, here's the net... one speaker appears to have been reconed but meters correctly. The other one appears to be original, but meters wrong. What the heck???

    Any ideas???
     
  2. portaflexer

    portaflexer

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    Speaker 1 possible recone - meters at 7.8 ohms

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  3. portaflexer

    portaflexer

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    Speaker 2 appears to be original - meters at 4.0 ohms

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  4. jnewmark

    jnewmark Supporting Member

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    I don't really know the answer, but as far using a multi meter to check ohm load on speakers, it really does vary. I have two JBL K140's that both register at 6.5 on my multimeter. I also have an old, no name Alinco 12 guitar speaker that is labeled 8 ohms, but registers dead on 4 ohms.
     
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  6. AstroSonic

    AstroSonic Supporting Member

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    Altec's (as well as JBL's and Stephens) had a aluminum dust cap that was the same diameter as the voice coil (and was attached directly to the voice coil former): in this case, 3 inches. Many recone kits and 'other' manufacturers often used smaller voice coils (1 1/2 - 2 inches), but used a 4 inch diameter aluminum dust cap, so they looked just like a JBL or Altec. Their dust caps were also a heavier gage and sometimes even just aluminized mylar. Whats the big deal? Well, the aluminum dust cap was adopted from home hifi speakers where it actually functioned as a dome radiator, extending the high frequency response of the driver. The JBL D130 was actually a passable full range hifi driver. Most implementations of the aluminum dust cap were ornamental (imitations), and not truly functional. So, most likely, the driver with the smaller, 3 inch dust cap and voice coil is original. Most 8 ohm Altecs of that vintage measured around 5 ohms (dc resistance). The specified impedance combines the dc resistance (dcr) and reactive components, and will always be higher than the dc resistance (which your meter measures). The impedance varies with frequency, typically having a strong peak at the driver resonant frequency, is nearly flat thru the lower midrange, then rises into the high frequencies. Choice of an impedance value is somewhat arbitrary. However, the dcr is always lower than the specified impedance. So a driver measuring 4 ohms dcr is not a 4 ohm impedance driver.
     
  7. portaflexer

    portaflexer

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    Thanks for the comments guys. At this point, I think it's reasonable to assume that speaker 2 is original and speaker 1 is a recone.
     
  8. portaflexer

    portaflexer

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    LOL. Thanks for catching that. I fixed the title.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

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    I suppose the question to ask is, how do they sound together? Running them the way that they are can be a plus.

    The DC resistance reading can vary as J pointed out. A 4 ohm speaker would typically read less than 4 so it appears that you simply have an out of spec 8 ohm speaker.
     
  10. portaflexer

    portaflexer

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    I don't have any problem with the sound. The cab sounds great. I just thought it was a bit confusing.
     
  11. kuratacus

    kuratacus

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    i have both a v4b 2x15 cab and a b25 2x15 cab and both have the 4 or 16 ohm selector switches on the back and were stocked with two 8 ohm speaker. i'd say one was reconed. and 8 and a 4 ohm speaker would make the cabinet 12ohms or 3ohms i think
     
  12. portaflexer

    portaflexer

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    8 + 4 would make it somewhere in the 3 ohm category... of course... speaker ratings always nominal (as a couple folks have already pointed out in this thread). I've seen those B25 cabs with the impedance switch. I always thought that was a cool touch. This one is actually an SBT cab, which means it's powered. The internal power amp is rated at 240 W @ 4 ohms. According to the schematic, both speakers should be 8 ohm and they're wired in parallel for a 4 ohm total load.
     

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