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Thread: New member & a question regarding impedance

  1. #1
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    New member & a question regarding impedance

    Hello all,
    In my quest to find an 8" JBL for a 1964 Fender Vibrochamp amplifier, I came across this very informative forum!

    I'm a guitar teacher/player, primarily focused on acoustic fingerstyle & bottleneck, but I've been known to play an electric guitar on occasion. My knowledge of electronics is an oxymoron, hence the following question:

    The early releases of the Vibrochamp actually had a JBL option fitted with the D208, and seeing as the '64 I just got has a very injured-sounding CTS 4ohm factory speaker, I'm doing an upgrade. When a reasonably priced D208 8ohm shows up somewhere, I'll probably have no choice………In any case…….

    How is it that many 16ohm speakers pictured on ebay, etc, have ohmmeters reading less than 6? I know that I can actually put an 8ohm unit in, and I am not concerned about the loss of power. I want a clean sound that can hold a very low tuning. I am not aware of any D208 being made in 4ohm, but that's why I am asking here.

    Thank you for any knowledge you can impart!

    Regards,
    Howard
    http://howardemerson.com/music2.html

  2. #2
    Senior Member rdgrimes's Avatar
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    The rating of the speaker, (16u, 8u, etc), is a different thing from the DC resistance measured with a meter. Drivers have a range of DCR that's considered normal, which you'll find in the spec sheets. 5-7u is fairly typical but each driver is different. Also good to note that a nominal DCR does not guarantee a functioning driver. But at least you know that the voice coil is not open or melted.

    Presumably you are dealing with an amp that has a load rating, so you want to stay with a driver that's rated the same as the amp. Don't put a 4u driver on a 16u rated amp, or a dead amp might follow.

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    Quote Originally Posted by rdgrimes View Post
    The rating of the speaker, (16u, 8u, etc), is a different thing from the DC resistance measured with a meter. Drivers have a range of DCR that's considered normal, which you'll find in the spec sheets. 5-7u is fairly typical but each driver is different. Also good to note that a nominal DCR does not guarantee a functioning driver. But at least you know that the voice coil is not open or melted.

    Presumably you are dealing with an amp that has a load rating, so you want to stay with a driver that's rated the same as the amp. Don't put a 4u driver on a 16u rated amp, or a dead amp might follow.
    Thank you for the quick reply! The Fender Vibrochamp has a 4u speaker output, so I think I'm pretty safe with just about anything readily available. 2u doesn't seem to show up very often.

    Aside from a rotted/torn edge or cone, how else could a driver not be functioning despite a good voice coil?

    Thank you for your indulgence!

    Howard

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    Senior Member rdgrimes's Avatar
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    Even with a "good" voice coil, there may be damage preventing free movement. Old drivers tend to accumulate crap inside the VC gap preventing free movement. Suspension issues also exist, either in the surround, the spider, or both. Rubbing voice coils can still be electrically intact.

    I'm sure there are alternatives to the D208 for you, other pro drivers and the like. Guitar people tend to prefer one driver to another, so try them all!

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    Quote Originally Posted by rdgrimes View Post
    Even with a "good" voice coil, there may be damage preventing free movement. Old drivers tend to accumulate crap inside the VC gap preventing free movement. Suspension issues also exist, either in the surround, the spider, or both. Rubbing voice coils can still be electrically intact.

    I'm sure there are alternatives to the D208 for you, other pro drivers and the like. Guitar people tend to prefer one driver to another, so try them all!
    Hi RD,
    I have a 2118H incoming. It's a re-cone, and it should be fine with a new offset baffle board. It's a 200 watt speaker so it will not be taxed too much. For $40 I'm not taking much of a gamble.

    Thanks for the explanation!

    HE

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    Senior Member macaroonie's Avatar
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    2118 will not do much at the lower end of the scale but it will be clean sounding . As you say for 40 its worth a punt.

    The Champ cab will take a 10" I believe if you increase the baffle cutout. In that case the go to JBL is the D/K or E 110. 6ohm normally
    Plenty of those around for not huge money and will give you a little more down below

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    Quote Originally Posted by macaroonie View Post
    2118 will not do much at the lower end of the scale but it will be clean sounding . As you say for 40 its worth a punt.

    The Champ cab will take a 10" I believe if you increase the baffle cutout. In that case the go to JBL is the D/K or E 110. 6ohm normally
    Plenty of those around for not huge money and will give you a little more down below
    Hi Mac,
    The 2118 arrived yesterday, and I've just made a new baffle board for it. Prior to doing anything I took out the old baffle, re-glued some loose Tolex, and cleaned the pots with Deoxit.

    I made the new board out of 3/4" plywood, and just off-set the hole slightly downward, yet it's otherwise centered. I'm just waiting for some vintage-correct grill cloth before I spray it flat black.

    Other than that? It sounds very, very nice, and it really responds to the bass control as well. Very, very clean, of course, and that's what I was aiming for. That's usually NOT what people buy Vibrochamps for, but……...

    So all in all not a bad fix for $40, and I can say it's JBL equipped, right? I still may do a 10" conversion now that you've planted the bug…….

    Thank you!

    Howard

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