10.5 Ohm '68 Bottom Cab??

[Milestone]

Hey fellas. I could use some help here...
I own a '68 bottom cab with 30 waters (102 014 cones and NO labels) that I bought last year. I took the back off the cab to do some investigating this weekend because I intend on restoring it. It's been recovered long ago, caster cups filled, piping channels filled as well. The wiring is series-parallel as it should be, solder connections are in good order, and all speakers are functioning and sound awesome!

I'm reading 8.2 to 9 Ohms on the speakers individually while they're still soldered in place. I'm taking the readings with a nice Fluke 787 behind the speaker tabs on the braided copper wire going directly to the coil. The reading fluctuates at the jack from 10.5 to 11 Ohms. Shouldn't this be a 16ohm cab? Did they make 8 ohm cabs in '68?

I'm using the cab with a early '69 metal panel SL and a 12,000 Metro build and have always used the 16 ohm taps on the amps. Now I'm wondering if I should I be running my amps at 8 ohms instead of 16 or is it safe for either? (especially the '69 SL). Suggestions?

Thanks,

Robert

[Scumback Speakers]

On the speaker frame there is a Txxxx code by the date code. What are those codes? Then I can help you figure it out.

[Milestone]

On the speaker frame there is a Txxxx code by the date code. What are those codes? Then I can help you figure it out.

Thanks Jim!

They're all T1281's with matching 18MA date codes

[Scumback Speakers]

On the speaker frame there is a Txxxx code by the date code. What are those codes? Then I can help you figure it out.

Thanks Jim!

They're all T1281's with matching 18MA date codes

Those should read 12.4-14 on a multimeter by themselves. If you've got 10.5, then one speaker is worn and is presenting more resistance, the wire itself is breaking down and presenting more resistance...or...it's built wrong. Celestion did have 12 ohm voice coils on some models and called them G12H30's, but isn't what yours should be.

Time to test them individually. Sorry!

Or say screw it and play.

[Milestone]

Those should read 12.4-14 on a multimeter by themselves. If you've got 10.5, then one speaker is worn and is presenting more resistance, the wire itself is breaking down and presenting more resistance...or...it's built wrong. Celestion did have 12 ohm voice coils on some models and called them G12H30's, but isn't what yours should be.

Time to test them individually. Sorry!

Or say screw it and play.

I'll order some new wire and rewire the cab. Thanks again Jim. I'll pull the speakers when the wire comes in and test them individually. A separate question for you tho Jim...what's your tip(s) for removing speakers from a cab this old that seem to be firmly stuck to the cab after the screws are out??

Thanks!

Robert

[Scumback Speakers]

You use a putty knife to wedge between the cork and the baffle board. Gently pry it up. Sometimes a blunt butter knife will work, too. Make sure you keep one hand on the speaker frame in case it pops off unexpectedly.

[neikeel]

More resistance??

- expected passive resistive load around 12ohms

- he is getting 10.5 ohms

- how does more resistance in one limb lead to a lower reading?

I would have though less resistance from one limb. Less resistance could be a short, but I believe that a complete open circuit or complete short (ie meltdown) is a more common phenomenon, of course Jim will have seen more failed Celestions than I have. Ones I see are totally dead (open circuit or jammed coils, or lifting glue on the spiders).

If you disconnect each speaker fully (no wires) what do you get for each one?

[Scumback Speakers]

When one speaker is out of whack from the others it affects the reading since they're all connected as a whole to output jack. So when one reads 16, the others read 12.5, it increases resistance reading by a touch. In this case it seems lower, which means that potentially one is an 8 ohm, three are 16.

I've seen so much goofy crap in these old cabs there really isn't any guarantee what's up until each speaker is measured by itself.

There's also the potential for faulty wiring as well.

As for the resistance failure, I've seen speakers fail with the reading going up (overheated to 18-20k ohms) and with loose wires in the voice coil barely touching the frame reducing the reading.

There are probably more reasons than that, but that's what I see the most of.

Hope that answers your question.

Jim