Sica 15" blown on gig- recone?

[Lynn Kasdorf]

I recently unearthed a cabinet I made that currently houses a 15" 4ohm Sica. I don't know the power rating, but the model number on the frame says this: BS 2.5PL 4ohm
I was using this on a gig with my Quilter 202 tone block. It sounded wonderful. But then, on a song, I kicked in a wah wah pedal and I had my volume pedal set it too high. When I "wahed", it got very loud and I head a radical distortion. (the rest of the band said that it sounded really cool, and how did I do it?)

So, after that incident, it sounds ok at low volumes, but develops a muddy distortion at higher volumes.

I assume that when a speaker gets damaged by over powering, the coil gets hot and burns or melts the cone, and this makes it thicker and it rubs on the magnet? Anybody know for sure what happens?

What should I do with the fried Sica- can they be easily reconed?

[Dave Grafe]

I assume that when a speaker gets damaged by over powering, the coil gets hot and burns or melts the cone, and this makes it thicker and it rubs on the magnet? Anybody know for sure what happens?

What should I do with the fried Sica- can they be easily reconed?

Other than poking a hole in the cone or cracking/bending/breaking the frame there pare two ways to destroy a speaker.

One is overexcursion, which happens when a BRIEF SPIKE in excessive electrical current causes the voice coil to jump the gap entirely and smash itself against the magnet on the rebound. This will be readily visible as the cone will be misshapen, sometimes in a spiral form, and generally will not move when pressed by hand. This is a function of the physical limits of the suspension system, both at the outer speaker surround and the inner spider where the voice coil meets the cone,

The other, more common failure mode happens when a CONSTANT OVERCURRENT or square wave heats the coil over time until the cylindrical core it is wound on loses its integrity and begins to warp. As it warps the coil wrappings begin to rub and arc in the gap, burning the wires, shorting adjacent windings and ultimately pulling the windings apart from the friction with the walls of the gap. This will often but not always result in a scratchy sound and feel when gently pushing on the cone by hand. This is a function of the coil design, i.e. wire gauge, number of windings, and composition of the core.

As an example the original JBL D130 had a paper cone surround, a narrow voice coil gap for high sensitivity and a voice coil wound around a light paper core for extended frequency response. These primarily failed from heat causing the paper voice coil core to lose its shape and from high peaks exceeding the limits of the cone surround. The D130F that followed used a rubberish "goop" to strengthen the surround and widened the gap slightly to reduce heat buildup and increase tolerance for warping. Both changes came at the expense of sensitivity but succeeded in doubling its rated power handling. The K130 model then replaced the paper voice coil core with a plastic one lees prone to warping and exchanged the gooped paper surround with a fabric "accordion" that increased excursion limits and durability. Again power handling was roughly doubled but as the gap was unchanged and the fabric surround less stiff this yielded an small INCREASE in sensitivity over the D130F design. The E130 design introduced a stronger, lighter coil core, further widened the gap and stiffened the surround to again increase power handling while relying on a stronger ceramic magnet to avoid a loss of sensitivity.

4 ohm voice coils are particularly vulnerable as they receive twice the power as 8 ohm speakers of the same design. Early Webb amps were notorious for blowing speakers because the stock 4 ohm speakers were incapable of handling the 300 watts the amp was capable of delivering into a 4 ohm load.

It is important to understand that the friction from square waves generated by overdrive distortion, whether via effects device or amplifier clipping, will easily create enough heat to burn up a voice coil at far less than its rated RMS power, while a smooth, undistorted signal can occasionally exceed rated power without doing any damage at all.

As for repair you might contact forum member Jeffrey Maxwell who does excellent reconing work, if the parts are available he can probably help you sort it out..

[Lynn Kasdorf]

Thanks for that excellent explanation!

[K Maul]

I hate saying this but nowadays it seems re-coning a speaker is close in cost to buying a new one. I could be wrong but usually only the really high end JBLs etc are worth repairing. That is what some techs have told me.

[Dave Grafe]

I hate saying this but nowadays it seems re-coning a speaker is close in cost to buying a new one. I could be wrong but usually only the really high end JBLs etc are worth repairing. That is what some techs have told me.

I would tend to agree with you on this

[George Piburn]

Those Seca's are Italian made, and I see zero parts in the USA for recone.

That Model is a Bass Speaker any way. They roll off drastically after 1000 to 2000 hz. D-130's go over 4000 hz for example.

For certain you would be better off replacing it with something already here in the USA.
__________________________________________
Back to the damage, not that it matters, I've also seen bubbles on the Kapton Former too from an over heat.