Adventures in Reconing

Okay, I don't really recommend doing this. I have to point out though that the posts by J M Fahey on MEF and TGP were invaluable to the process.

My Twin came with a pair of Fender-labeled Utah speakers with torn (and moldy) cones. That sort of thing tends to happen when you ship a "135W" amp with a pair of 50W speakers, and then store it in a basement that's swamp-adjacent for thirty years.

Anyway, they were useless, so I got the bright idea that I could try reconing them and if it worked I'd have useful speakers, and if it didn't, I'd only be out $50.

Getting to this point was the hardest part.

So I cut the cone and surround out of the frame, measure them, and go looking for parts. I decided to go with the parts sold by Weber, as this was a guitar speaker and they sold guitar speaker parts. There are a couple other vendors, but not many as there's some weird shadowy cabal of speaker reconers and unless you get into it and start a business the vendors won't sell to you. It's basically crazy.

I decided to go nuts and order Kurt Meuller cones, in essence making the end result similar to Weber's 12F150B. The spiders Weber sells are cup-type while the original was flat, but this didn't turn out to matter too much. 

Alright, so cleaning the old gunk off of the basket was a huge pain. I used a heat gun, razor blades, I tried a couple solvents.. eventually I got it all off. Then it was just a matter of cleaning the voice coil gap, shimming the voice coil in the gap so that the coil is centered on the top plate, put the spider over the voice coil, and put the cone over that. Boom, first dry fit. 

This is actually after gluing, but you get the idea.

Now, ideally - and if you're using the correct parts - the spider and the throat of the cone will meet perfectly, so you can attach them both to the voice coil with one bead of adhesive. If you're a jerk like me and buy the wrong parts, they need a little massaging. In this case, I had to cut back the throat of the cone a fair amount, but I got it wrong from both sides - the spider was too tall and the cone was too deep. 

On the first speaker I reconed, I didn't do quite a good enough job of this and as a result the spider is pushed in at rest - not great for efficiency nor longevity. 

Also it's upside-down, which makes it hard to hear.

I did a lot of research - well, "research" - before starting this. Like I said at the top, J M Fahey was a huge help. There were a bunch of other people with opinions, but they tended to lead me astray. Like the people who suggested gel super glue for bonding voice coil/cone/spider - this worked very poorly for me. Also it left a mess when I used it to attach the dust cap.

Epoxy works much better. Contact cement was used to attach the spider, cone, and gasket to the basket. Solder the voice coil leads to the tinsel wire, solder the tinsel wire to the terminals, and boom, done. 

The second one came out nicer.

Now here's where the real craziness sets in - what to dope the surround with? There are two kinds of dope I've seen on guitar speakers: the clear stuff and the black stuff. The black stuff seems similar to what's used to seal cloth surrounds and I'm pretty sure that's just butyl rubber in a solvent. The best and cheapest candidate I've found to try is actually windshield adhesive. I haven't bought any, because I found a good DIY alternative for the clear stuff. 

Unlike a cloth surround where doping is necessary to block airflow, doping on paper surrounds is done to prevent cone cry and overall tame the breakup modes of the cone itself. If applied lightly it can contribute to that notch at 1.5kHz, and if applied heavily it can neutralize that notch at the expense of overall sensitivity. And you can look at what Celestion's done or what Jensen's done over the years, but I don't have the test equipment to optimize this process and I'm sure there are books somewhere about it.

In any case you want the doping compound to penetrate the paper and stay flexible. Some DIYers use rubber cement (thinned even) or white glue (PVA) but supposedly these don't stay flexible enough. After a bunch of research I found an adhesive known as EVA (ethylene vinyl acetate, as opposed to polyvinyl acetate). It's like regular white glue, but the addition of ethylene means it retains much more flexibility. You've seen EVA in at least one form - hot glue sticks. Finding an easy source of an EVA emulsion glue was a little trickier, but eventually I stumbled on Helmar Super-Tac.

Here's a blob of Helmar Super-Tac that I dried so I could play with it.

This blob here actually dried to a silicone-like rubberiness. It's pretty awesome stuff. Really thick like this, the surface dries much faster than the center so the water can't escape. Yes, that's right, it's water-based, which is a huge boon for DIY'ing. Also new applications of the glue (or water) dissolve the dried glue, which is great if you're applying multiple layers. Maybe not so great for ship building. 

I ran a few experiments with dilution ratios and I eventually used a 2:1 dilution of water:Super-Tac and applied about 6 coats to the surround of each speaker. 1:1 would've probably worked just as well. A straight application right out of the bottle might work too, but one coat is probably enough. I also found it was better to let the coats dry 8-24 hours before applying the next one. I rushed the second speaker and applied a couple coats within 4 hours of each other. It looked dry, but it wound up trapping a little extra water for a milkier look. This eventually wormed its way out though.

This is in the Peavey cab that I just stained.

Anyway, this resulted in what I'd guess is a light-to-moderate level of doping. I doped the first speaker and compared it to the undoped second side-by-side. And yep, a hair less sensitive but more tamed treble and a little more in the midrange. Once I liked it, I doped the second speaker to match, and they're now resting comfortably in a Randall 412 with a pair of Eminence OEMs from the 90s. 

I've left a whole lot out because I'm sick of typing. I ran a bunch of experiments on doping using the old cone material from the Utahs, though it turns out the cones I got from Weber are way more absorbent. Liquid latex never really soaked into the Utah, but maybe the Kurt Meuller cone would be different...

Super long story really short: for DIY doping of paper surrounds, try an EVA emulsion glue like Helmar Super-Tac.