The Celestion G12M & G12H Creamback Project

Coming from a background in High Fidelity sound reproduction, speakers targeted at producing sound (as opposed to reproducing it) can be a bit puzzling: these speakers are often chosen for their “unique character” (politely speaking) as opposed to linear frequency response or low distortion. Peter Thorn quotes John Suhr in a column on Premier Guitar: “speakers are like a filter—the final EQ that shapes our guitar tone”[1]. I’m reasonably sure that merely adding a top-of-the-line EQ to my pedal board will not end my search for the tone I’m looking for, but eventually I was also reasonably unsure that my heavy duty JBL K120, E120, K130, and 2135 I had collected on eBay, along with a quad of actual classic EVM 12L, are optimal for all tones.

Enter the infamous Greenback speaker, the Celestion G12M Greenback, whose magnet on the back happens to be wrapped in green plastic, and hence its nickname Greenback. It popped up every so often in one of Premier Guitar’s “rig rundowns” and similar sources of collective wisdom that eventually I had to overcome my bias and have a look: Stamped speaker basket, smallish 1.75″ voice-coil, cheapish looking soldering tabs for electrical connections, and all the power it can handle is 25 watts? You gotta be kidding!

I’m using a classic AlNiCo JBL K120 whose heavy duty basket and 4″ voice coil make the Greenback look like a toy and whose lovely push-on connectors make the Greenback’s soldering tabs look like a bean counter’s delight. Its 100 W power handling can cope with the 35 W output of my 1964 Fender Tremolux amplifier without breaking a sweat, and the 1×12 cabinet is smaller than the original Fender 2×10. So why rock the boat?

Well… because I couldn’t get my rig to sound anything like the classic rock tones from the 70ies and turn of the 80ies I grew up with, no matter what PlexiTone pedal, Marshall amp, or Angus Young Signature pickup—installed on a 2014 Gibson Les Paul Traditional—I threw at it.

Inconveniently, however, the Celestion baskets are ever so slightly larger than the JBLs and EVs I had used before and hence they didn't fit the baffle cutouts of any of the test boxes I had previously built. Even more inconveniently: Celestion doesn't publish any T/S parameters, arguing that they don’t apply to the large signals occurring in producing (loud) rock music. True, but if nothing else, the basic T/S parameters should give me at least a ballpark figure what size sealed enclosure to build to avoid under- or overdamping the speaker.

For now I’ll have to use a “guesstimate” obtained from inspecting the W×H×D dimensions specified on various manufacturers’ websites, and to make it worthwhile building another set of speakers I’ll try to learn a few things while I’m at it, namely how to:

and last but not least, I’ll use the recent Creamback versions of the G12M and G12H, with power handling rated at 65 W and 75 W, respectively. From what I read on the “world wild web” they have a voice coil that can handle the extra power which in turn suggests that they must be heavier and hence they could sound ever so slightly different, but for now this will have to make do. Following are some snapshots taken during the project:

I addressed the issue of cutting 5′×5′ sheets of Russian birch plywood with a bunch of two-by-fours, a straight edge, a couple of clamps, and a circular saw upgraded with a 7.25″ 60 tooth fine finish saw blade. My local hardware store, from where I sourced my amazing SawStop table saw, suggested a track saw instead, at about 4 times the price. While there’s nothing wrong with a track saw, either solution requires working on the floor and since most of the cutting can be done on the table saw, the circular saw made more sense.

My first impression with speakON connectors was great: They make a solid connection that is hard to accidentally “unconnect” because of the twist required to lock them into place. The plugs exert a firm grip on the cable and the particular sockets I used are designed for speaker enclosures, providing an airtight seal. I can see using them for most if not all future speaker projects.

Flush-mounting speakON sockets required some careful planning: Perfectly round holes, even with rebates, can be routed with a plunge router and a Jasper Circle Jig or similar, but this necessitates affixing a piece of sacrificial lumber on the back of the work piece to anchor a pivot pin. Not easy on the fully assembled and finished enclosure! Doing it before assembly in turn called for laminating the finishing door skin (my ersatz veneer) to the outside of the rear panel but I was unsure if this could produce a smooth cut into the door skin. Hence I tried it out on a spare piece of lumber and door skin, guiding the router very carefully around said pivot, and taking several turns to slowly increase the plunge depth. It worked!

Cutting a 45° miter joint wasn't something I had ever attempted before, and my first cut ended up being slightly less than 45°. It turned out that the table saw had to be calibrated. To glue the pieces of hardwood, making the cover-up of the end-grain look like a picture frame, I first clamped all 4 pieces in place but without glue, before removing one piece at a time, adding glue, and re-clamping it. It may help to clamp the miter joints with big parallel cabinet clamps or similar, but I still had to do some sanding to make it look smooth. Practice makes perfect.

Last but not least, my experience with crimping has been mixed so far. They use color coding but not for what I had expected: can I get red for + and black for −? Nope, they are blue if they can accept 14 AWG stranded wires, + or − aside, and the part that connects to the soldering tabs on the speaker or the speakON socket comes in different sizes, but all these sizes are blue.

The speakONs use 3/16″ as specified by the manufacturer while the Celestions may be using 0.205″ as discussed on the internet[2]. I found 3/16″ to be a reassuring fit on the speakONs while 0.205″ was more or less loose on the Celestions; in fact I managed to “wiggle” a 3/16″ onto one of the terminals but not any of the other ones.

I ended up using a pair of pliers to give the 0.205″ sized crimping terminals a couple of squeezes until they fit in a somewhat reassuring way. If these weren’t enclosures for testing various speakers, I’d be more than happy to just solder the wires to the terminals. And the variability encountered upon squeezing the crimping terminals makes me wonder what kind of kludge I’ll have to use to swap any of the speakers…

So… how do they sound? Well… the first impression didn’t exactly knock off my socks. It does seem to take some time to break them in, because unlike any of the classic speakers I had eBayed before, these came new, straight from Parts Express.

Meanwhile I did get goose bumps a few times. Not a bad sign! But I’ll give them a bit more time before publishing any measurements.

Stay tuned!


  1. Peter Thorn, Tone Tips: Speakers—The Final Frontier, Premier Guitar, 04-Aug-2015.
  2. Celestion Push-On Tabs - What Size Female Disconnect Crimp Terminal?, the GEAR PAGE, 27-Apr-2015