As mentioned in my article covering the 10” line of Celestion guitar speakers, swapping speakers is the most definitive and dramatic change that can be made to an amp’s core tone short of heavily modifying the amp circuitry itself. Speakers are often a good way to give a middle-of-the-road tube amp that final 10% and turn a good amp into a great amp. The cost of a decent speaker can easily compose 30% of the cost of a mid-level tube amp from a pure material standpoint, so it is understandable that most amp manufacturers are going to settle for the speaker that does the amp justice but comes in at a certain price point.
Because the tone of an amp is so heavily influenced by the speaker projecting its sound, stock speakers have become intrinsically tied to the DNA of classic guitar sounds… be it Silverface Fender, 60’s Marshall, or a Soldano SLO. Pressed to call out the speaker/amp combo that comprises “that sound”, most players start with one of the most iconic pairings in the history of rock and roll; the Vox AC15/AC30 with a Celestion Alnico Blue speaker. Today, Vox offers a (killer) reissue of the AC15 with a choice of speakers at varying price points, including a Celestion G12M Greenback, Celestion G12M-65 Creamback, or Celestion Alnico Blue. The most affordable model includes the Celestion G12M Greenback.
The Celestion G12M Greenback is a fantastic speaker to start with, and excels at bringing punch and growl to low wattage amps with heavier distortion as well as presence and clarity when wired in a 4x12” configuration and driven by a higher wattage head. After a few days of break in, the Vox AC15 with the Celestion G12M Greenback has a wonderfully punky sound that deviates slightly from the vintage Vox combination with a Celestion Alnico Blue in that the mid-range focus shifts slightly lower and the high-end is slightly rolled off. The G12M Greenback compliments heavier grit and has a slightly grainy breakup quality without sounding blurry or buzzy. In short, a great combination for copping classic rock and lo-fi garage rock tones.
However, the elusive Vox sound, as presented in classic recordings, is different than that. It’s jangly, searing, and rich in even the cleanest of settings while oozing quirky character. To see how much of that character truly came from the speaker, I pulled out stock Celestion G12M Greenback and dropped in a new Celestion Alnico Blue.
The Celestion Alnico Blue produces warm lows, a smooth mid-range with an emphasis in the upper mids and a bell-like high-end that sparkles without getting too aggressive. The combination of these characteristics creates the signature jangle in late-clean/early-breakup signals that sizzles and builds layers of harmonics around the upper-mids and treble while the bass anchors the tone. The Celestion Blue is a little tight on the top-end and spikey in the mids when first installed, but a few hours of break in quickly mellow these out and by the 20 hour mark the speaker is silky smooth across the frequency range.
With the new speaker in, all that's left is to replace the reverb tank, remount the chassis in the cabinet, place the tubes back in, and seal the cabinet back up.
The difference between the Celestion Greenback and Celestion Blue is significant enough that even an untrained ear will notice. Even before it was broken in, the Celestion Blue had more volume and a much more harmonically complex upper mid-range and top end sparkle. A room mic set to record the difference from the exact same location with the exact same settings clipped on some of the higher frequencies with the Blue, lending proof to the volume increase and additional treble. Brighter, louder, and richer in clean-to-early breakup settings, the Celestion Alnico Blue completely transforms the AC15 and evokes Beatles, Brian May, and The Edge perfectly. While both performed well (if differently) in high-gain settings, the Celestion G12M Greenback seems better suited to that more aggressive distortion by adding focus and a bit of roundness to the edges. The Blue, on the other hand, will grab on to every bit of distortion it can and scream it out, making high-gain a little more unruly.
Chris Loeffler is a multi-instrumentalist and the Content Strategist of Harmony Central. In addition to his ten years experience as an online guitar merchandiser, marketing strategist, and community director he has worked as an international exporter, website consultant and brand manager. When he’s not working he can be found playing music, geeking out on guitar pedals and amps, and brewing tasty beer.