According to Guitar Player Magazine’s Art Thompson:
"Anyone into vintage British guitar tones should be jazzed over Celestion’s reissuing of the original G12M and low-resonance G12H 12" ceramic-magnet speakers. Hand-built in England using authentic glue formulations, edge treatment, and voice-coil-former material—and designed to match the coil and magnetic-flux specifications of the ’60s-era speakers—the 20-watt G12M and 30-watt G12H are visually accurate, as well, sporting period-correct solder-only tag panels and vintage-style Thames Ditton labels on their green plastic covers."
Read Art Thompson’s review.
The G12M Greenback emerged in the early to mid-sixties. It was Celestion’s first ceramic magnet speaker and had greater power handling (20 Watts as opposed to 15 Watts) that the Celestion Blue (used by Vox) which had an Alnico magnet. It was also cheaper to produce and cheaper to the consumer. Soon after Celestion produced the G12H (which were given green or black cases, depending on availablility of plastic!) This model had a heavier magnet than the G12M and so was capable of handling more power. This time 30 Watts. The increased magnetic flux (the level of magnetism within a magnet) also altered the tonal quality of the driver.
There were two G12Hs produced with different resonance frequencies, 75 Hz (or cycles per second) for lead guitar and 55 Hz for bass. Some more adventurous lead players experimented with the low resonance version and found it produced a very desirable tone, with a “darker”, more “rounded” quality.
The G12M and G12H were speakers that helped forge the sound of the electric guitar during the classic 1960s British Invasion.
The third addition to the range was the G12-65. Identified as the voice of 80s rock and blues, this speaker is often thought of as one of the greatest 4 x 12 speakers ever produced, bringing together vintage tone with modern, high power-handling capability.