Celestion Greenback 20 Watt G12M Explained

Celestion Greenback 20 Watt G12M Explained

The Celestion greenback 20 watt G12M and 25 watt G12H are some of the most sought after pre-rola speakers. However it is easy to be fooled by the labels on these speakers, everything is not always as it seems.

Here I’ll give a quick overview of the main differences between the 20 watt and 25 watt G12M (and equivalent G12H), and how to identify the imposters with the deceiving labels on them!


When Celestion first introduced the greenback range in the mid 1960’s, the G12M only had a power handling of 20 watts and the G12H only had a power handling of 25 watts.

Celestion greenback 20 watt G12M and 25 watt G12H
20w G12M and 25w G12H

In late 1967 Celestion upgraded the voice coils across all speaker models to allow for roughly 5 watts more power handling. This gave us the familiar 25w G12M and 30w G12H – and we can assume the alnico speakers also increased from 15w to 20w (or 17w to 22w according to some sources).

Celestion greenback 25w G12M and 30w G12H
25w G12M and 30w G12H

Difference in voice coils

The “5w less” speakers had a white coloured voice coil former. These speakers are known to struggle with their rated power handling.

The “5w more” speakers had a brown coloured voice coil former, made from a more heat resistant material.

white coil vs brown coil
white coil vs brown coil

There are subtle sonic differences between the two types of voice coil. The white coil speakers are the most sought after by collectors and discerning tone hunters!

Transition dates

The transition from the white former to the brown former did not happen all at once. It was a gradual transition over several months.

Whilst repairing speakers I have seen the brown formers as early as June 1967, and I have seen the white formers as late as Jan 1968. However, most speakers do seem to have the brown former by Nov 1967, and a white former as late as Jan 68 is pretty rare – I have only seen one example.

Label transition

Even more confusing, the labels did not change “with” the voice coils. Celestion only altered the labels several months later – around April 1968. So there was a long transition period when the speakers had the ‘5w more’ brown voice coil, but they still carried the older ‘5w less’ labels.

The labels only changed several months after the voice coils.
The labels only changed several months after the voice coils.

So when buying speakers just be careful of this. Dealers will always ask a premium if they have the ‘5w less’ label on them (20w G12M or 25w G12H). However, if they have the brown voice coil former they are really no different to the later pre-rola’s except for the slightly different cork gasket.

To be 100% sure of a white voice coil, the speaker ideally needs to have been made before June 1967. Early greenbacks from that period are particularly rare due to lower production numbers. That is one of the reasons why they are more valuable to collectors.


25w G12H vs 30w G12H

On G12H speakers look for doping on the reverse side of the cone. This is usually a good indication the speaker has the white voice coil former. A G12H without the reverse side doping is most likely a brown former speaker – a 30 watter.

25w G12H speaker with doping on the reverse side of the cone
25w G12H speaker with doping on the reverse side of the cone

20 watt G12M vs 25 watt G12M

Sonic differences aside, there are no visual clues on the G12M speakers to determine what type of voice coil it has, without dismantling the speaker. This is definitely not advisable unless the speaker needs repairing. Some people have suggested using a strong flashlight to look through the dust cap. I have not tried this myself yet, but apparently it works so long as the dust cap is not too clogged with dirt.

Wiring and Polarity

Some new information has also come to light regarding the wiring of the voice coils and polarity of the magnets. Thanks to research undertaken by Michele Bisignano and Maurizio Montani in Italy.

It appears that the white voice coils are wired in such a way that requires the magnet to be in south polarity. The later brown voice coils require the magnet in north polarity.

You can check this using a small magnet on the back of the speaker.

Late 1967 - North polarity, 1966 - South polarity
Late 1967 – North polarity, 1966 – South polarity

*photos courtesy of Michele Bisignano*

So far I have only tried this on a handful of speakers myself and it does seem to check out. However, I would like to try it on a good few more speakers from the transitional period (late 67) to confirm it fully.

Sonic differences

The speakers with the white coil formers are the best sounding greenbacks Celestion have ever made in my opinion. They sound clearer, have better note definition, feel more responsive, and have that ‘woody’ tone in spades.

Rare 'white former' greenbacks circa 1966 - dig deep!
Rare ‘white former’ greenbacks circa 1966

If you are a ‘plug straight in’ kind of guy, then you will probably love the white former period greenbacks too. Just be careful with them. The key to getting the best tone from them is to give them plenty of power. It can be a fine line between ‘optimum tone’ and blowing the speaker!

18KM = 18th Oct 1967 - with the white coil former
18KM = 18th Oct 1967 – with the white coil former

The brown coil speakers will give a warmer and thicker sound at lower volume levels and do not need to be pushed as hard to sound at their best. However, they lose some of the magic of the earlier speakers in my opinion. If you use a lot of distortion or other effects then you will probably prefer these later speakers.

Ultimately, the type of greenback you prefer will come down to personal taste, your playing style, and the other elements of your rig. Use your ears and keep experimenting!


  1. Michael J. Wane
    November 11, 2018 / 9:37 am

    Hi Brian!
    Great info here! I have a spring-ish ’68 bottom cab, basketweave with skinny handles and what I believe to have original speakers (date codes 6/68. The labels on the greenback covers say 20 watts, but there are traces of glue over the number ’20’ on all the covers. When I got the cab, there was one tiny gold sticker in the bottom corner of the inside of the cab that said ’25’. Looked like it had been glued on at the factory over the ’20’ and fallen off!
    Interesting tidbit just the same…

    • Brian Harding
      November 11, 2018 / 4:58 pm

      Hi Michael, glad you like the article. Yes from about April 68 to July 68 Celestion put little ’25’ stickers on the G12M labels, and ’30’ stickers on the G12H. This is how Celestion first showed the increase in power handling. Nowadays those stickers have usually fallen off and can be found floating around in the bottom of the cab somewhere, but the square shaped sticker residue is usually still visible on the label. I guess having new labels made for every speaker model was too much of an expense back then. You can view/read about the label changes in my other blog post here.

      regards, Brian

  2. Marko
    October 9, 2020 / 4:34 pm

    Did this 20w G12Ms also come with 55hz bass cones? Never saw even a picture of one and would really love to know. Cheers, Marko

    • Brian Harding
      October 9, 2020 / 5:23 pm

      Hi Marko, yes they exist, but they are rare. The earliest T1511’s (55Hz G12M) I ever saw were made either June / July 1967. I think this is roughly when Celestion first started making them, and when Marshall first started using them instead of Goodmans in their 4×12 bass cabs. I haven’t seen any from 67 myself for a good few years now. June / July 67 would be right on the transition period of the white former to brown former. So whilst you might see T1511 with the 20w labels on them into mid 68, there are probably very few of them around (if any) with the actual white formers.


  3. May 5, 2021 / 8:05 pm

    Hi! My father have 4x G12M 20w, former version with the white coil, he thinks they’re from 1966. Do you know roughly the estimated the value of them? And any tips on where to sell?
    Thanks in advance and thanks for a great article!

    • Brian Harding
      May 7, 2021 / 9:07 am

      Hi, sorry for the late reply. Their value really depends on their condition, so if you can email me with some photos to info@bygonetones.com I can help you out with a valuation.

      regards, Brian.

  4. Tomo
    March 13, 2023 / 11:51 pm

    Hi Brian.
    This site is a bunch of valuable information!
    Polarity info is very unique.
    I’m just sharing my pvc greenbacks info.

    G12H 25W x2 Apr 66
    x2 May 66
    Jan 67

    G12M 20W Aug 67
    x2 Dec 67

    • Brian Harding
      March 14, 2023 / 11:34 am

      Hi Tomo, thanks for the additional info. Yes an interesting discovery about the polarity change. They sound like some nice early speakers you have. I would keep hold of them because they are getting very difficult to replace thesedays once sold. What model are your G12H? T1217s?

      regards, Brian.

  5. Tomo
    March 16, 2023 / 7:04 am

    Hi Brian,

    Yes, these are all T1217.
    Nice compressed tone.
    Someday I want to try 25w T1281, but it’s very hard to find though.


    • Brian Harding
      March 16, 2023 / 8:42 pm

      Hi, yeah I’ve been looking for some early T1281’s myself for a tall B cab. I found some recently in poor condition that will do temporarily, but would like to find a mint original quad. Maybe dreaming thesedays. Should never have sold the ones I had in the past. Brian.

  6. Louis Campione
    December 19, 2023 / 1:02 pm

    Hi, I want the most inefficient quality speakers for my JTM 45 head. The amp is loud, i want speakers are not as loud, but still sound rich. What do you recommend in this day and age?

    • Brian Harding
      December 30, 2023 / 4:43 pm

      Hi, you probably want smaller speakers, either 10″ speakers, or 12″ speakers with a smaller magnet. Maybe try a pair of G12S from the 70’s – model T1417 or T1517 and see what you think. Brian.

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