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Fractal Audio AMP models: Brit JM45 (Marshall JTM 45)


* EDIT: Up-to-date information is available in Yek's Guide to the Fractal Audio Amplifier Models *

Brit JM45: based on Marshall JTM 45

The JTM 45 was Marshall’s first amplifier in the ‘60s. Its design was entirely copied from the Fender Bassman. It was a hit, and a vintage JTM 45 is worth a lot, listing #6 on Vintage Guitar’s list of most valuable amps. Some production models were named JTM 50.

Vintage Guitar:

“Despite being derived virtually point-by-point from Fender’s 5F6-A Bassman schematic, British components give the JTM45 a tone all its own.”​

It is a different kind of Marshall tone than the later Plexi amps. Wikipedia:

“For all of its differences with the Bassman, the sound of the JTM 45 is still described as "like a tweed Fender"; it has more sag and less crunch than the later Marshalls, and is favored for blues and rock rather than for hard rock and metal. The JTM 45 does not deliver the famous Marshall "crunch" that became so sought after.”​

Eric Clapton played a JTM 45 combo known as the Blues Breaker on the famous ’66 Beano album by the John Mayall Bluesbreakers. More JTM 45 history and more about Eric Clapton's amps and guitars.

The JTM 45 was re-issued by Marshall, with 6L6 tubes:

“The very first Marshall amp, made back in 1962, was the JTM45. It was an instant hit and launched a whole new generation of groundbreaking guitar players and sounds. Since then, connoisseurs of tone have sought out rare original JTM45s from the ’60s. Now they shall seek no more, thanks to this meticulously accurate re-issue. It was GZ34 rectification that was the key to the JTM45’s sound, and this is a feature painstakingly reproduced in this re-issue. The way the rectifier interacts with the other valves causes subtle harmonics to shift and smoulder beneath every note, producing the unmistakable, original, and much emulated Marshall sound.”​

The JTM 45 uses KT66 tubes and is 30 watts. It has four inputs, two per channel.

It provides two channels: Normal and High Treble, and two inputs per channel. Fractal Audio has models of the High Treble channel and of the jumpered inputs.

Jumpered channels: some players use patch cables to “jumper” the inputs (2nd input of channel 1 goes into 1st input of channel 2). This enables them to have the benefits of both channels at once. This is modeled in the “Jumpered” model of the JTM 45. This model has two Drive controls. Set them at the same position, or keep Normal Drive lower than Treble Drive.

The High Treble channel is bright. The amp is also heavy on bass. Like a Plexi’s Normal channel, it’s not unusual to turn down the Bass control all the way to prevent the tone getting flubby.

Apart from the Volume controls for both channels, the amp has Bass, Middle, Treble and Presence controls.

The JTM 45 doesn’t have a Master Volume, so keep the Master control in the model dimed. Without a Master Volume control, the JTM relies on power amp distortion. To achieve a nice warm overdriven blues or rock tone, you need to turn up the drive a lot, and crank the tone controls. This makes it a very loud amp in real life.


"They had no Master Volumes so people rarely got the Drive past 3.00 since it would melt your face. Without the specter of having your skin flayed off as is afforded by a model of the amp, the temptation is to turn the Drive way up. When you do this, the low notes get very muddy. Single notes can form an almost perfect square wave which will sound like a synthesizer.”

"Our model is based on Channel 2 which is the bright channel. Also our reference amp has a 100 pF bright cap. Many JTM45s did not but I think they sound better with one. A JTM45 with a bright cap is similar to a Superlead. If you want the Channel 1 sound with an Axe-Fx use the Brit JM45 Jump model and turn the Treble Drive knob all the way down."​

Unlike its cousin the Fender Bassman, the JTM 45 was designed to be used with a 4x12 cabinet with Celestion G12M (greenbacks) or G12H speakers.

Greenbacks have a sweet midrange and good bass reproduction. The rear of those speakers was green, and the nickname “greenbacks” was born. Another popular speaker in those days (and still): the G12H, a 30 watts Celestion speaker that compresses less than a greenback and has a flatter response (“Jimi Hendrix” speaker).

You check this page for the stock "greenbacks" cabs, suitable for the Marshall models. Personal favorites among the stock cabs are: 54 and 55 (Cab Pack 20), 58 and 59 (Cab Pack 8), 103 (Cab Pack 2), 131 and 132 (Cab Pack 14, I think…).

Click to open the current re-issue’s handbook.

A selection of YouTube videos:

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If you'd like a modernized version of the JTM 45, try the Dirty Shirley model. Dave Friedman based his amp on the JTM 45. It has a master volume.
It's my preferred amp model at the moment, for everything from clean to hard rock. A single amp model. I just drop Depth and Negative Feedback a little, and vary gain per scene.
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Rocket Brother

Power User
Another great write up Yek :)

The JTM 45 is a great amp - both the real amp and the Axe Fx model.

Claptons famous woman tone came from a pushed JTM45 2x12 combo (Bluesbreaker combo) with greenbacks and a Les Paul with the tone knob dialed back - wailing emotional blues tone.
The dialed back tone knob is key to achieving the woman tone.

I second your recommendation to check out the Dirty Shirley as a bit more modern and rocking alternative to the JTM45.
I've had the Dirty Shirley as one of my main go to amps for clean to mean since it was introduced in the Axe Fx, and I've been oh so close to buying a real Dirty Shirley online twice, based purely on how great it sounds in the Axe Fx.
I've played full nights of blues and rock on just one Dirty Shirley preset and just controlled my gain level with my pick attack and the volume knob. Great great amp.
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Johan Allard

Power User
I can just chime that I'm on the Dirty Shirley fan bandwagon. It's been my go to amp for a while and have also been thinking of actually buying a physical Dirty Shirley as well. I saw a video from NAMM I believe where Dave Friedman was talking about the possibility of building a small 20w Dirty Shirley combo and sounds like an awesome idea. I hope he does and I may well have to get one :)

Love these write ups Yek. Got me trying a few models I've never really tried before like the Boutique model. Thanks for your hard work and dedication!

Rocket Brother

Power User
I can just chime that I'm on the Dirty Shirley fan bandwagon. It's been my go to amp for a while and have also been thinking of actually buying a physical Dirty Shirley as well. I saw a video from NAMM I believe where Dave Friedman was talking about the possibility of building a small 20w Dirty Shirley combo and sounds like an awesome idea. I hope he does and I may well have to get one :)

Love these write ups Yek. Got me trying a few models I've never really tried before like the Boutique model. Thanks for your hard work and dedication!

Dave has indeed made a 20W Mini Dirty Shirley (the DS-20) - it's readily available to buy from his dealers.


Power User
Awesome! I mutated a dirty Plexi 50 preset I had into a gorgeous clean JTM45 preset in less than 2 minutes using the information in the OP.


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I'd stayed away from this model because it always seemed flubby to me. Made me wonder what people saw in it. The tips here really helped me pull something beautiful out of it. Thanks @yek!


Fractal Fanatic
This is a go-to amp for me; thanks again yek ... love these threads!

Will have to play with the DS :)
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Another great write up! Thanks Yek.

One comment on "woman tone". I've always thought of it as the post-Bluesbreakers/Cream era tone, when Clapton was playing Marshall 100w amps and was more about dialling back of the tone controls on the guitar rather than the amp. Clapton explains it in this video.

Regarding the JTM45 on the Bluesbreaker album (one of my all time favourite albums), there's lots of material on the internet about how loud the amp was when it was recorded. Here's a quote from the excellent book "Strange Brew: Eric Clapton & The British Blues Boom 1965-1970" by Christopher Hjort (itself a quote from a Mojo article):

Gus Dudgeon, engineer "Nobody had ever come in wanting to play that loud, not even when I did the Stones audition" and from Mike Vernon, producer "I went to talk to Eric...Is this absolutely essential? Because Gus is having kittens, he doesn't know how to record it. He's never had to deal with anything like this volume in his life. Can you turn it down? And Eric said very politely, ' No, I can't, because if I turn it down, the sound changes. And I can't get the sustain I want' "​

Also from the same book but this time a quote from much later (1998) interview:

Eric Clapton "When they tried to set up recording, I wouldn't let them put the microphone anywhere near my amplifier....I intuitively knew it wasn't going to sound good miked close"​

One of the great things about the Axe is we can get "that" sound without "that" volume :)

Alex C

I discovered this model just a few days before this (excellent) write-up, and it instantly jumped to the top spot as my favorite in the box (for now!).
After engaging the CUT switch and turning bass right down, it sounds clear and full and extremely touch-sensitive; with DRIVE set around 4 [EDIT: more like 5] I can pick lightly (or roll the guitar volume down) for beautiful cleanish, sparkly sounds, and dig in for smooth grind and punchy power chords. I feel like it easily enters the lighter end of Plexi territory, but without some of the shrill or honky edge that the Plexis can retain on cleaner settings (in my experience). A drive block (FAS Boost here) takes it to the next level without sounding mushy or woofy.

I'm using my current go-to cab block, a stereo cab with Factory Cab 60 (4x12 FRACTAL GB M160) on both sides, 50/50, with a 57 on the right side only.

I didn't know the full history with Clapton and some of the older-style, about-to-explode, dark and sputtery tones, and in a way I'm glad, because I might not have given this amp a fair chance. The last video that Yek linked is much more representative of the smooth, clear, grinding sounds I'm getting from this model (but with fewer oblique bends here).

Yek, thanks again for doing this series. I remember a few years ago we were all trying to figure out an "amp of the week" feature or subforum to cover all the models, and it got too complicated pretty quickly. This is exactly what we were looking for all this time; thanks for your continued efforts.
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Love this model... however not sure where to set the LF parameter to emulate the 2x12 combo. I use the OH 2x12 Alnico SIlver IR with some Speaker Drive, Dephase and Motor Drive turned up a little. Any ideas? Use the 10% rule (in this case 82.5Hz)? Thanks in advance...

Could've had a mid-70's version of the 'Bluesbreaker' Combo - a 2187 2x12 combo (1987 Treble Channel/1986 Normal Channel, Greenbacks)... opted for a used Carvin X100 head instead (High Gain! Rock Out!) - Ah the stupidness of youth.
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Fractal Fanatic
I had a good play with this model yesterday - Fired up my trusty old strat and the black box gave me Little Wings to fly on :)


I've worn my fingers down to stumps on the jumpered amp block over the past few days. It's amazing how you can set it so that it has elements of clean, and then bursts of fizziness and harmonics that just bloom out of nowhere. I'm finding that I don't like it quite so much with the drives turned way up, and I've been using the input trim tied to a scene controller to set the overall distortion. I might try adding a second scene controller onto the Treble Drive, and see how a combination of raised input trim and treble drive can tweak a less buzzy sound out of higher gains. I've noticed that I don't really like the super high drive demo's on the youtube clips that were posted too. So maybe it's just my taste.

I've tamed the flubbiness with pretty high Lo filters on both the amp and the speaker cab - around 170Hz seems to do the trick for me. I've increased the sag a lot, which seems to emphasize the dynamics that cause the fizz and harmonics to pop out. I might mess with the character a little bit, setting the frequency closer to 600Hz to balance out some of the shrillness that occasionally comes through. In the end, I suppose I'll end up with something that doesn't actually sound true to the original amp, but I'm hoping it will capture the feel without the warts.


I'm not sure which thread I read it in, but the JTM45 is very close to the Sunn Model T amp and gets great stoner metal sounds when the Master Volume is cranked up. I tried this the other day and it sounds monsterous!


Fractal Fanatic
Dave has indeed made a 20W Mini Dirty Shirley (the DS-20) - it's readily available to buy from his dealers.
I just picked one up used - should be here soon hopefully. I had a Pink Taco for a long time and it kicked ass. I've been using a Badger 35 for my "real" amp lately - we'll see if it is dethroned :)
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