Marshall Limited Edition JTM45/100

The original version of this world famous amplifier was built in late 1965 at the request of the young rock musicians of that period. Jim Marshall was being asked to provide bigger and louder amplification to accommodate the harder edge and aggression of this younger generation’s take on rock and roll and blues. He was only too happy to accept this challenge!

His team of engineers set to work using a JTM45 with two power sections as the basis for the very first Marshall 100 Watt head and in November 1965 these formidable amps with accompanying 8x12 cabs made their stage debut. The original 8x12s proved too bulky for transportation and were returned in favour of stacking two 4x12 cabinets, one on top of the other. Thus the Marshall ‘stack’ was born.

This authentic replica stack and head celebrates the fortieth anniversary of this much documented episode in rock music history and is part of an ongoing series, featuring hand wired re-issues of revered historical Marshall Products. The reason we have decided to do this is simple - public demand!

At a time when the face and sound of popular music was rapidly changing, players were constantly pushing back and redefining musical boundaries. The JTM45/100, with its relatively simple controls, features and circuitry was there on the front line at the beginning of this music revolution.

Our goal for this handwired re-issue is both obvious and simple - to make it as identical to the late 1965 made originals as possible. To this end the circuit, cosmetics, construction, sound and all-important dynamic characteristics all have to be 100% correct - and they are. We went to incredible lengths to achieve maximum authenticity and are delighted to report that our suppliers were equally as exacting in their tasks.

The JTM45/100 is sold as a full stack which includes the JTM45/100 head, 812T40 and 812B40 cabinets. This is a limited edition of 250 units. All units are individually numbered and come with a numbered certificate signed by Dr. Jim Marshall OBE and held in an attractive leatherette folder. Also included are a full set of dust covers.

This head first saw the light of day in late 1965 and was affectionately named the JTM45/100 because of the continued use of the JTM45 preamp section and also its front panel. Because original JTM45/100’s are extremely rare and collectable, we had to travel to various parts of the world to examine other originals in addition to closely examining many photographs kindly provided by the lucky few owners. Slight circuit variations were apparent between models due to Marshall’s ongoing development of the 100 watt amplifier at that time.

The circuit we have revisited for the JTM45/100 is vastly different to the later circuit we use for our critically acclaimed, late 60s Plexi re-issue, the 1959HW. The most relevant technical departures in question lie in the dual output transformers, Power Supply Topology and the use of KT66 power valves just like the originals.

As you can see from its front panel layout, the JTM45/100 is an extremely straightforward amplifier. Its two channels - High Treble and Normal - each have two inputs (High and Low sensitivity) and separate Loudness (volume) controls. Both channels share the amplifier’s four tone controls: Treble, Middle, Bass and Presence. Like all Marshall valve amplifiers, the JTM45/100 performs at its very best when turned up to deliver your personally preferred dynamic range. Due to its power rating and astonishing projection, it can be a formidable live amplifier in even the largest of venues. A truth bore out by its impressive history time-and-time again.
The speaker cabinets in the Anniversary Stack, 812T40 and 812B40, have been designed to recreate the majestic look of the original 8x12s when stacked. We could, of course, have replicated the 8x12 but in consideration to your back (and your roadies!) we decided that dividing it into two 4x12s was much more sensible! Just like the original 8x12, both cabs are loaded with Celestion’s highly celebrated T652 Alnico Speakers. The angled top cabinet also features a semi-open back, just like the genuine 8x12 reference we took our dimensions from.

Technical Information
Valve compliment: Three ECC83s (12AX7s) in the pre-amp and a quartet of KT66* power valves working in push-pull. (*KT66: these output valves were standard issue on mid sixties Marshall Amplifiers yielding a fat, round tone that has a distinctly vintage flavour).
The way the three ECC83s in the pre-amp are utilised is as follows -V1 is the pre-amp gain stage for the High Treble and Normal channels. Each half of the valve (the ECC83 is a dual-triode which means it is literally two triode valves in one) acts as a dedicated gain/input buffer stage for each channel. V2 is common to both channels which are passively summed at the input of this valve. The first half of V2 acts as a second gain stage, which boosts the signal further and directly drives the second half of the valve which is configured as a cathode follower - a unity voltage gain device which acts as a high input impedance buffer, providing a low output impedance to drive the pre-amp’s passive tone network. V3 is the amplifier's phase invertor and sits at the very end of the pre-amp section’s tone circuit. V3 divides its input signal into two separate output signals that are inverted with respect to each other - hence the name ‘phase invertor’
(a.k.a. ‘phase splitter’). The two resulting signals are then fed to the push-pull power output stage of the amplifier.
Tone Circuit: In typical Marshall fashion, the JTM45/100’s shared tone network is passive and interactive. By interactive we simply mean that the settings of the Bass and Treble controls affect the amount of mid-dip (cut) available via the Middle control.
Tag Boards: The tag boards used in the hand wired re-issue are exactly the same as those in the original in terms of thickness and matrix pitch. The material we're using is made exclusively for us and is ‘UL recognised Micom type Marshall EM42.’

Transformers: The mains (power) and output transformers are vital to the performance, sound and feel of an all-valve amplifier. Because of this we spent a great deal of time studying and analysing the constructional methods and materials used in the original transformers so we could match everything as closely as possible. We also went to great lengths to ensure that the all-important electrical characteristics and performance of the originals were duplicated exactly. In order to achieve these vitally important goals we worked extremely closely with our associates in the R&D department of Drake - the company which supplied us with the output transformers used in the original JTM45/100

Mains (Power) Transformer: In the original JTM45/100 the mains transformer was an “off-the-shelf” device purchased from a highly regarded general electronic components supplier. Referred to as the “RS De Luxe” this transformer is no longer available, but with the help of Drake’s expertise we were able to replicate the performance of the original ‘De Luxe’ of which we have several genuine examples. We paid particular attention to an important electrical effect called ‘regulation’ which is the variation in behaviour of the HT voltage and current from the transformer and is dependant on load demand (i.e. how loud you’re playing!). This behaviour affects how the amplifier compresses and reacts to saturation when played at high volumes - hence its importance.

Output Transformers: As already mentioned, the pair of output transformers used in the original JTM45/100s were supplied by Drake, as are those in the reissue. Fortunately Drake still had their original documentation intact after all of these years, a factor which obviously helped ensure that the reissues are 100% accurate.

Power Supply: This circuit has the same exact filtering on the H.T. (high voltage DC, a.k.a. the B+ voltage) line as the original versions, a factor that contributes to the JTM45/100 having a unique, dynamic characteristic all of its own.

Chassis: We are using a box-section chassis made from 16-gauge mild steel with butt-welded corner joints. The steel is also passivated giving lifelong resistance to corrosion.

Front Panel: Extra thick, gold coloured Plexiglas (actually Perspex, causing some people to refer to it as ‘Perplexi’!), exactly as the original.

Rear Panel: Made from cream styrene with gold print, just like the original references.

Head Cabinet Construction: High-grade, flawless (knot-free) Baltic birch-ply with finger locked joints for maximum strength. The main cabinet frame (sides, top and bottom) are 15mm ply, the front baffle is constructed from 12mm ply while the back of the cabinet is 9mm ply. All edges have a 15mm radius.

Head Cabinet Cosmetics: The Marshall Block logo, black Levant covering, original venting, beading and piping
JTM45/100 Head