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Overview: Historical,Tone and Tech Talk
The JTM45/100 is a Class-A/B, all-valve, 100 Watt head with two channels (High
Treble & Normal) that share tone controls. 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
As 'the Guv'nor, ' Dr. Jim Marshall, has already stated, 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 JTM45/100's organic, vibrant, sweetly distorted, harmonically rich and thick musical tone emanates from its power
valves being driven into a dual 50 watt output stage.When 'cranked, ' the JTM45/100 produces its own unique roar
with an inspiring 'punch you in the chest' clout. It reacts superbly to the guitar's volume control / player feel
combination and, not surprisingly, responds exceedingly well to picking dynamics too. From 'in-your-face' when you play
with unbridled aggression to subtle 'in your ear' blues caresses when you pull back and turn down...just like the
celebrated masters of the day.
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
Wherever and whenever possible, we have gone back to the original manufacturers for all
components to maintain the highest quality and to ensure maximum authenticity.
Components:
Three ECC83s (12AX7s) in the pre-amp and a quartet of KT66* power valves working in push-
pull.All valves are of the highest quality available and are subjected to our usual meticulous grading and testing
Valve compliment:
processes.
these output valves were standard issue on mid sixties Marshall Amplifiers yielding a fat, round tone that has a
distinctly vintage flavour.
*KT66:
2
The way the three ECC83s (labelled V1,V2 & V3 in the photo) 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.
ECC83 VALVES
V1
V2
V3
KT66 POWER VALVES
3
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