The serial number on your Amp is located on the back panel of the chassis.
Some amplifiers made between 1979-1981 have them on the front panel.
January 1965 - June 1969:
These are more difficult to date than post 1969 amps. The best way to estimate the year is by looking at the cosmetics and certain electronic features which are explained below:
The JTM45 metal block logos now changed to plastic white script with gold coloured plating.
Do to their brittle nature, the Gold Logos brake easily and are quite commonly replaced with white logos
Power Amp Tubes: KT 66, 6L6, 5881
Back Panels are Cream.
JTM 45 Output Transformer: Drake or RS Spares Used. The Drake (p/n 784-103) has a primary impedance of 8,000 ohms. The RS Spares output transformer mainly used in the JTM 45 Amp has a primary impedance of 6,800 ohms.
Handles changed to plastic with larger end cap.
The first 100 watt (JTM 100) amps appear late 1965. Early examples where sent out with the old JTM 45 panels as new ones were not available yet or to use up surplus stock. These amps had two 50 watt Transformers and a Solid State Rectifier instead of the Gz34 Rectifier. The early JTM 100 cabinets had small Vox style air vents but where later changed to larger ones do to the extra heat being generated from the amp.
Cabinet feet are small, round and grey
The JTM45 logo changed to JTM50 on 50 watt amp panels and JTM 100 on 100 watt amp panels.
Power amp tubes changed to EL34's or 6550s due to the KT 66's poor supply and greater expense.
Gz34 rectifier phased out
Transformer uprated. 100 Watt production was now standardised with a single 100 Watt Drake (p/n 784-084) output transformer wound for a primary impedance of 4,000 ohms.
The EL34 equipped JTM 50 Marshall amplifiers were fitted with the Drake (p/n 784-139) output transformers that had a primary impedance 3,400 ohms.
All cabinets now had corner locked joints and were signed inside as a method of quality control.
Chassis all upgraded from aluminum to steel construction around autumn 1967
The first 200 Watt Majors appeared, with just treble, bass and volume controls, “active” tone circuitry and KT 88 valves.
"Reverse" or "Black Flag" JTM Logo features on the amps front panel for roughly six months (Can also be seen late 1966)
Back panel changes from Cream to Gold Plexi Glass.
JTM 100 Power Tranformers change from Drake to Dagnall.
Gold logos changed to white
The JTM series is now changed to JMP. JMP stands for Jim Marshall Products. JTM stands for Jim Terry Marshall.
The 200 watt Major was modified to match the normal panel layout and the active electronics where dropped.
The output transformer is rotated 90 degrees to reduce hum.
Amp Housing lip or shelve under control panel changes from straight to curved.
Plexi Glass Panels started to change to Brushed Aluminum early 1969
Cabinet feet change from small grey to larger round black ones around mid 1969
July 1969 to 1992:
From July 1969 to 1983 the date letter was put after the serial number.
From Jan 1984 to October 1992 the model letter is first, then the date letter second.Both now placed before the serial number.
Letters I & O were omitted to avoid confusion with numbers 1 & 0.