Marshall JCM900: every version
Every version, how to tell them apart, how to date one, and what to check before you buy. 3 versions, 1990–c. 2000.
The JCM900 replaced the JCM800 in 1990 and pushed preamp gain further. It came in two lines: single-channel heads with two footswitchable master volumes (the 2100 and 2500), first with diode clipping and from 1993 with an extra ECC83 instead (SL-X), and the two-channel Hi Gain Dual Reverb (4100 and 4500). The model number and the label on the panel identify most of them; the output valves changed during the run.
The versions, in order
Hi Gain Master Volume (Mark III) 2100 / 2500 (1990–1992)
JCM900
One channel with two gain ranges, diode clipping and two footswitchable master volumes.
How to spot one
- One channel with two footswitchable master volumes for rhythm and lead levels.
- Solid-state clipping diodes in the preamp, the main difference from the later SL-X.
- Three ECC83 valves in all.
- Model 2100 is 100 watts and 2500 is 50 watts.
Specs
| Power tubes | 4 × EL34 |
|---|---|
| Rectifier | Solid-state (silicon diodes) |
| Phase inverter | Long-tail pair |
| Preamp tubes | 3 × ECC83 (including the phase inverter) |
| Channels | 1 |
SL-X 2100 / 2500 (1993–c. 2000)
JCM900
The Mark III's replacement: the clipping diodes are gone and a fourth ECC83 adds a valve gain stage.
What changed from the Hi Gain Master Volume (Mark III) 2100 / 2500
- 1993: the clipping diodes were removed and a fourth ECC83 added an extra valve gain stage.
How to spot one
- SL-X on the panel; the name stands for Super Lead extra gain.
- Four ECC83 valves instead of three.
- No clipping diodes: the extra gain comes from the added valve stage.
- Still one channel with two footswitchable master volumes.
Specs
| Rectifier | Solid-state (silicon diodes) |
|---|---|
| Phase inverter | Long-tail pair |
| Preamp tubes | 4 × ECC83 (including the phase inverter) |
| Channels | 1 |
Hi Gain Dual Reverb 4100 / 4500 (1990–c. 2000)
JCM900
Two channels, each with its own volume and reverb level, sharing one EQ.
What changed from the Hi Gain Master Volume (Mark III) 2100 / 2500
- A second channel and a reverb level for each channel, with a shared EQ, in place of the single channel with two master volumes.
How to spot one
- Two footswitchable channels (A and B), each with its own gain, volume and reverb controls.
- Shared Presence, Bass, Middle and Treble controls.
- Two gain controls on the lead side, the first marked 0–10 and the second 10–20.
- A series effects loop with a level control.
- Model 4100 is 100 watts and 4500 is 50 watts; output valves are EL34 or 5881 depending on the year.
Specs
| Rectifier | Solid-state (silicon diodes) |
|---|---|
| Phase inverter | Long-tail pair |
| Preamp tubes | 3 × ECC83 (including the phase inverter) |
| Channels | 2 |
JCM900 buying checklist
Date it
- JCM900 serial numbers use Marshall's barcode-style format, which encodes the year of manufacture.
- A Mark III label means 1990–92; SL-X means 1993 or later.
- Inspection or test dates inside the chassis can narrow down the year.
Check it’s original
- Output valves vary by year on these amps (EL34 or 5881), so either can be original; check that the bias suits the valves fitted.
- Diode-clipping mods and removals are common on Mark III amps; a Mark III with no diodes has been modified.
- Footswitches are often missing; the channel and master-volume switching need one to use the amp fully.
What usually needs work
- These are printed-circuit-board amps: check for cracked solder joints at the valve sockets, jacks and pots.
- The filter capacitors are now old enough to need checking or replacing.
- Noisy channel switching usually points to the switching parts or the footswitch cable rather than the valves.
Questions buyers ask
What does SL-X mean?
Super Lead extra gain: from 1993 the 2100 and 2500 replaced their clipping diodes with an extra ECC83 gain stage.
Which JCM900 has reverb?
The Hi Gain Dual Reverb 4100 (100 W) and 4500 (50 W), with a reverb control for each channel.
Did the JCM900 use EL34s or 5881s?
Both, depending on the year and model. Either can be original.
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