Marshall JCM 900 MkIII Hi Gain Master Volume
In the mid 1990′s I picked up a JCM 900 MkIII model 2100 100 watt head. The MkIII amps were made between 1990 and 1992. I decided to make this page simply because I could never find anything out there about these Marshalls. So, through much research and experience, I’m providing the most thorough information I can.
When it comes to these amps, you have to look past the name. As with anything, there’s gems and there’s duds. The Marshall gems as we all know are the Plexi’s 1959/1987, the JMP 2203/2204 and then JCM 800 2203/2204. I’m sure there are more than those but these are the ones that come up right away. As for the JCM 900′s, unfortunately I always believed there were more duds than gems. But, for two years, they produced one hell of an amp, a gem, the MkIII 2100/2500. Don’t confuse these with the SL-X, which means Super Lead eXtended. The SL-X amps are also known as the MkIV Master Volumes.
The JCM 900 series amps were first introduced in 1990. They came in two flavors: Hi Gain Master Volume and High Gain Dual Reverb. The Master Volume amps were a further development of the original 2203/2204 Master Volume amps and they were called the Hi Gain Master Volume MkIII amps. The Dual Reverb amps were based on the 2205/2210 amps. (Courtesy: Dr. Tube)
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Features:
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Controls:
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Complete MkIII Model Listing: |
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210X Models: Preamp: 2x12AX7, power amp: 1x12AX7 and 4xEL34
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250X Models: Preamp: 2x12AX7, power amp: 1x12AX7 and 2xEL34
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The JCM 900 impedence switch on the back of the amp is set up as follows: (thanks glassman)
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Answers to common questions I get:
Q: Ok, so what’s with the Preamp Volume and Gain Sensitivity? I thought they were the same thing like the JCM 800′s?
A: On the JCM 800′s, there’s one Preamp/gain control, the Preamp Volume. On the MkIII’s, there’s two. The Preamp Volume adjusts the primary gain (1-10). The Gain Sensitivity adjusts the secondary gain (11-20). Now, the Gain Sensitivity can take you from JCM 800 mild crunch to completely insane overdrive. It really does kick it in to high gear. The way I run it is with the Preamp Volume at 10, Gain about half-way. This is a really nice middle ground for good ‘ol rock-n-roll. I then use a booster to slam the front end harder. Granted, that booster doesn’t boost the volume any really but it sends a higher level signal in to the Preamp which makes it overdrive some more. Sometimes I’ll play with the Preamp Volume still on 10 but the Gain at 11 and use an overdrive like the MXR Wylde Overdrive or Boss SD-1 and slam it that way. There’s my clean (sorta clean anyway) or dirty.
Q: There’s two Master Volume controls. Is this a single channel or two channel amp?
A: The MkIII models are single channel amps. The two controls allow you to have totally independent volume levels for rhythm and lead playing, which is only footswitchable. Without a footswitch, Master “B” controls the overall volume. You can use the Marshall P801 footswitch or any other single button footswitch to change between the two Master Volume levels.
Q: What is the Output Mode Switch (50/25 on the 2500, 100/50 on the 2100)?
A: This answer comes right from the manual: Switches the amplifier from high to low power output. The low setting configures the output stage to triode operation, which gives half the rated output (i.e. 25 Watts on 50 Watts models – or 50 Watts on 100 Watt models). The high power position gives pentode operation for the full rated output.
Q: What’s the Direct Out and Recording Compensated Out for?
A: The Direct Out is an unfiltered signal directly from the Preamp which allows you to connect to an external power amplifier. The Recording Compensated Out is a filtered signal for connecting directly to recording equipment or a PA system. *Note: Even when using these line-outs, you still must connect the amp to a speaker cabinet of the proper rating. Powering up the amp, or any tube amp, without a proper load connected (meaning a speaking cabinet) will cause harm to the amp, possibly blowing the output transformer.
Q: Is it true there are diodes or chips used to get some of the distortion in these amps?
A: Yes, it is true. There are diodes in the Preamp by way of a diode bridge rectifier. The secondary gain stage has a diode bridge rectifier which assists in achieving the higher gain levels. Signal going through the rectifier is controlled by the Gain Sensitivity control. By turning the Gain down all the way, the rectifier is essentially bypassed. Increasing the Gain increases the amount of signal going through the rectifier. The overdriven tube warmth remains. Think of it as a built-in tube screamer. There is also an IC chip in the Preamp but it is at the tail end after the tone stack, most likely to help drive the effects loop. This arrangement is nothing new and many amps make use of it, with some going as far back as the 1980′s.
Q: What about the JCM 900 SL-X? Isn’t it the same amp?
A: Yes and no. The reason I say this is because the circuitry in the SL-X is different from the MkIII to a point. The most obvious difference between the MkIII and the SL-X just from looking at them from the outside is the additional 12AX7. This gives the SL-X more gain but in my opinion, takes away some of the character. Looking at the schematics, the preamp is different between the two, more specifically, from the input to V1 to the start of the tone stack. From there onwards, the two share the same tone stack, effects loop and power amp. The SL-X was produced between 1993-1999. Many of the SL-X amps came with 5881 power tubes instead of EL34′s when Marshall switched over in late-1993 to early-1994. This change is another reason why to me, the SL-X lost some of that Marshall mojo. My advice for the SL-X is the same as for the 4100/4500 Dual Reverbs; if you want one, get one with EL34′s.
Q: How reliable are these amps?
A: I’ll jot down my experience and let you be the judge. My MkIII 2100 came in to my possession in the mid-1990′s (it’s a 1990 model). It still had the stock tubes, showed some wear on the tolex (from gigs no less) but otherwise was solid and sounded great. Fast forward to the present. It’s still solid, still shows that tolex wear (hey, it’s character) and still sounds great. The only thing I’ve had done is routine maintenance – re-tube, bias adjustment and the potentiometers cleaned. Every so often I’ll dust off the outside of the chasis in the back.
Note: As with any tube amp, some components get worked harder than others and just simply wear over time. That being said, my 2100 lost its power transformer in 2006 and needed a replacement. Yes, they are worth fixing.
Q: Why did Marshall make these for only two years?
A: I wish I had an answer for this one. My only guess would be because of the increasing demand for higher gain amps at that time. Mesa Boogie had just introduced the Dual Rectifier and as Marshall’s main competition, they needed to come out with something to go up against it. This could be why the SL-X was born. I still think the MkIII is better than the SL-X and believe they will hold a better value down the road, but only time will tell.
Just a final word. If you are not familiar with the inner workings of your tube amp, don’t try to perform any maintenance on it. Take it to a qualified tech to have the work done. There are lethal voltages in these things even when turned off. Those big filter capacitors can hold enough voltage to kill you. I think a bench fee is money well spent to keep you breathing and your amp running smooth.
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More Info: |
Schematics: |
Sound Clips:
These clips are of my personal MkIII 2100. There is no post EQ at all. What you hear is the raw recording.
EQ settings on the amp: Presence = 2-3, Bass = 8-10, Mids = 3-4, Treble = 6-7. The Preamp level is on 10.
For the track named, "New sound clip March 2009", this one is post-repair of the power transformer, new filter caps, new bias supply components, as well as new tubes; EH power and preamp.
The configuration is as follows:
- Guitar (Frankenstrat) straight in to the amp.
- Ibanez delay/echo in the loop on the echo setting.
- Effects loop volume box after the delay/echo in the loop.
- Presence at zero
- Bass at 4
- Mids at 5
- Treble at 5
- Gain Sensitivity at 18 (remember, it goes 11-20 on the dial)
- Preamp at 10
- Master Volume at 4
- Cabinet used is an Avatar Vintage 2×12 with Celestion Classic Lead 80 speakers at 16 ohms.
The Master Volume is a little touchy between 2 and 3 so the effects loop volume box lets me get past that and then choke it back down to reasonable levels.
CAUTION: Do NOT put any volume box between the amp and speaker cabinet. They are NOT attenuators!