took this from the amptone site
hope your hungry 8)
Early JCM-800's with diode clipping
By the early 1980's, Marshall was producing amps that mixed tube and transistor distortion. The transistor distortion was accomplished by adding "diode clipping" circuits among the tube preamp gain stages, which gave the amps a lot more distortion, but sacrificed warmth. The tubes before the diodes helped to push guitar signal into the diodes, where they were clipped (distorted), and the tubes after the diodes both amplified and "warmed up" the sound somewhat. These and later Marshalls had a bright, buzzy distortion, especially at low volumes. When cranked up loud, the power tubes would begin distorting, thickening up the sound to reduce the harsh, thin, buzzy distortion. Another unsual (for Marshall) feature was that the EQ was positioned early in the preamp, just after the first gain stage. This would be changed on later JCM-800 models: moved to after 3 gain stages, just like on the 2203 amps. And the new 4210/2205 amps sported (had) an effects loop, something new for Marshall. This allowed guitarists to put their delay pedals after the preamp distortion for cleaner delays. Finally, Marshall used a post phase inverter master volume for these amps. Marshall would later change back to a master volume after the EQ (but before the phase inverter) much like the 2203 amps.
Some metal players loved these new Marshalls, whereas many older rock and blues players hated them.
The 1980's marked a confusing period for Marshall, as they were simultaneously still making 1. non-master volume amps: circuits 1987 (50 watt model) and 1959 (100 watt model), 2. 1970's style Master volumes (circuit 2203-all tube signal path), and 3. the new models with tubes and transistor distortion mixed, like the 50 watt 4210.
Many players call all 2203 style master volumes JCM-800's, but these 2203 amps were being made long *before* Marshall began calling their amps JCM-800's. Also, there were JCM-800's being made which used diode clipping, and thus were *not* the older style 2203 circuit.
So, if you hear someone talking about a "JCM-800", he could mean an all tube 2203 master volume circuit (first produced in the 1970's, and which Marshall continued to make in the 1980's). Or he could be talking about one of the newer circuits with diode clipping, first made in the 1980's. You could ask him "What model number are you refering to?". Or "Are you talking about the 2203 circuit with an all tube signal path, or the circuits with the mix of diode clipping and tube distortion?" If he says something stupid like "All JCM-800's are the 2203 circuit! No JCM-800's have diode clipping!", then you can tell him he's full of shit. (i.e. he's either lying or just plain ill-informed) You can say, "Marshall's 50 watt combo (model 4210) and 50 watt head (model 2205) both use diode clipping. Just look in the "History of Marshall" book.
Marshall's later JCM-800's
Marshall modified the new design, and a 1988 schematic for the 2210 (head) and 4211 (combo) amps show an extra tube gain stage after the diode clipping. The diode clipping itself had been altered to a more complex arrangement-more like a diode bridge rectifier, and the EQ was repositioned to after the third gain stage. Much like the 2203 amps.
For Marshall's 25th Anniversary, they came out with the Jubilee amps, favored by Slash or Guns and Roses, and recently reissued as the "Slash" amps. These are among the most popular JCM-800 amps, which is somewhat ironic, as they probably have to greatest use of diode clipping. Which at low volume, to my ears, makes the amps sound too trebley. When cranked, however, they can get an aggressive, yet warm tone. Listen to any Guns and Roses record, or the guitar solo in "My Mama Said" by Lenny Kravitz. You'll notice Lenny's rhythm guitars sound edgy and bright, but when Slash starts his solo, his tone is thicker and meatier with less high end "buzz" and more "throat." It could well be the EL34's being overdriven.
Marshall's JCM-900 amps continued to rely on pretty much the same formula as the later JCM-800's. There were lots of differences (some of which added gain), but the basic style remained the same: two gain stages in series, diode clipping, third gain stage and EQ and master volume. Some JCM-900 amps used IC's (transistor opamps) for additional gain. Some claim that Marshall began using lower quality components in these JCM-900's, which reduced reliabity. I've heard complaints that the output transformer blows up more easily at high volumes than the output transformers used in earlier Marshall amps.
Marshall designed these amps with the late 1980's metal players in mind, so they have a lot of gain and thin, buzzy transistor clipping mixed in with the tubes. Some amp techs find old JCM-900's used for low prices, and peform extensive modifications to these amps to convert them either to the 1960's style circuits (#1959, #1987), or the 1970's #2203 circuit. These mods are not cheap, but they give the amp a warmer all tube sound.
Doug Hoffman makes some point to point wired boards perfect for converting some JCM-900's to an early Marshall/tweed Bassman circuit. It still requires a lot of work, but less so than making the boards yourself.
Bogner "FISH" preamp (Brown Channel) Marshall 30th Anniversary (Lead Channel)
As many rock and metal players' tastes matured, they began realizing that while they loved the *amount* of distortion the new Marshall amps had, they didn't like the *quality* of it. They wanted the aggressive tone of Marshall amps, but with more underlying warmth, much like Eddie Van Halen's sound. Eddie had led people to believe that his Marshall amp had been heavily modified, so that seemed the natural route for other guitarists to take.
Old Marshall heads from the 1960's and early 1970's were used for both their stock sound, and the fact that their construction made them very easy to work on. Newer amps used printed circuit boards, which were hard to modify.
Guitarists and amp techs began adding master volumes and additional gain stages. Some simply changed some of the resistor and capacitor values in 2203 amps to increase the gain of exisitng stages. Others took the more radical approach of adding entirely new gain stages, which greatly increased the amount of distortion. One of the more popular circuit configurations that came out of this "hot modding" phenomenon was the "4+0" circuit: gain stage, gain control, three more gain stages, EQ, master volume, then the power amp.
Not all of the modificiations sounded good. In fact, many great sounding stock amps were butchered into amps which were noisy, unreliable, and bad sounding. Guitarists found that it's not hard adding gain-what is hard is making the gain and resulting distortion sound MUSICAL.
Some of the more talented amp techs with sharp (good) ears were able to create some good sounding circuits. One was a Mr. Reinhold Bogner, whose "FISH" preamp was popular in the early 1990's. Unlike Boogie amps, each channel was almost completely independent. The highest gain "brown" channel used the 4+0 configuration, the two other distortion channels (Strato and Shark) used the lower gain 3+0 circuit. The Brown and Strato channels are very similar to Marshall circuits, with a shared Marshall style EQ after the gain stages. The Shark channel is a lot like a Vox AC30's Top Boost channel with an extra gain stage. Even the EQ on this channel is very Vox like. It doesn't completely capture the Vox AC30 sound, of course, because its lacking the AC30's power amp and speakers, which are a very important part of the Vox sound.
There were also other amp techs who either modified Marshalls to the 4+0 style preamp, or began making their own amps and preamps with this type of circuit.
Marshall eventually decided to use some of these ideas for it's 30th Annivesary amp (model #6100). While the amp does use some solid state devices, most of the preamp uses an all tube signal path. Gone are the diode clipping circuits common to the JCM-900 and many JCM-800 amps.
The lead channel uses a 4+0 circuit. It is unusual in that after the second, third, and fourth stages, there are "cathode followers". Cathode followers are usually one of the two triodes in a 12AX7 set up to lower the impedance of the signal. Some amp techs claim cathode followers add more "crunch" to the sound. It is normal for a Marshall style circuit to use a single cathode follower just before the tone controls, but not after each gain stage.
http://www.harmony-central.com/Guitar/guitar-amp-evolution.html