Found this....
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.
I'm not sure I agree with everything said in the article but it should shed some light. Hope this helps.
