Originally Posted by Brett Blackmore

The JCM 800 was the result of taking a plexi and adding a more advanced valve based pre-amp (more advanced than the earlier Master vol models) which gave you the advantage of having an all-tube distortion plus the extra distortion from a master volume type preamp. The drawback was the fact that it needed to be played loud to get both the main-and preamp tube distortion. If you could not crank it up then you needed to add more gain thru a pedal or preamp. The advantage is that it has a beautiful creamy-distorted sound when cranked up and there is a lot of push and bottom end - afterall the motor inside it is a Plexi... eg. M.Schenker only uses 50w Marshall JCM 800's.

The JCM 900 was supposed to be the new JCM800 but with more gain and a wonderfull pearly clean channel. Yet they made 2 mistakes. One was to use transistors on the B channel to get more gain out of the preamp stage. Somehow the natural response (I feel) was throttled and the tube compression is lighter. The transistors do not make it sing and secondly, they did not make enough gain - the JCM 900 is NOT high gain, but a medium gain. They corrected this later with the ensuing SLX model by adding one more valve. (ps. I am saying transistors but they might be diodes etc.. I am not a technician). Anyway, thru more luck than planning this amp was coincidently launched as the new emerging grunge wave arrived. It became a non-shredding favourite, especially amongst rhythem guitarists or punk bands. (it was the highest selling model until the JCM2000 arrived). I know I have critized its transistor staged gain design, but at the end of the day this amp truly sounds great - is has that true Kerrang!! Marshall sound and do not forget it has a wonderfull clean channel which can crunch very close to the older non-MV Plexis. I can play AC/DC or Hendrix on this til' the cows come home. Probably the A channel is NOT affected by the transistor gain and it is the original JCM 800 channel. I guess there are users who crank this channel and add a o/d pedal and find that sweet spot... Nirvana..

The DSL & TSL amps after the JCM 900 had no more transistors in the preamp stage, they went back to the JCM800 design and used additional preamp tubes instead.

Cheers Brett