There should be no differences between a JCM800 2204 that has vertical inputs and a JMP 2204 (aside from the very first circa '76 JMP 2204s that lacked a gain stage). The circuit boards are the the same, the component values are the same, the iron is the same. Sonic differences are most likely related to tubes/bias points. Tim I really like the MV Marshall I built myself. I got the MV preamp schematic here on ampage, and then took the 50W Tweed (2 ohm Bassman) power amp schematic from the same page, and combined them, and a SS power supply, and I've got the sweetest amp I've ever played. All true point-to-point wired, using terminal strips. Here's a link to the homebrew page I made about it: http://geocities.com/teleman28056/mv_clone.html Chris B The plate voltages between the late 70's JMP 2204's and the JCM-800 2204's are different. The JMP's have plate voltages of around 385 volts, while the JCM-800's are around 450 volts. The JCM-800's are crunchier sounding because of this. The JMP's are a little warmer. 100 watt Marshalls, from what I've heard, tend to have higher voltage on the plates. With this and the OT's, 2204's often sound smoother and creamier than the 2203, like Dai said. If you prefer a punchier, more dynamic, rougher sound, get a 2203 and pull a couple tubes. If you want a more compressed, singing tone, get a 2204. Todd Agree with all that, but additionally the 2203 has LOTS more bass than the 2204. The OT in the 2204 is definitely a bit too easily saturated. I'll bet an OEI tranny would help out lots. I think you underrate the differences between the MV JMP models and MV JCM. The late JMP is almost the same as the early JCM, with one minor difference (cap across a gain stage). There are definitely iron differences between models with the same circuit design. The JCM pots are a lot better, more linear. 1979/80 JMPs appear to have the same pots and volt/ohm selectors as the JCM. Much more convenient/reliable. From the tonal differences I would put money on the OT design having changed slightly in the early '80s. The very first production JMP circuit had a heap less gain than the later one - but is a very good crunchy sounding amp. AFAIK it was only available in 1975. Only ever seen one. The best MV Marshall I have heard was a '76 2203, and seemed to have the gain of the later JMP style circuit. Never took it apart - but it really rocked. If the voltages changed with the same transformers and circuits, what gives? Seems unlikely to me that the same circuit on the same PT should have a different B+. I therefore assume the transformers differ. Liam Do yourself a favor and add a 100K resistor from the wiper of the Master Volume pot. A better detailed explanation is in O'Connor's TUT. Marshall MV's do sound 'fizzy' at real low volumes, but the 'mod' brings back the bottom end when you have to keep the MV down around '2'. I did that to all my JCM800's and won't have a Marshall that doesn't have this mod. Preamp gain is cheap and easy to get if you want more 'buzz' while you're in there. I think most people would say the pre-PI MV is the best for a 2203/2204. There's enough gain in the preamp to get plenty of distortion without having to put the MV after the PI. Plus, the pre-PI MV is elegantly simple. A post-PI MV adds parts and complicates the workings of the feedback loop. Michael Tousek Yes, Sir. Mr. O'Connor does a far superior job explaining the impedance of the EQ circuit and the low-frequency roll-off created by the Master Volume control. In a nut shell; without a resistor in there, the roll-off happens about 1KhZ with the Master set on "1", and raises to about 45Hz with the volume on "5". Adding the resistor lowers the cut-off frequency to about 72Hz with the volume on "1". I add that resistor, and the 'Resonance' control (also from TUT, and 'borrowed' from the 5150) to add some serious 'oomph' to my JCM800's. I have a 2X12" combo that seems to sound 'fuller' than a straight head, so the Resonance idea wasn't needed. Someone earlier asked about the preamplifier resistors and the .68uF cathode bypass cap. That's part of the thin tone in JCM800's, to my ears. I raise that cap while lowering the 10K cathode resistor seen earlier, being careful because microphonic 12AX7's show up big time here. I sometimes need to shield a wire or two, and add a snubber cap across a plate load. But this is all 'stuff' that can be removed if you want the stock amplifier, and no one ever needs to know.