But, the power stage in that amp is basically identical to the old 800 series so the base sound is there for the taking. I myself love to boost the input to poweramp via a rack EQ in the amp loop, and that way it yields really great old Marshall sounds with ease. It also helps I have a 4x12" with two 25W Greenbacks from the late 60's and an Eminence Texas Heat plus a Marshall C35 there - well broken in and certainly not metal speakers.
Anyway the mods.
First thing to do was snap off the C83 cap as per instructions from Santiago, the amp's designer himself.
"Removing C83 on the board will be the solution for what you are after. Just snap it off of the pcb and both gain and treble response
are lowered while retaining the bite and not getting muddy. I also think that the gain control will be more usable in all its range and
those modes more dynamic"
And that it certainly did!
The second change was to solder a .68uF polyester capacitor parallel to R97 resistor. That tamed the extreme high end shriek and warmed up the lead sounds quite nicely.
Then I removed and replaced the R58 resistor with a 180K resistor for much less negative feedback which causes the power stage to start overdriving much more easily and gives that pleasant power tube crunch WAY easier than stock.
Finally, as the original JCM series amps come with a real choke and the JVM has a simple component there to do the job, I replaced that - the R106 resistor - with a real choke, a Hammon 4H 225Mv choke. That improves the clarity and low end response some.
These mods were stupefyingly easy to do, despite I'm quite a hack with electronics, mostly because the good folk at the JVMforum were very helpful and had great examples and guides available. The amp is now stupefyingly good, exactly the kind of sound I have always wanted and aimed for.
I have owned a whole lot of amps ranging from several Fenders like The Twin etc. to Ampeg, many Marshalls like the 2203 and JCM800's, Ceriatone's EL84 custom jobs, several tube rack systems like Marshall JMP-1 and Peavey Rockmaster, and loaned and played a shitload more amps in search of my tones...and, well, I just cannot any more imagine a tone I'd like better than the modded JVM. I've had it now for two years and every time I play it, no matter what setting, it's just pure greatness. Especially with active pickups the tone is so clear and organic and warm it's outta this world. Full chords ring out with perfect string detail even at high gain settings without muddying up. I just love it to death.
Here she is in a pic a few years old already:
The guitar there is Davette, my main axe, with EMG 85 + SA and my own design body shape; that's an entire story in itself how it came about.
Here's a small clip I made with the SG I recently rebuilt completely. I'm playing at a low volume so you can hear my picking quite well, but then again, it's a good example how the JVM, despite being a 100W monster, really sounds awesome at any volume and doesn't need to be cranked like an old JCM series.
[video]https://www.youtube.com/watch?v=G5okJYzIBjM[/video]
