by fuzznut http://music-electronics-forum.com/t12476-2/

The rockmaster lives again! I had one sitting in my attic for 10 years. 
I recently pulled it out of there, and fired it up (with a Peavey 
classic 50/50 amp and 4x12 x-pattern V30s & G12H30s). I was then 
immediately reminded why it's been in the attic: it has this 
certain midrange peak that is very annoying after a couple 
of minutes, and no matter what you do, you can't dial it out. 
I was about to stow it away again, but then I remembered something 
about my marshall JMP-1 when I was poking around in there and modding 
it. In the JMP-1, there is a low pass filter after the distortion 
circuit. I remember removing it, just to see, and it sounded like 
dookie without it. I went hmmm, I wonder if the rockmaster would 
sound better with a low pass filter in there. So I found a suitable 
spot, across R400 with a .0047 cap, and... WOW!!! What a difference!! 
That annoying peak is gone, and the sound is much more even, and fuller. 
Between the edge control an the presence control on the power amp, the 
highs are much nicer now and not fizzy.

So whoever has a rockmaster, I suggest trying this. The .0047 cap is 
not written in stone, it depends on your gear, but I found that for 
me the annoying midrange starts leaving at .0022, and above .001 I 
have to compensate the HF rolloff too much with the edge control. 
Try it!!

BTW, I only use crunch and ultra with the gain pulled. With the 
gain not pulled, the circuit uses negative feedback to reduce gain, 
and to me it sounds less lively, although a bit fatter, than the 
pulled gain, true cascaded circuit.


Yesterday I did a lot of work and there was a lot of discovery happening. 
I liked the sound so much, I duplicated the circuit in my "test bed" 
amplifier chassis. What I found was:

1- The Rockmaster is, save for a few minor changes, the same circuit 
   that is in the XXX and the JSX.
2- It already has a low pass filter (C500 and R600). This is why the 
   bass response is so great. But it's slope is not enough to get 
   rid of that annoying midrange. 
3- The JSX has the extra low pass cap, like what I did, except 
   that they put them on the wiper of the volume pots.
4- The XXX and JSX both have "C14" at .0022, instead of .001, and this 
   is the chosen component on those amps in which to add more "meat". 
   In the JSX, you can double it to .004, in the XXX you can bypass 
   it all together, which must sound like hell if you use too much gain.
5- All these amps, the last gain stage is moderately loaded down, 
   an example would be the 56k resistor to ground in a trainwreck express. 
   It is my understanding that this has the effect of giving compression 
   and sustain, which might be why this preamp "sings" so well. I don't 
   know, I'm just a bum on the street. 

Kommentar fra annen bruker som tester filtermod:
OK so I put an .0022 across R400 and I really like it- as long as I dont 
pull the gain boost on either the crunch or lead channels. Toned down the 
edgy upper mids and treble nicely. It becomes too upper mid/trebly 
aggressive again with either gain boost engaged. 
Thanks Bob