I want this JCM 900 to be heavy as FUCK !!!! I still would like to have clarity though. I must be able to determine the notes. Not nu-metal fuzz.

So basically all I have to do is the tubes and my pick-ups ?

If so I got it covered all must do is get all of this installed.

VOLUME UP!
bass UP
MIDS UP
treb were ever you want it.
Gain (see treb)

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get a pedal or something before hand to give ur guitar a lil boost, thta will give you more gain... also get an eq or something so u can really control your sound. and yeah i duno much bout tubes but get some high gain pick-ups like emgs or super distortions etc

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Maybe put some weights inside?

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i'd suggest getting some hand picked jj's from eurotubes.com he has some 12ax7's that have about 10% more preamp gain than the average 12ax7.

also sovtek 12ax7lps's are pretty high gain. i have one in my jmp-1.

you have the 4100 model right? i'd look into trading it in for a 2100(sl-x). the 2100 has an extra preamp tube, and a second gain knob. much more brutal. but it doesnt have reverb(more than a fair trade imo).

one thing a like about the jcm 900's... the eq isnt very responsive. ie. cut the mids to zero and you still wont get a super scoop sound. you can kinda just knock the knobs around at random and get interesting tones.

my jcm 900 settings for heavey stuff was everything at ten. sounded great, but it might be to loud for some.

if all else fails, you can stick a proco rat in front of it.

does your 4100 model have a half power switch? the 2100 does... it's really nice.

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Yeah.

Try either the Proco Rat in front of it (Dismember/Entombed sound)
or a Big Muff (Defleshed sound: http://www.lcrr.de/defleshed/music/gar.mp3)

I know for a fact the Defleshed uses a Marshall with a big muff. I asked them and got an answer.

Concrete will do the trick.

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Tasty.

I do not think that I have a half-power switch, I will check next time.

(i leave at a my friends that is where we practice)

I have a 4100, and it has a half power switch. It changes the tubes from penetode to triode operation. The gain in the amp is only good for adjusting the tone of the distortion. You can get a Metallica sound on like 7, and Morbid Angel Slayer sound on like 8- 8.5. I had it modded once, but it didn't get the amp gain I was after so I unmodded it.

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Bass = 10
Mid = 0
Treble= 10
Gain = 100%

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you forgot presence and volume also at 10

normally even thinking of these setting's on most setups just makes me laugh.

but on a jcm 900... it actually sounds good. not ultra scoopy.

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pres-4
bass-8.3
mids-3.8
treb-6
gain-8
volume-6-8

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Get yourself a good set of tubes. JJ ECC83S for the preamp section and either the EL34L's or the KT88's for the power section. Next step would be to have a full set of SOZO tone caps installed, then have someone rip out that shitty stock Output transformer and install a Mercury Magnetics or Heyboer OPT in its place. After that if this amp doesn't breath like a fire breathing demon from hell then its your playing. Do the first two suggestions though it will make the amp sound like a whole new beast!