JCM900 4500 Mod: C27 Jumper or...

[NoelH]

Hey folks, a Newbie here.

I just made a deal for a JCM900 4500 head. I looked at some info around the WEB and found that the wimpy/thin lead channel tone came from a pre-eq section you could take out of the circuit by snipping out R7 and R43. DONE! And I have to admit, it sounds GREAT now! Simple and very effective. It is still a little shy in bass frequencies however.
I understand one more mod will get me more bass response: Jumpering C27. Been thinking, I could just put in another cap in parallel, therefore increasing capacitance. That should also get me more bass response, correct?

If you have a recommendation on this, I'd love to hear it. BTW, I also have a choke on order to plumb in.

Thanks for any help!

[J M Fahey]

Jack the Ripper used to say: let's go part by part.

1) that pre-distortion EQ is nothing more than a standard Marshall tone stack, fixed set to treble on 5, bass on 10 and mid to 20 ... because the fixed resistor is 2x what it should.
2) by clipping R7 you basically put it out of the circuit and recover some gain, you can call that a slight boost.
3) audio now has 2 paths : treble through C27-R41 and bass through R43-C10-R40
4) if you clip R43 you block the bass path, so signal gets thinner.
C27 in series with R41+R35 is a high pass cutting below some 300Hz, very audible.
Believe it or not, this is actually *good* .
You increase C27 by any significant amount and distortion (which was already boosted) will probably become muddier or farty, unless you, say, play with a Strat or similar.
5) most people love Plexis and hate JCM900 , which strangely have the same power amp and drive the same 4x12", because Plexis are always used full blast, balls to the wall, clipping the power amp (and smashing your eardrums) while Master volume amps and even worse, SS preamp/distortion ones such as JCM900 are used at much lower volume.

Personally I wouldn't increase Bass but play amp LOUD , then you tell me.

Try it

Unbearably loud?

Well, they were designed for Stage use, preferably at a Stadium or open air Rock festival, not too happy at home, garage or small Club, which is where most "normal" people play.

[NoelH]

JM,
Thanks for the info. Actually when I clipped out R7 & R43, the overdrive became much smoother, and pleasing to the ear. Got rid of a bunch of 'hash' in the treble frequencies. And, at moderate volumes, the amp is sounding quite good. Perhaps because it is the 50 Watt version?
If I understand some previous posts, I took out the Pre-EQ section of the overdrive channel by removing R7 & R43. Is that correct?
All I want now is to increase bass response slightly.

Any thoughts on this?

Thanks,
Noel