Jcm 900 mkiii 2501 mods

mrjones2004x

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So I’m taking my mk3 900 from 1991 to Marshall tomorrow to have some of the pots changed and a general check over of valves and cap etc but he asked if I wanted to put the amp back to stock. I mention it was modded.

it has a bunch of mods and I do think it sounds amazing but I wonder how it would sound back stock?

I play it with preamp volume on 8 and sensitivity on 20. It’s pure thick and ballsy. If I roll back the sensitivity it’s thin and anemic. Like terrible but I do t use it like this. Wonder how it would be stock ???

mods I can see is

D1, D2, ZD1, ZD2, disconnected.
A capacitor on one of the valve bases has been snipped? Says 22 on it. Assume it’s v1. Not sure which is which. Only one preamp valve has a cap.
R6 on power board has had a extra resister put in parallel. Both resisters are 4.7k? (Yellow, violet, red, gold)I read this is for the negative feedback So 2.35k ohm overall.
Can’t spot any more

Anyone think I should go back to stock? And suggestions.

thanks
 
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tschrama

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Keep the mod… seems to be a good mod to het tube overdrive instead of diode clipping.
 

mrjones2004x

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Thanks, I do love it’s sound. Maybe I’ll find a stock one to purchase one day. think the pot changes will fix the odd volume drops. Been crackling a while on adjustment
 

JCM-Andy

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Hope you got it fixed and still are happy with how it sounds.

I have an Mkiii 2500 dating 1990 with a similar mod, not done by me, with D2 + BR1 out (assume the BR1 is also out in yours), but D1 and the two Zener diodes (ZD1 and ZD2) still in. As I understand, the Zeners in the front won´t influence much unless you would push the front very hard > 10V P-P. An extra voltage divider has also been added in mine, between stage 2 (V1b) and 3 (V2) to lower the voltage into V2. Anyhow, I have the same experience as you – it sound best with the sensitivity on full (or almost on 20).

Cannot really say how it compares to stock, it was already modded when I got it.

Cheers!
 

mrjones2004x

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Hope you got it fixed and still are happy with how it sounds.

I have an Mkiii 2500 dating 1990 with a similar mod, not done by me, with D2 + BR1 out (assume the BR1 is also out in yours), but D1 and the two Zener diodes (ZD1 and ZD2) still in. As I understand, the Zeners in the front won´t influence much unless you would push the front very hard > 10V P-P. An extra voltage divider has also been added in mine, between stage 2 (V1b) and 3 (V2) to lower the voltage into V2. Anyhow, I have the same experience as you – it sound best with the sensitivity on full (or almost on 20).

Cannot really say how it compares to stock, it was already modded when I got it.

Cheers!

BR1 is in in my circuit but I think from reading it’s out due to other things being removed. I may be wrong tho.
For p90s I normally have preamp at 6 and sensitivity up 3/4 to full.
Looking forward to trying it on Friday when I get it back with its new tubes, pots and jacks. Hopefully no more crackles and volume drops. Also had the occasional glassy high noises which I’d guess was from a tube somewhere.
 

JCM-Andy

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Thanks, yes probably BR1 is just ‘out of circuit’, but not entirely removed. Just saw your other thread about bias - think it will sound killer with the new tubes, correct bias and those other parts fixed.

Also find the Low mode quite useful when playing at home. Still very load, but are able to crank the master pretty high to get into the good sounding region (using a 412 Marshall cab).
 

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