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marshall mod?


 
5/25/1999 10:27 PM
todd
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marshall mod?
I read of a Marshall for jcm900's using the power supply filter caps to get more bass. "On one cap both taps are used. On the other, only one of the taps is connected. Get more bass response by wiring the red tap to the yellow tap" Does this sound correct? Has anyone tried it? thanks
 
5/26/1999 6:53 AM
Liam
Todd,  
 
It might reduce power amp "sag" a tiny bit, but probably won't dramatically increase the bass. Some car audio uses big caps to improve bass handling, but they use ENORMOUS caps. I think they're acually 1 Farad (that's a million uF in our terms).  
 
If you want more bass, either use bigger bypass caps on the cathode resistors, or see if you can apply any of Trace's JCM800 mods (there are a bunch of them in the archives) to the JCM900 circuit.  
 
I can't see that you'd do any harm by connecting the extra capacitor tap, except for the possibility that the HT fuse may blow on power up. If anyone else can think of reasons why this might cause problems, I'd be pleased to be corrected. (My brain's a bit slow today.)  
 
Liam
 
 
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5/30/1999 10:21 PM
Jon
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Liam,  
 
I was unable to find the mods for the JCM-800 series. Could you give me another pointer please? I've got a 2205 that I want to mod.  
 
Thanks,  
 
Jon
 
6/1/1999 2:54 PM
Doc
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Stored energy in a capacitor is 1/2(CV^2), where C is in Farads, V is volts. A car electrical system is 12v (really more like 14v with the engine running). A typical guitar amp runs a B+ rail 400 to 500v. So you can see if you square the 500v (=250000), you won't need nearly the size capacitor as you would with the car system (14x14=196).  
 
One farad on the car will hold 98 joules, where 40uf on a 500v amp will store 5j.
 
5/26/1999 8:59 AM
Matt D
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I performed this mod on my 50w JCM 900 HiGain Dual Reverb head. I believe it did improve low end slightly. Overall, this amp lacked tone and no circuit tweaking provided acceptable results. I'm much happier with my Bluesbreaker RI. This reveals my tone preference, perhaps the 900's work well for other folks...  
 
Matt D
 
5/26/1999 9:06 AM
Liam
Matt,  
 
I didn't like to say it, but their not my favorite amps either. The earlier models (especially pre JCM800) and reissues just seem to have a heap more usable tone. I'm glad to hear it doesn't damage anything doing the mod!  
 
BTW Ace from Skunkanasie gets a smoking tone from his JCM900 Hi Gain head, so I guess they do work for some people.  
 
Liam
 
5/26/1999 1:26 PM
Matt D
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Liam,  
No doubt that the guy who bought my 900 head was just thrilled with the tone! He liked the buzzy lead channel (the channel I NEVER kicked in!). But he is a metal player and it worked for him.  
As far as the cap mod goes, I was assured it was safe by a friend, Ed Pennypacker (formerly of Sovtek), who also told me not to bother with any other mods such as OP xfrm substition.  
 
Matt D
 
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