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JCM 800


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3/31/2005 9:52 PM
Tim
JCM 800
Hey I recently picked up a 50w JCM 800 combo the high input wass so so bright I use a strat with Kinmans. I have removed the bright caps 470pfx2 and the one across the Pre-Amp volume its much better tonally however the preamp gain has lost alot of go I have to really crank it up to 9 to get it sounding the same.  
 
The Bass controls are nearly turned off. I am usually a Fender amp man but this amp is very versatile. Any mods I should know  
 
Cheers  
 
Tim
 
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3/31/2005 11:21 PM
John G
Tim,  
When a customer brings me his amp in to get the treble under control, I usually leave all the top caps in place and re arrange the eq mid control to that of the JCM800 Model 4140( schem can be found at DR Tube.com ).Don't forget to add the 4n7 cap from the middle control wiper to ground.The controls now work like a Fender ie when all controls are at minimum you get NO sound out.  
Dont forget pre tube type makes a big difference to tone and treble content.  
John G
 
4/1/2005 12:36 AM
John G
Tim,  
Sorry I should have written JCM 800 Model 4010.  
John G
 
4/1/2005 1:50 AM
Tim

What Pre Tubes do you recommend they currently have the stock Marshall ones this amp was full of cobwebs all I have done is replaced the pwr tubes svetlana el34 and a grid resistor that was blown
 
4/1/2005 3:12 AM
Chris@CMWamps

Hi Tim  
 
The mid-control-mod is a very good suggestion for a "budget-mod".  
The scratchy/bright sound from lots of post '68 50 Watts Marshalls is due to the outputtransformer. If you do replace the Drake with a plexi replica from ObsoleteElectronics or MercuryMagnetics you'll get way sweeter/nicer highs to start with. The overall sound at low volumes will also be greatly improved.  
 
Hope this helps, have a nice weekend in advance,  
 
Chris
 
4/1/2005 3:58 AM
Chris@CMWamps
ooooooooooooooooooooooops
:)  
Forgot to tell:  
when you do use a better OT most of the time you can re´nstall those caps for a nice gain/sizzle without a piercing high end.  
 
Mo'coffee :D  
 
Chris
 
4/1/2005 9:02 AM
Dai Hirokawa

quote:
"when you do use a better OT most of the time you can re´nstall those caps for a nice gain/sizzle without a piercing high end."
 
 
does kind of seem like it, after going from the stock JCM800 OT to an OEI.
 

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