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Re: Sagging JCM800 pre-amp or blocking distortion proble

9/30/2005 4:18 PM
Jon BlackstoneRe: Sagging JCM800 pre-amp or blocking distortion proble
Doug -  
 
I've got a 4210 which I have extensively modified. It's that early one with NPN transistors doing the channel switching.  
 
I rearranged the 'boost' preamp to be like a plexi (tone stack after cathode follower), and replaced the baxandall tone stack in the clean channel with a "leo's ladder". (One with a little less loss than a fender.) The clean channel sounds great.  
 
At one point I had the boost channel going straight to the phase inverter for a pure Marshall thing, but I guess I decided the extra stages were benign, 'cause I put it back to mixing in before the reverb.  
 
Interesting that you mention the dry bypass resistor being 680K. I was a little mystified by the schematic saying 6M8 for this. My amp had a 680K in it, but it looked like a previous owner had put that in. I settled on 1M, which gave me ample reverb and still seemed to hit the PI pretty hard. Was there a mistake on my schematic?  
 
I don't have a problem with leakage betweeen channels. I took out the parallel caps on the mix resistors - maybe those cause the leakage people seem to get.  
 
My reverb return hums. I might install an FET follower inside the reverb pan to fix that.  
 
I put larger coupling caps around the post-PI MV. It seemed to me that those .02uF's in series, with a resistance to ground in between, were looking like a textbook HPF.  
 
Jon Blackstone  
Blackstone Appliances

 
Replies:
Doug Mine had the 680k resistor as well,... -- 10/5/2005 9:13 AM