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Re: Marshall Mods to JTM 45


 :
10/3/2000 1:21 AM
Ian Anderson
Re: Marshall Mods to JTM 45
quote:
"Almost no audible effect if you don't have a bypass cap accross the lead channel resistor. "
 
 
So why would anyone want to change the resistor value if there is no apparent benefit or indeed change in tone or anything at all????  
 
 
... Ian
 
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10/3/2000 2:40 AM
Peter S

Ian,  
It's the same kind of thinking as the people who say a JTM-45 needs 32uf caps to sound good. The original early ones had 270k like a Bassman.....they were earlier, so they must be better. The only audible difference is a little better channel separation, which is kind of moot because all you need to do is rotate the volume control on the channel you're not plugged into to get maximum gain out of that channel. I think Marshall switched to the larger values because they started adding brightness caps accross the resistor on the bright channel. The larger values gives you more of an effect with the cap. I can't realy see any other logical explanation for it. I always use the 470k resistors but it realy doesn't matter in my case because I never bypass the resistor with a cap......it makes the bright channel too bright and the muddy channel too dark.  
 
Peter
 
10/3/2000 3:19 AM
Bryan

Hey guys, thanks a lot for all the information. Sorry if I repeated an oft asked question, but I am new to the site and am primarily an effects building kind of guy. This should keep me busy for awhile.
 
10/3/2000 10:33 AM
Dave H.

quote:
".....it makes the bright channel too bright and the muddy channel too dark."
 
 
Thatís how it sounds to me. Perhaps thatís why people like to bridge the channels to balance the mud with the bright. I like to set it up with no caps across the mixing resistors and no bright cap. I split the cathodes and use different bypass caps to make the channels sound different. If you donít have caps across the mixing resistors Iíd think the only tone difference between 270k and 470k would be the earlier HF roll off with 470k and the Miller capacitance of the next stage but you probably canít hear it through guitar speakers.  
 
Dave
 
10/3/2000 12:33 PM
Trace

quote:
"Perhaps thatís why people like to bridge the channels to balance the mud with the bright. "
 
 
I think most people jump channels to get the extra gain. A great deal of why those amps mush up when the amp is dimed is that the PI is being pounded to death and just goes to mush. It's not a bad idea (for those who dime the amp) to change the .1uF (on the flip side of the PI) to a .022 to help eleviate this problem. Also when the bass channel has 220uF, 250uF or 330uF you're going to get much the more that channel is cranked up.  
 
Trace
 
10/4/2000 12:08 AM
Ian Anderson

quote:
"I think most people jump channels to get the extra gain. A great deal of why those amps mush up when the amp is dimed is that the PI is being pounded to death and just goes to mush.  
 
It's not a bad idea (for those who dime the amp) to change the .1uF (on the flip side of the PI) to a .022 to help eleviate this problem.  
 
Also when the bass channel has 220uF, 250uF or 330uF you're going to get much the more that channel is cranked up. "
 
 
Will this only tame the bass mushyness when the amp is cranked or will it cut the bass at any volume?  
 
I tend to run the bright channel on 8 and the normal channel on 7 with the channels bridged.  
 
 
... Ian
 
10/4/2000 3:09 AM
Trace

quote:
"Will this only tame the bass mushyness when the amp is cranked or will it cut the bass at any volume?"
 
 
It will reduce some bass at any volume. A 220uF or 330uF is a pretty huge cap. most guys find that a 5uF is plenty. The other changes help. When the power tubes start breaking up they also add other harmonics to the tone.  
 
quote:
"I tend to run the bright channel on 8 and the normal channel on 7 with the channels bridged."
 
 
I think it'll help you. I always left the bass pot off on those amps (stock that is)...same for the Treble. I just turned the mids to 6. Plenty of high-end and plenty of low-end at higher volumes (both channels at 5 or higher)  
 
Trace
 

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