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Trace's Marshall Mods


 
6/4/1999 9:13 PM
Ethan Walker
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Trace's Marshall Mods
Hi Folks, I picked up an 84' JCM 800 (2204) today. It had 6550's stock, so I rebiased and added Svetlana EL-34's, cleaned pots, etc. Then I tried a few of Trace's mods from a previous post with good results-lowering the R6 cathode resistor seems to work well. I'm going to mess around w/the slope resistors tomorrow. Here's the question: has anyone tried altering the input resistors as mentioned in M. Thompson's "Great Tube Amp Mods" book? For the record, I'm after a stock MV Marshall sound with more gain. Any comments are welcome-Ethan
 
6/4/1999 9:35 PM
phil
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Maybe you could change the cathode-follower that feeds the tone stack to a fourth gain stage? You could add almost any amount of gain by doing that... but also run the risk of oscillation, etc.  
 
Anybody ever tried a mod like that?
 
6/5/1999 6:41 AM
Liam
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I'm not sure you'd gain too much. I always thought the cathode follower stage was meant to be a buffer to stop the tone stack loading the preamp stages.  
 
I got shot of the input resistors entirely on my 2203. Just run screened cable to the grid of the first tube. I was told it'd probably oscillate. It doesn't (or at least if it does it's over 10 MHz). It does give a bit more gain.  
 
Trace's cathode bypass cap mods are superb. They alter the tone and the gain structure. Just awesome.  
 
Liam
 
6/5/1999 7:35 AM
SpeedRacer
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I'm not sure what he means either; the gain of a CF stage is less than 1 by definition (100% feedback on the cathode..) It is a buffer for the tone stack, and by chance is a key ingredient in the tone of the pre-ampp by the way it distorts.
 
6/5/1999 9:01 AM
phil
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My thought was to convert the catode follower into a standard voltage gain stage by adding a 100k plate resistor, and 1k (or higher) cathode resistor.  
 
The idea is Kevin O'Connor's, from TUT.
 
6/5/1999 9:18 AM
Trace
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Phil;  
 
Hey there! Sorry I haven't replied sooner but I've been in Moving hell!!! I moved to a different city and this is the first time online in about a week or so.  
 
I can send you a schematic of the full blown 2203/2204 mod if you like. There's 3 different ones that I basically do, of course depending on what someone is looking for. When you say you want a stock master volume tone what is it you are looking for exactly? More gain? More beef? More Bass?  
 
I agree with Speedracer on the one issue. I don't think that'll do much for you to be honest. If you are looking for more gain there's a few other things you can try. You could also try Kevin's idea and see if that works for you. It all depends on the tone you are looking for.  
 
Can you name a few tunes that have the tone you are shooting for??  
 
Hollar back;  
Trace
 
6/6/1999 10:17 PM
Trace
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Phil;  
 
I've messed with this but it's really not the place to get more gain in my humble opinion. It really messes up the tone pretty bad.  
 
Has anyone else had good luck modding this??  
 
:-)  
Trace
 
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