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Some quick Marshall Mods...


 
8/30/1999 5:28 AM
Trace
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Some quick Marshall Mods...
I've had some fellas emailing me about some Marshall mods so I thought I would post them here. Tboy, will the archives be up soon and if so will we have axcess to the info before the Ampage had "went down?"  
 
 
 
These are just some things to beef up a Marshall in general and give it a "warmer" tone. If anyone has any questions please feel free to post them here so we can all have fun! (ha, ha) I'd LOVE to hear from anyone else who has any mods. It's always a blast to try new things! :-)  
 
 
 
These aren't really earth shattering but they do yeild great results.  
 
 
 
This pertains to the older Models up to and including the JCM800's (2203-2204)  
 
 
 
This mod is for the EQ section.  
 
1.) Remove the .022 caps that are connected to the slope resistor and replace with .033's.  
 
 
 
2.) I also like to replace the slope resistor and treble cap on the later 800's with a 47K / 270pF combo.  
 
 
 
3.) For a thicker tone that's really cool for leads and really fat chords solder a 250pF cap to the .022 that goes to the grid of the PI tube. You can "hard-wire" it or wire to a switch.  
 
 
 
4.) After all that good stuff is done remove the pair of 220K's (that are just after the PI and are connected to the .022 caps) and replace with 100K's for the JCM800's what have the higher plate voltages and use 82K's for the older 4-input models...of course make sure to rebias the amp.  
 
 
 
 
8/30/1999 10:26 PM
Jim S.
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For mod #3, you don't mean to solder that 250P cap *in parallel* with the .022uF, do you? If so, the net capacitance would be .022250uF (assuming 100% exact cap values), which makes no audible difference from .022. Of course, wiring the two caps in series would only serve to drastically thin out the tone. Do you mean to connect one end of the 250p cap to ground to reduce high frequencies? But it seems like that might only serve to muddy things up.  
 
 
 
Please explain this mod in more detail. Thanks!  
 
 
 
Also, for mod #4, it sounds like you're referring to the bias feed resistors, right? I've never heard of going lower than 100K for these. Would 82K really sound much different than 100K?  
 
 
8/30/1999 11:58 PM
Terry
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Jim,  
 
 
 
I have a Traynor YBA-1 Bassmaster that had a 68K resistor and in my efforts to "Marshallize" the amp I changed it to a 220K. It is my understanding that increasing this resistor value will increase output. Also, I've read others advising to lower the value from 220K to 100k and that would add to the tube's life. BUT, how does either affect tone, etc.  
 
 
 
Also, I understand that some people have added a post PI master volume using a dual pot rated at 100k. And I've heard negatives about making the resistors in question the variable type. I'm all ears for someone to clarify this all up. I don't believe there is much current drawn through these resistors to begin with so am not certain as to the effects on bias. Have not experimented and documented it yet.  
 
 
 
Would reducing the resistor value also allow for bass roll-off? And, what about output? I'd also be interested in hearing more on the 250pf mentioned in item #3.  
 
 
 
Take care, T.B.  
 
 
 
 
8/31/1999 12:54 AM
Trace
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quote:
"I have a Traynor YBA-1 Bassmaster that had a 68K resistor and in my efforts to "Marshallize" the amp I changed it to a 220K. It is my understanding that increasing this resistor value will increase output. Also, I've read others advising to lower the value from 220K to 100k and that would add to the tube's life. BUT, how does either affect tone, etc."
 
 
 
 
@@@ It does increase the tube life (as I understand it) and it also helps to "brown-up" or "warm-up" the tone of the preamp. If a Marshall has a lot of harsh high end then this helps trim those harsh "highs" I also think that this makes the pre-amp work harder.  
 
 
 
q{also, I understand that some people have added a post PI master volume using a dual pot rated at 100k. And I've heard negatives about making the resistors in question the variable type. I'm all ears for someone to clarify this all up.  
 
 
 
I've installed these and I have one on one of my own Marshalls. These are simply dual-ganged (two pots back to back so to speak) 100K pots. The "Train Wreck Pages" call for a liner taper but I've use the audio tapers without any problems.  
 
The idea behind this is to keep the phase inverter's tone and in the older 4-input models the phase inverter's tone is pretty important. The phase inverter actually "breaks up" or distorts and if you install a Pre-PI on these models you loose a lot of the tone of the amp (IMHO) When the dual-ganged 100K's are turned to "10" the amp is restored to it's original "fully-cranked" tone (well 220K's would be totally stock but you get the idea :-)  
 
 
 
~~~I don't believe there is much current drawn through these resistors to begin with so am not certain as to the effects on bias. Have not experimented and documented it yet.  
 
 
 
@@@ It doesn't really have any grave effects as far as I can tell. Perhapes someone else can elaborate on this more?  
 
 
 
~~~I'd also be interested in hearing more on the 250pf mentioned in item #3.  
 
 
 
@@@ I pretty much answered this in the other post but I forget to mention that you can use values between 250pF to 470pF. It really depends on whay you want to hear. I like a 250pF to 330pF in the older Marshalls. I use this on 800's as well but it depends on what someone is looking for.  
 
 
 
Another nice touch for the JCM800's (2203/2204)is  
 
 
 
1.) Change R6 from the 10K stock value to 4.7K  
 
2.) Place either of the following in parallel to R9 (820ohm cathode resistor)  
 
 
 
A.).68uF = very Marshall'esq  
 
b.) 22uF = great for "crunch" or "heavier" tones  
 
c.) 1uF = more Soldano'ish so to speak  
 
 
 
If you want a more Soldano-ish tone you might liek to go with C. and replace C1 (.68uF) with a 1uF.  
 
 
 
 
 
Sorry if I'm rambling...it's been a long day. I hope this helps someone get a little closer to the tone they are looking for. Music is a brotherhood...."Share the knowledge" I always say! (ha, ha)  
 
 
 
Hollar back!  
 
Trace
 
8/31/1999 12:28 AM
Trace
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quote:
"for mod #4, it sounds like you're referring to the bias feed resistors, right? I've never heard of going lower than 100K for these. Would 82K really sound much different than 100K?"
 
 
 
 
Hey Jim! I hear the differnce but maybe it's because I sit in a studio listening to amps under mics a lot. I tried this on the 4-input models and this is where I think it works the best. 100K's of course work and I think sound fine on the Master Volumes  
 
 
 
quote:
"For mod #3, you don't mean to solder that 250P cap *in parallel* with the .022uF, do you?"
 
 
 
 
Jim, I usually wire this to a switch and pop it in and out when needed but I do put it on parallel and it's a cool "mid-boost" that sounds really cool for leads and fatter rhythm tones. Again, I hear the difference but who knows if someone else does!! (ha, ha) I may be loosing my mind or sniffing too many solder fumes. :-)  
 
 
 
Hope this made sense...it's been a longgg day!  
 
Trace
 
8/31/1999 3:01 PM
Jim S.
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Sorry, but I can't help but be very skeptical...  
 
 
 
When you add a 250p cap in parallel to a .022u cap, you're effectively increasing the capacitance by only about 1% !! That's significantly less than the tolerance of most film capacitors (usually 5% or 10%). I'm really having a hard time believing you or anyone could hear the difference 1% makes. In general, I find that changing the value of a coupling cap by any less than 20% up or down yields only a rather subtle difference in tone. Assuming you really can hear a tonal change by adding 1% more capacitance, does it really seem to fatten things up THAT MUCH?
 
8/31/1999 3:04 PM
Jim S.
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Are you sure that cap isn't a 250n, rather than a 250p? (250n = .25u). This would make more sense.
 
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