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slope resistor mod


carryonplease

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misc21e.jpg

 

So im changing the value of my slope resistor to 47K. But im really confused. The above picture is how the preamp section of my amp looks. First of all, can anyone tell me which of those resistors in the slope resistor?

 

But that probably can't be answered, so i can tell you, on my amp, the resistor which is green in the center of the picture is red, and labeled 36K. But according to these schematics, it doesn't seem like that is it, unless im reading the schematic wrong which is very very possible.

 

rbmk1204.gif

 

Can someone shed some light on this? Apparently i am supposed to have 2 resistors that are in the 30K range. One at 36K and one at 30K. And i dont see any other resistors with a "3" except for the one that says 36K. and according to the schematic on this website. The 30k resistor is the one to change.

 

http://www.geocities.com/lupe7_99/redbear.html

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i believe that's a mistake. the diagram on that website is telling you to change the pot from 30k to 47k. i'm pretty sure the green 36k is the red resistor. let me know how it turns out cuz i'm about to do the mod myself. fyi i've seen a jcm 800 schematic with a 56k in that same spot so i'll be trying that too.

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the diagram on the website tells you to change the 30k pot that says mid next to it on the schematic. it ment to tell you to change the 33k

 

not to hijack your thread but how do u go about adding the 25k resistor to the mid pot to turn it into a 50 k pot?

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you replace the pot. if you add a resistor, you'd have a base resistance plus the 30K sweep.

 

if you want to change the slope resistor, replace the 33K resistor (based on the schematic, anyway) to whatever new value you want to try. i like 47K, but i use slightly different pot values.

 

it's just a resistor swap. if you don't like the results, you can always change it back.

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you replace the pot. if you add a resistor, you'd have a base resistance plus the 30K sweep.


if you want to change the slope resistor, replace the 33K resistor (based on the schematic, anyway) to whatever new value you want to try. i like 47K, but i use slightly different pot values.


it's just a resistor swap. if you don't like the results, you can always change it back.

 

 

would that be ok if u just wanted more mids? i ask because if you look at the pic the pots are weird and connected to pcb board

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This is a really cool FREE program that can give you theoretical frequency response curves for what happens when you change the slope resistor or other components in the tone stack with other values. It does this over the sweep of the pots. I think it's a great tool for understanding what will happen when you mod stuff in the tone stack area of an amp:

 

http://www.duncanamps.com/tsc/

 

the tone stack model you want to click on would be the "Marshall" one.

 

It seems to show that adding a 47K resistor instead of a 33K will tame the bass/mid end a little bit.

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im hoping changing from 33K to 47K will tame the bass. i run the bass on zero right now and it is still too much. And since it will hopefully bump the midrange frequencies up a notch i will get more cutting mids rather than low mids, which get pretty sloppy on this amp. farty is a good way to describe it. And i would actually enjoy an overly aggressive or focused tone.

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for anyone wondering, i did the change from the 36K to the 47K slope resistor and was fairly happy with the results. It didn't necessarily tame the bass as much as i had hoped but it bumped the range of the middle frequencies up a notch and it sounds better, plus i have more volume and also more gain somehow.

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for anyone wondering, i did the change from the 36K to the 47K slope resistor and was fairly happy with the results. It didn't necessarily tame the bass as much as i had hoped but it bumped the range of the middle frequencies up a notch and it sounds better, plus i have more volume and also more gain somehow.

 

 

47k actually cuts more mids than 36k

that's why it gets more aggressive

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  • 1 year later...
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Very old thread but I came across it and wondered if anyone had ever continued this on. I was wondering if anyone has ever used the the marshall/redbear tone stack layout (only thing different is the bottom routing on the mid and an extra capacitor on the redbear stack) but put in Fender values? How much is the rest of the circuit going to effect the sound?

 

I guess anything is reversible, but I tend to like the Fender sound but am poor and only could afford a RedBear I picked up cheap. I can say I like the amp overall but I do hear the signature bass tone. Based on some testing with the tonestack calculator the output signatures are pretty much the same and with those values you can better control the low end.

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Personally when I modify a tone stack, I usually mess with the caps instead of the slope resistor.

 

I had a blackface Super recently with too much bass and lower mids I replaced the .1 uF bass cap with a .022, and the .022 mid cap with a .046 and it fixed the problem perfectly.

 

It retained a lot of shimmer and gain that way so I was able to drop the preamp tube down to 5751 a la SRV ad it sounds great. I'm experimenting with KT66 tubes in it right now for a little more power and headroom. We'll see if the PT can handle it.

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