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Slope resistor -- its effect?


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10/10/1999 7:57 PM
Dave M.
Slope resistor -- its effect?
I'm specifically interested in the "how and why" of it in my Marshall metal panel 1987. My amp has the standard 33k in place. I see from looking at a couple JTM-45 schem's (Doyle's book; my only resource at present) that those models used 56k. To add more info to this mix, I've considered trying a 47k in my amp.  
 
So, I'm curious about how different values can, may, or actually do affect the tone -- primarily in my amp, and/or in general in guitar/bass amps.  
 
TIA for all the help,  
-Dave M.  
 
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10/10/1999 11:28 PM
Steve A.
Re: Short answer
Dave:  
 
    There was a thread over at the old AX-84 forum on slope resistors that had at least 30 posts and got into all of the technical aspects such as RC filtering, etc.  
 
    However I like to simplify things and I just consider the slope resistor to be like a voltage divider that splits up the signal into the treble and non-treble frequencies. By using a 47k or 56k resistor, less of the signal goes to the bass and mid caps so the overall sound is brighter and usually cleaner. With the 33k resistor the sound is thicker and heavier, with much more of the lower frequencies.  
 
    Some amps have included a variable slope control so you could dial in the amount you want. For a Marshall style stack I would add a 25k linear pot (wired as a variable resistor) in series with a 33k fixed resistor so that you could adjust it from 33k to 58k. (For a BF Fender style stack you might try a 50k linear pot on top of a 68k resistor so that the range would be 68k to 118k.)  
 
Steve Ahola
 
10/11/1999 12:46 AM
Trace

quote:
"By using a 47k or 56k resistor, less of the signal goes to the bass and mid caps so the overall sound is brighter and usually cleaner. With the 33k resistor the sound is thicker and heavier, with much more of the lower frequencies."
 
 
 
I've found this to be the opposite...as far as the over sound being brighter with more bass with a 33K verses a 47K or 56K (Steve's gonna beat me bloody ;-)  
 
With Marshall's I find that a 47K get's thicker sounding and a 56K is even thicker yet, more bass, etc. Did you maybe flip this around when you were typing Steve?  
 
quote:
"Some amps have included a variable slope control so you could dial in the amount you want. For a Marshall style stack I would add a 25k linear pot (wired as a variable resistor) in series with a 33k fixed resistor so that you could adjust it from 33k to 58k. (For a BF Fender style stack you might try a 50k linear po on top of a 68k resistor so that the range would be 68k to 118k.)"
 
 
Very good idea Steve :-) I think if you look in the "Schematics" section of the Ampage here you can see it on the Marshall 2000.  
 
Trace  
Steve Ahola  
 
10/11/1999 5:35 AM
Steve A.

Trace:  
 
    Hmmm... I've hardly ever just switched the slope resistor by itself since I usually change it along with the treble cap. But when I have, it has always seemed like the higher values are brighter than the lower values... Then again I'm usually working on tweed and BF Fender designs more than Marshalls. Can we get a third opinion on this?  
 
Steve Ahola  
 
P.S. I just rewired the 3 way tone stack switch for the Abnormal channel of my Son of Pro Maniac frankenamp mod, and finally added in the third voicing for the tone stack: 1010pF treble cap and 33k slope resistor, which is very full and thick compared to the 720pF/47k combo (brighter but still down'n'dirty) and the 330pF/56k combo (which is as light and fluffy as Aunt Jemima's pancakes). All of those caps are silver micas and the preamp design is like a cross between a tweed bassman and a Marshall.  
 
    So what value treble caps are we talking about in the Marshalls you've worked on? I've seen everything from 250pF to 0.001uF listed on the schematics. BTW do you use a bright cap across the initial volume controls, and if so, what values do you generally use? Thanks!
 
10/11/1999 6:28 AM
A.S
Third opinion
My favourite is 56k and itīs brighter than 33k.  
Iīve seen that variable slope on a Marshall JCM 800 m. 2000.  
I has a 10k resistor in series with a 100k pot wired as a rheostat. Alters the mid scoop between 400Hz to 1kHz and moves the bass between 100Hz and about 50Hz.  
 
A.S
 
10/11/1999 8:18 AM
Steve A.

A.S.  
 
Iīve seen that variable slope on a Marshall JCM 800 m. 2000. It has a 10k resistor in series with a 100k pot wired as a rheostat.  
 
    So with that control the slope resistance can be varied from 10k to 110k? I never tried anything lower than 33k on a Marshall-style stack, nor higher than maybe 91k. Are the extreme positions of that control very usable? Or would it be better to stick to the tried and true: 33k to 68k?  
 
    Thanks!  
 
Steve Ahola
 
10/11/1999 9:38 AM
A.S

I donīt really know how it sounds because I havenīt tried it. But I guess that if Marshall used this device on that model and that model only, then it might not be such a great hit but Iīm gonna have to give it a try just because we brought it up. Iīll be back some day with the results.  
 
A.S  
 

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