Slope Resistor value in Marshall tone stack?
jwaterfield
how it works. From what I gather, it governs the midrange frequency of
the amp. I could be completely wrong....?
I own a JCM 900 Dual Reverb (schematic here:
http://www.schematicheaven.com/marshallamps/jcm900_dualrev_100w_4100.pdf).
The slope resistor is R61, if I'm not mistaken.
Here's my question: what would happen to the sound if I replaced it
with a larger value (say, 56k)? A smaller value (10k, or so)? Does it
cut/boost certain frequencies, or does it affect the range of the tone
controls...or both? What exactly would changing this resistor do?
Thanks in advance!
Geoff C
news:1170390846.3...@p10g2000cwp.googlegroups.com:
> I've been trying to research exactly what the slope
> resistor does, and how it works. From what I gather, it
> governs the midrange frequency of the amp. I could be
> completely wrong....?
>
> I own a JCM 900 Dual Reverb (schematic here:
> http://www.schematicheaven.com/marshallamps/jcm900_dualrev_1
> 00w_4100.pdf). The slope resistor is R61, if I'm not
> mistaken.
>
> Here's my question: what would happen to the sound if I
> replaced it with a larger value (say, 56k)? A smaller value
> (10k, or so)? Does it cut/boost certain frequencies, or
> does it affect the range of the tone controls...or both?
> What exactly would changing this resistor do?
>
> Thanks in advance!
>
>
Rather than trying to describe it, I suggest you visit
www.duncanamps.com and downlaod a neat utility called 'Tone
stack Calculator'. You can fiddle with the resistor values and
watch the graphical response change.
It is really easy to use.
RonSonic
Like the other guy said get the Duncan tone stack calculator.
I'll say that it will take less time for you to go tack some resistors in there
than it would for me to type about it.
Ron
rob
wrote:
>I've been trying to research exactly what the slope resistor does, and
Current value is what...33K or so if I remember. Higher values will
induce a wider notch in the frequency response, of course. Lower
values will have a narrower notch, but if the resistor going to ground
from the mid rolloff is low, it will present a sort of 'phased' effect
(this is the 6.8k resistor in Fender circuits. I don't remember the
value in JCM900).
That effect is in fact caused by phasing of signals going through the
treble and bass sides of the circuit and then effectively adding
again. Frequencies at the center of the range are shifted in opposite
directions so re-adding can cause a very deep cancellation.
This is often part of the sound of a Marshall amp. Fenders have a
wider mid scoop to begin with, so the narrow notch effect is not as
pronounced. You can hear that by varying the mid control on any Fender
amp that has one.
Yes, the slope resistor will affect the perceived range of the bass
control. A lower value slope resistor will provide more bass for the
bass control to work with. Again, check the range of the bass control
on a Fender vs Marshall. The range of the treble control will not be
affected much by the slope resistor.
Also the slope resistor forms a voltage divider, working against the
bass pot. If you raise the value too much, you'll be losing signal
through that voltage divider.
There is a cap going from the slope resistor to ground that rolls off
mids (that is the whole point, after all). You could also alter the
value of that cap. Some of the same effects, but you won't have the
negative side-effects re the voltage divider.
From there you can try some of this yourself, but that should give you
a starting point and an idea of what to listen for. You might even
consider a switch for the mid-cut cap. You'll probably want to keep
the values spaced about a 1.3 : 1.0 ratio or so. Your 2:1 example for
the slope resistor value is too wide. If you decide to keep the
switched caps, bridge each cap with a high value resistor to guard
against popping when it is switched.