Slx 2100 bright cap location

[rpurdue]

hey all. I tried the search and came up dry. I have a 2100 slx and dig it, but the high end can be very harsh. Does anyone know the bright cap number to try some different values?

 

[anitoli]

Reducing the treble and presence doesn't eliminate enough high end?

 

[rpurdue]

Reducing the treble and presence doesn't eliminate enough high end?

No, it has that stringy quality still that a bright cap gives.

 

[RickyLee]

Hmmm. Seeing your thread, I was thinking @Cadorman would have chimed in on this one as he has been working on his SL-X quite a bit lately.

That darn schematic that is on the innerwebz is brutal to read. And then my SL-X has been stored away for quite a few years. I will see if I can decode that schematic later today . . . .

 

[RickyLee]

The bright cap across the gain pot (I am not sure which gain) is oddly in the R22 location.

 

[mickeydg5]

VR1 is the Preamp Volume and R22 1nF is the bright capacitor in the Issue #5 schematic.

 

[RickyLee]

VR1 is the Preamp Volume and R22 1nF is the bright capacitor in the Issue #5 schematic.

That is a bit odd huh? You would think VR1 is Gain and VR2 is Preamp Volume.

Always thought this preamp was interesting, as in that cathode follower right after first stage. The 6100 CH3 uses quite a few cathode followers as well. That is partly why I like these circuits as they are very unique. Many people ripping out that SL-X topology.

 

[mickeydg5]

Cathode follower circuits are quieter and have other advantages.
Whether you day gain or volume in these cases because they mean and do the same thing.
More volume = larger signal
More gain = larger signal.
It si just the naming Marshall gave the controls but I am sure the second control does provide more distortion artifacts.

 

[ampmadscientist]

hey all. I tried the search and came up dry. I have a 2100 slx and dig it, but the high end can be very harsh. Does anyone know the bright cap number to try some different values?

C13 (470pf) and R22 (1n) are both bright caps in series.
When 2 caps are in series, the value will be smaller than the smallest of the 2 values.
Final capacitance = 319 pf

Remember that 1n = 1000 pf.

Calculator:
https://www.digikey.com/en/resource...sion-calculator-series-and-parallel-capacitor

 

[mickeydg5]

Still R21 and C13 form the resonant filter while capacitor R22 is varied across the top side of the potentiometer, like a bright capacitor.
Its the same in all those Marshall amplifiers.

 

[RickyLee]

I was going to mention the treble peakers/attenuators C13 & C15, but figured he wanted that cap across that first gain pot.

 

[mickeydg5]

Well that is the bright capacitor or does the job that people consider to of a bright capacitor.

 

[RickyLee]

Yeapers. Thinking that might just be what he is looking for . . . .

 

[ampmadscientist]

Yeapers. Thinking that might just be what he is looking for . . . .

There is something else
that nobody has thought of before. Which is actually what everybody was looking for.
But that is another story...

 

[mickeydg5]

There is something else
that nobody has thought of before. Which is actually what everybody was looking for.
But that is another story...

You talking about the opamp master volume sets?

 

[ampmadscientist]

You talking about the opamp master volume sets?

No, I'm talking about the coupling between the stages.
And I am talking about what Leo Fender did, but never realized what he did. He never knew what he had done.

 

[mickeydg5]

No, I'm talking about the coupling between the stages.
And I am talking about what Leo Fender did, but never realized what he did. He never knew what he had done.

Which Fender?
The SLX looks like basic style Marshall coupling.
Spill it man!

 

[ampmadscientist]

Which Fender?
The SLX looks like basic style Marshall coupling.
Spill it man!

It's the amusement that I figured it out and nobody else did.

 

[mickeydg5]

It's the amusement that I figured it out and nobody else did.

It is nonexistent until you spill the beans.

 

[ampmadscientist]

It is nonexistent until you spill the beans.

I'll try to explain it.
Because the layout of the circuit board, two capacitors are side by side.
And what Fender did not realize at the time, was that these capacitors are coupling together through the air space in between.
So look at the tone stack layout:


And what is happening is that because the 250pf treble cap is directly adjacent to the .1 uF Bass cap, the two caps are coupled together through the air space, forming a additional capacitor (shown in red).



They didn't realize that this provided a solution to the coupling between the stages of a tube amp. Essentially the solution was accidentally invented because of the circuit board layout.