#1 · Aug 15, 2008 21:43 UTC
The Marshall tone stack is, really, just a derivation of the Fender tone stack with some different values. We've already covered how the tone stack works, on a Fender, in quite some detail, so I'm just going to kind of gloss over that part, here, and focus on the more relevant differences.
The first difference is that tiny little cap, at the top of the stack, that feeds the Treble / Bass / Middle controls. In the Fender it was a nice small value of 250/270pF (depending on amp and model). On the Marshalls you'll see 450/470/500pF values in this spot with the 470 being the most used. I think the differences, here, had to do more with availability of parts, in the early, days and not so much the tone of the amp. These parts would vary by 20-25% anyway, so a 450, 470 or 500pF capacitor, in this position wouldn't make any audible difference unless, of course, your name is Eric Johnson. Then it would.
This 470pF capacitor, in this position "extends the reach" of the treble control further into the midrange region giving it more control over more of a frequency range than the "Fender". In fact, this extends that control deeper into the part of the distorted tone we like to call "Crunch".
Another notable difference, in the Marshall amp, is the fact that the slope resistor is WAY smaller. Most Marshall's have a slope resistor of 33K, versus the Fender value of 100K. This allows the Bass / Mid control to have a bit more impact when you're twistin' the knobs, as what it did on the Fender. That said, though, remember that Fender amps were pretty much flat response all the way through the amp (with the exception of the tone stack). These Marshalls have everything shaved off below 720Hz, twice before anything gets to the tone stack. Then, we have the added "imbalance" of all the harmonics generated by the distortion so they're at a low frequency disadvantage to begin with. This helps to compensate that "problem".
The final difference is in the values of the caps feeding both the mid and the bass controls - especially the one feeding the "mid" control. This value is much larger than that in the Fender amps. This has the direct effect of "lowering" the "mid-cut" produced. Of course, the high-pass filter for the treble has shifted down, as well, so moving the mid-cut down, in frequency, only makes sense, too.
And, thus ends the Marshall preamp topology...
Enjoy.
Dar
The first difference is that tiny little cap, at the top of the stack, that feeds the Treble / Bass / Middle controls. In the Fender it was a nice small value of 250/270pF (depending on amp and model). On the Marshalls you'll see 450/470/500pF values in this spot with the 470 being the most used. I think the differences, here, had to do more with availability of parts, in the early, days and not so much the tone of the amp. These parts would vary by 20-25% anyway, so a 450, 470 or 500pF capacitor, in this position wouldn't make any audible difference unless, of course, your name is Eric Johnson. Then it would.
This 470pF capacitor, in this position "extends the reach" of the treble control further into the midrange region giving it more control over more of a frequency range than the "Fender". In fact, this extends that control deeper into the part of the distorted tone we like to call "Crunch".
Another notable difference, in the Marshall amp, is the fact that the slope resistor is WAY smaller. Most Marshall's have a slope resistor of 33K, versus the Fender value of 100K. This allows the Bass / Mid control to have a bit more impact when you're twistin' the knobs, as what it did on the Fender. That said, though, remember that Fender amps were pretty much flat response all the way through the amp (with the exception of the tone stack). These Marshalls have everything shaved off below 720Hz, twice before anything gets to the tone stack. Then, we have the added "imbalance" of all the harmonics generated by the distortion so they're at a low frequency disadvantage to begin with. This helps to compensate that "problem".
The final difference is in the values of the caps feeding both the mid and the bass controls - especially the one feeding the "mid" control. This value is much larger than that in the Fender amps. This has the direct effect of "lowering" the "mid-cut" produced. Of course, the high-pass filter for the treble has shifted down, as well, so moving the mid-cut down, in frequency, only makes sense, too.
And, thus ends the Marshall preamp topology...
Enjoy.
Dar