The Bassman-style tone stack serves two purposes: to compensate for the treble and bass attenuation that inevitably occurs at each stage of amplification and to allow the guitar player to adjust the frequency response using bass control RB, middle-range control RM, and treble control RT. The result is an overall gain reduction combined with possible bass and treble boost.

Fender Bassman 5F6-A tone stack schematic

Fender Bassman Book

Reference: The Fender Bassman 5F6-A by Richard Kuehnel

The 5F6-A parts values are

R1 = 56k
RT = 250k
RB = 1M
RM = 25k
C1 = 250pF
C2 = 0.02uF
C3 = 0.02uF

The JMP50 Model 1987 parts values are

R1 = 33k
RT = 250k
RB = 1M
RM = 25k
C1 = 500pF
C2 = 0.022uF
C3 = 0.022uF

The Marshall JTM45 was a Bassman 5F6-A clone that eventually evolved into the legendary Marshall designs of later years. It can be argued that increasing the capacitors C2 and C3 by 10 percent was driven more by part availability than tonal considerations. The substantial reduction in R1 and the doubling of C1 were clearly deliberate. In the pages that follow we will examine the effects of these changes on bass, midrange, and treble response. Along the way we will gain a better understanding of what makes Leo Fender's design one of the most widely copied guitar amp circuits. Finally, I will describe a simple, step-by-step method that I use to calculate the required parts values for a particular frequency-shaping objective.

How Marshall's Modifications Affect Low-Frequency and Midrange Response