Switchable Tone Stack
The two main tone stacks used by Marshall over the years was first the 56k slope resistor and 250pf capacitor combination – this was used exclusively until 1968 on all their amps. Later on Marshall started using the 33k/500pf combination, and this is the one used on the 1959slp, 1959HW and 1987x reissue amps and basically all their flagship Lead models since 1968.
Some people associate the 56k/250pf with the “classic Marshall tone” as it was used by Eric Clapton on his early recordings, The Who when they first got their hands on Marshalls, and even Eddie Van Halen’s “magic Marshall” is said to have used this tone stack. However, it’s also been said that until Marshall started using the 33k/500pf combo, Jimi Hendrix had all his amps worked on to put this combination in. Even more important though is what sounds best to you, and what better way to find out by being able to switch between the two with the flick of a miniswitch. Therefore, I’ve worked up these diagrams to make installation of the switch a cinch. Use whatever wire colours you want, I’ve just used whatever seemed to make the diagram clearest.
Method 1
First off is the most obvious way of doing it, and with each setting you will only be using one resistor and one capacitor. Some people might prefer this setting in case they’re using carbon comp resistors and want to maximise resistor distortion, or want to keep the circuit physically as well as electronically the same in each setting. The downside to this method is that you may get a popping noise when flicking the switch, as for a split-second the switch is breaking the contact between the resistor/capacitor and the rest of the circuit.
click the thumbnail to enlarge the diagram
Method 2
The second diagram shows the other way you can switch between two tone stacks, by adding components in parallel with the tone stack on the board. For this to work, you must be using 56k/250pf as the tone stack on the board, and use 82k and 250pf for the parts mounted on the switch. By putting the two resistors in parallel (56k + 82k), you reduce the value of the resistance to 33k, and by putting the two capacitors in parallel (250pf + 250pf) you achieve the capacitance of 500pf. Because in this method there is always a resistor and capacitor connected to the rest of the circuit, you eliminate any switching noise.
click the thumbnail to enlarge the diagram
Copyright © Richard Baines 2006
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