Give your tonestack a second job

[Mark Hammer]

I was reading one of Gerald Weber's columns in the current (Los Lobos cover) issue of Vintage Guitar on the busride to work this morning, and realized something that had been staring me in the face for a few years...apparently a little too closely to be able to see.

In 1981 or so, Steve "T-Boy" Morrison (the brains and heart and coding chops behind Ampage) had a nice little article in POLPHONY magazine about modding Fender amps, in an issue that had a number of tube amp mods.  One mod of his I tried was carried out on my old blackface Tremolux, to great success, which is largely the reason I recommend it here.

Some background...

The standard Fender/Marshall tonestack is a passive affair that essentially "leaks" signal to ground in a selective manner.  The end result is that the *appearance* of treble/middle/bass cut and boost is created although what really happens is that enough mids are leaked to create the sense of bass and treble being hiked up.  (Note that in those older Fender amps with just a treble and bass control, they often have the identical tonestack circuit but stick a fixed resistor - usually 6k8 - where the midrange pot would normally be on later models)

This approach results in passive signal loss, which subsequent gain stages are intended to recover from.  The signal loss, however, depends entirely on one common path the ground, through the midrange control.  As that resistance value increases, there is less signal loss, and more midrange "boost".  Since the tonal characteristics produced by the tonestack depend  on the "selective signal leakage", after you reach a certain point of increasing the resistance of that mid-determing component (fixed or pot), all of the mid-scooping the tonestack is intended to produce disappears, but just as important, the signal level is now noticeably hotter in addition to being beefier.  Make it an infinite resistance (i.e.,  open circuit) and the tonestack ceases to have any noticeable effect (a small effect remains but you need to minimize or maximize the controls to hear any impact, and even then it is minimal).

The mod that Steve described in the article involved simply using one of the many existing jacks on your average Fender amp to provide a footswitchable link between the midrange resistance and ground.  Make the link and you get your tonestack back.  Break the link and you get a meaty boost of a a couple of db, suitable for soloing.  As noted, I installed this on my Tremolux and was impressed with the degree of versatility it added for so minor a change.

The connection with stompboxes.....

A great many projects and commercial pedals intended for producing overdrive or more obvious distortion include Fender/Marshall style tonestacks.  Alternatively, many people will insert them post-build when the gain of the device (e.g., Joe Davisson's "Blackfire" design) makes the passive loss to the tonestack something you can easily live with.  The principle remains the same, and very often the component values are identical too, so implementing Steve's tone-bypass is easy to do with a single SPST toggle switch or stompswitch.  I suppose if you can tolerate the current drain, using a SPDT or DPDT switch to provide LED status indication might be appropriate too.

The usefulness of such a mod is of course going to depend on the tonal characteristics of the guitar itself and how you like to set your amp controls.  In some instances, I can imagine that the tonestack is really needed to make the guitar sound "normal", given the amp settings or speaker characteristics, and removing the tonestack makes the tone undesirable, even as it yields more drive for subsequent stages.  For instance, on smaller amps/speakers, the illusion of bigger bass may depend a great deal on taming the mids with a tonestack-produced scoop, and removing that scoop just makes everything sound like its played in a cardboard box.

As well, the usefulness of the mod will depend on where it is placed/located in the signal path.  In amps you can  be fairly assured that it occurs at a point where lifting the ground connection will result in driving another section of the amp harder and producing more delicious grind.  In a pedal, that can vary.  If the tonestack is followed by one or more gain stages (and the ROG amp simulators provide examples of this), then the ground lift will produce more overdrive *within* the pedal.  If the tonestack is placed at the very end of the signal path in the pedal, such as just before the output level pot in a Blackfire, then no additional distortion is produced within the pedal, just a tone change and volume boost.  That boost may well make a subsequent pedal (e.g., chorus) behave badly for both tonal and level reasons.

Please note that this requires a certain type of tonestack to be useful in the manner described.  Baxandall-type tone controls will behave differently as will the simple treble-cut types.  There is some likelihood, however, that lifting the ground connection on mid-shift type "contour" controls (found on many solid-state low-to-mid power amps from Fender, Marshall, Crate, and on some of the Marshall pedals) will produce the desired effect.  In these latter instances, since the mid control is an entirely separate circuit from the treble/bass stack, you get a mid/level boost while still retaining the treble/bass function.

Bottom line: This is a simple, simple mod well worth experimenting with in certain contexts.  There WILL be some circumstances where it might take away more than it adds, or complicate things more than simplify them, but in general it is simple enough to just try out and see if you like it.

Alternate recipes:  Nothing wrong with sticking a second much larger value pot in parallel with the midrange pot.  In many instances, the standard midrange pot on a Fender is 10k.  Stick a 250k pot, wired as a variable resistor in parallel.  The joint resistance will be largely determined by the existing midrange pot for most of the secondary pot's range, but you should be able to adjust the degree of boost/tonal-change that occurs when you break the midrange pot's ground connection, using the secondary pot.  So, instead of going from 7k to open circuit, it may go from 7k to 180k; enough to produce a discernible change without totally transforming the tone.

[petemoore]

SO.. on the amp you modded, you just had a long ground wire, through the 'extra' jack, into a switch [on/ off] then back into the amp to provide ground / not provide ground to the 'midrange resistor'?
 In other words you're not having to run a long signal wire [just ground]out of the amp to do this, so that it lends itself to a low noise/low tech mod ... don't really need to shielded wire [or use internal switching] if merely running a long ground to alter tone the cct. function...[?]

[Mark Hammer]

If doing the remote-switched mod to an amp, you definitely need a shielded cable from the jack to the switch since extraneous noise will happily work its way back up the midrange and bass-control pots.  Indeed, standard reverb and tremolo defeat switches on traditional Fender amps worked exactly the same way - ground/unground - and required a shielded cable out to the switch.  

If you did this with a switch in a stompbox, shielding would be unnecessary in the majority of instances, assuming the sort of layout that reduces instability.  I can think of instances, though, where adding a switch as an afterthought might require the leads to the switch to pass by parts of the circuit which might induce oscillation.

[aron]

This is an old mod and a useful one. I have found it useful to parallel a capacitor with the treble cap (in the case of a ladder circuit) or to use a totally different bypass capacitor when switching out the tone circuit.

[christian]

To make Marshall/Fender-type tone-stack active one, you feed the output into inverting amplifier(low gain, probably adjustable from 1-10?), and use the output of this amplifier as the ground connection (negative feedback).
I tried this out and it gives a good deal of accent to the sound. Too much gain and it doesn´t sound pretty anymore

Also tried this with BigMuff tone-stack. Heavy Metal Thunder?