The Watering Hole

The topic if this section - the Fender Tone Stack Implementations...

There are three basic "tone stack" implementations in Fender's most popular amp lines.   Granted, there are a few other implementations (especially in the less well-known Bassman's) that Fender used, but there are three basic ones:

1) Single tone control - a.k.a. "high cut"
2) Dual tone control (Bass / Treble)
3) Triple Tone Control (Bass / Mid / Treble)

I will explain each tone control section in a separate post, within this topic.

So, the first one - "high cut".  This is a common implementation of a tone stack found in many amps, not just the Fenders.   It amounts to a "high cut" network systematically removing more, and more, high frequencies from the resulting signal (your guitar).  

There are some things to note, here, that you may not have thought about, in times past.  If you look at the "Frequency Response" chart, to the right of this little gem, you'll notice that the output is relatively flat from the low frequencies (left) to the midrange frequencies (right).  Above the midrange the frequency response starts to "roll-off".   This is what is known as a low pass filter, as it allows low frequencies through and high frequencies are "shunted".  

Notice the tone control.  This control is a variable resistor, or potentiometer (a.k.a. "pot" - though I hesitate to use that latter term ;) ).   There are 3 "legs" on a pot.   The top, the "wiper" (the one with the arrow) and the bottom terminal.   The "wiper" will "travel" as you rotate the control from the top to the bottom terminal.   When the wiper is moved all the way toward the top there is 0 resistance between the wiper and the top terminal and maximum resistance between the wiper and the bottom terminal.  When the wiper is moved the other way the reverse occurs.   If the wiper is in the center of the control there is, essentially equal resistance between the wiper and the top terminal and the wiper and the bottom terminal.  

A couple of other things to point out, here.  The dual, horizontal "bars" under the "TONE" control, and the vertical, parallel, bars between the "PREAMP" and "VOLUME" controls are what are known as capacitors.   Capacitors have interesting charactaristics.  

1) They only pass AC (signals).  They block, and do not pass, DC signals.  

2) As the frequency gets lower, and lower, the capacitor looks more and more like an open circuit (blocking low frequencies).  

3) As the frequencies get higher and higher, the capacitor looks more and more like an open circuit (looks more like a "short" for high frequencies).    

Some "tomes" would have you believe that all capacitors are "shorted" at high frequencies and that is not the case.  All capacitors show some minimal amount of "resistance" (correct term would be reactance, or impedance) at high frequencies.   They do not short out, although in some cases, it's almost proper to think of it that way.  Technically, though, it's not correct.  Just be aware.

You can think of a capacitor as a "frequency dependant resistor" having less resistance at high frequencies than what it does at low frequencies.   That's the easiest way to put it, I think.

Back to the "hi-cut" circuit.  As the tone control wiper is moved toward the bottom terminal of the control, there will be less and less resistance provided by the pot, to the signal, effectively "shorting" the signal from the wiper to the bottom terminal of the pot.   This means, then, that most of the signal is now across the capacitor.   It will "block" low frequencies and "short" high frequencies.    

When the wiper is at/near the top terminal of the "TONE" control you will get a frequency response similar to that on the far right - almost all of the high frequencies will be allowed through the amp.   When the wiper is at/near hte bottom terminal of the "TONE" control the frequency response will be similar to that of the far left curve, with much of the high frequency content being "shunted" or "removed" and only the lows and lower midrange being allowed through the amp.  

One final note, here, and that's the overall response of this tone stack and the amp.  As you recall from the previous "Fender" section, ALL preamp stages in the Fender amps are designed for flat frequency response.   If you look at the resultant output of the tone stack you'll note that it's "flat" from the bass side all the way through to where the hi-cut control (TONE) removes some of the high frequencies.

You'll see, when we get to the next two tone controls, that most amps DO NOT exhibit this "flat" response through the midrange.   This is, sometimes, also referred to as "smooth through the midrange", as well.   You can obtain a similar response, with the standard tone controls (Bass / Middle / Treble) by setting the "Mid" to 10, the Bass and Treble to "0".    

That should about cover the "hi-cut" version of the tone stack.  Gibson, Marshall and others have all used this circuit, or variations thereof, to produce the hi-cut.  The larger the capacitor the "lower" into the midrange the hi-cut will dip.    The larger the tone control pot the more "control" you'll have over the cut - to a certain extent.   A smaller value capacitor will "extend" the high frequency response even when the tone control is set for full cut.    

Enjoy...

Dar

Dual and Triple Tone Controls

I'm going to include BOTH in this topic, as the dual control tone stack, and the triple control tone stack are, almost, essentially the same design.    Fender just replaced a fixed value resistor with a pot in the triple control stack.  

So, the basic design of this tone stack consists of 3 capacitors (one of which is almost 250pF), 2 or three potentiometers (Bass / Treble / Middle) depending on design, a "slope" resistor (almost always 100K) and, well, that's it.  What more does one need.

So, what gives about this tone stack and how does it work... I'm going to try to be very "non-technical" about this stack and see where that gets me.    :o

The output of the first stage preamp enters the tone stack in the upper-left corner and is sent to both the capacitor at the top and the "SLOPE" resistor.   The signal "splits" at this junction.  The top capacitor and ALL of the resistors (variable and/or fixed) on the right side of this tone stack, forms what is called a high pass filter.   The capacitor effectively "blocks", or significantly reduces, low frequencies will easily passing high frequencies.  On most Fender amps this capacitor is typically 270pF and because the thing is so darned ol' small only the "glassy", high frequency harmonics of your guitar strings are going to get through the thing easily.

In the early days of country, which occured before rock 'n' roll, PA's were relatively non-existant.  Certainly the 64 x 16 mixer hadn't been invented yet and, as a result, typically, only vocals were amplified by the PA and the instrumentalists were left on their own to "crank up and be heard.  The most common complaint from guitar players, in that era, was the fact that no matter how loud they cranked their amps they couldn't be heard.  

The obvious answer to the entire perplexing problem was to make the high frequencies louder than the low frequencies.  As could be seen in the previous less on the single tone control, that tone control only had the effect of making the high frequencies softer, leaving the low frequencies alone.   That didn't quite fit the bill.   So, enter the multi-control tone stack, which is made up of two low-pass filters and one high-pass filter (which we just looked at).

One of the things we learn in mixing/recording technology is the fact that if you cut the bass frequencies the treble will automatically seem louder EVEN IF you didn't turn up the volume after you cut the bass.    The same goes for cutting the treble...   so, the Fender tone stack introduced some psycho-acoustic magic called the "slope resistor".   This resistor, typically 100K in value, worked in conjunction with the 250pF capacitor and the Bass/Mid controls, to effectively "split" the high/low frequency ranges of the signal entering the tone stack.  The larger this resistor value the less effect the Bass / Mid controls have on the resultant output, causing a marked increase in Treble.    The smaller this resistor value the greater the effect that the Bass / Mid controls have on the resultant output, causing less of an increase in Treble and a more "thumpy" output.

Because Fender amps were designed specifically to run clean at pretty much any volume (most of them were), this differentiation, or split, between the upper/lower frequencies was a requirement.   In an amp that distorts, the distortion generates a higher ratio of high frequencies (remember ALL harmonics are a minimum of 1 octave higher than the fundamental that caused the harmonic to be generated) by default.  

Now, as you're aware, the more bass you have in the output signal the more "farty" your power amp gets.  It takes roughly 10x the power to amplify low frequencies than what it does to amplify the high frequencies, so you want less bass in the output, if you can help it.  Plus, too much bass "masks" the guitar with everything else and it doesn't allow the guitar to stand out.

So, Fender put the "slope resistor" into the tone stack and the separation was born.  In the early days many Fender amps had two tone controls the Treble and Bass.   The "mid" was fixed at 6.8K which corresponds to having your "Middle" control, on most Fender amps, set to about 3 and just using the Bass / Treble controls to deal with the frequency shaping.  

As I mentioned above, and you've probably already forgotten, Fender used a high pass filter (Treble Control) with two low pass filters (Mid / Bass controls).   So, the "slope up" toward flat response for the treble occurs at some frequency and the slope down (cutoff) for the low pass filters (Mid / Bass combo) move down at some other frequency.  This caused a "notch" in the response and "separated" the Bass/Mid frequencies from the "Treble" frequencies.  Interestingly enough the Fender "notch" occurs right about where their speaker cabinets would resonate the most and sound "boxy" or somewhat "boomy".  

For those that are into recording/mixing, you know this as the "mudrange" of frequencies, that range around 230-400Hz, or so, that when boosted turns your track(s) to sludge and mud.    Well, Fender cut it right out and separated it, right from the get-go.

It's also interesting that when you listen to many of the older Fenders with just a single tone control (hi-cut), that you'll hear a more "boxy" representation of the Fender sound!  That "boxy" sound comes directly from that "smooth through the midrange" lack of 300Hz, or so, cut created by the addition of bass / mid controls in the amp.  

A few other quick things to think about... the mid-control frequency will shift and the mid control is "swept" moving the notch lower as the mid is turned down and higher as the mid is turned up.   As the mid is turned up it's adding more and more high frequencies into the tone stack and this is how it "complements" or "interacts" (as LINE6 puts it) with the Treble control.

A final note about this particular tone stack... the "Treble" control acts as a "mixer" control.  When the wiper is all the way to the top terminal it's pretty much only getting signal from the 270pF capacitor on the top (all Trebly like).   When the wiper is at the bottom terminal it's pretty much getting all the signal from the Bass / Mid section.   In that position, the Bass / Mid control act more like a Bass / Treble control (try it sometime, you might like it).   When it's in the middle, it picks up signal from both sides about equally.

The "output" of the tone stack comes off the Treble control wiper and that feeds into the next stage - whatever that is.  On the 5F6 series of Bassmans it feeds into the last stage of the preamp prior to the splitter. On pretty much every other Fender it feeds stage 2.  

Enjoy...

Dar





 

A few other quick things to think about... the mid-control frequency will shift and the mid control is "swept" moving the notch lower as the mid is turned down and higher as the mid is turned up.   As the mid is turned up it's adding more and more high frequencies into the tone stack and this is how it "complements" or "interacts" (as LINE6 puts it) with the Treble control.  

note:  this is where downloading and playing with the Tone Stack Calculator I recommended in the other thread, can be really handy.  Gives you a graphic view of what actually happens to freq. response as you turn the knobs, and has standard Fender, Marshall and Vox stacks built in.  Also, gives you a view of just how much level is lost when you throw a tone stack into a circuit...  

Check out that Tone Stack Calculator while reading these things and it will make even more sense when you SEE how they work.  IMO.  

Outstanding!

Thanks again!

Randy  :)

Very imformative, Dar.  Thanks.

Howie, a link here for the Tone Stack Calculator would be nice.  There are a lot of "other threads".  Or a point to the thread in question.

I found it.  It was an older thread that was before I made it back here.

http://www.chargermusic.com/wh/cgi-bin/yabb2/YaBB.pl?num=1217003053

Sorry Deano!  Didn't realize you hadn't seen the tone stack calc thread.  It's good stuff.  BUT, the amp voicing is MORE important, as Dar has said, in determining overall tone, the calc just shows what that amps tone controls do from "there."