Fender AA764 Tone Stack Mod (Champion 600 in this case)

Discussion in 'Amp Central Station' started by AxemanVR, Aug 13, 2020.

  1. AxemanVR

    AxemanVR Tele-Afflicted Ad Free Member

    Dec 27, 2011
    Minnesota USA
    In a nutshell I removed the "R18" resistor in my 2009 Fender Champion 600 Reissue amp and replaced it with a 500k trim pot.

    WARNING: Be very careful whenever working on any electrical device, since they can still hold a lethal charge of electricity even when unplugged! If you're not sure about what you're doing then I suggest not doing it...
    NOTE: This mod should work on any Fender type tonestack with a similar Fender AA764 circuit design.

    Some of you may recall my on-going Champion 600 Reissue project that I dubbed "ChampTone" (here's the original thread in case you're interested):


    01 Before After.JPG

    *What I kind of failed to mention in that thread was how some guitars sounded better than others when played through that amp - which hopefully explains my sort of wishy washy, back and forth struggle with it...

    Anyway, after transplanting the Champion 600 guts into a much larger Fender Princeton 65 amp cabinet (in order to accommodate a 12" speaker) and adding a few components to reduce some unwanted noise, most of the subsequent modifications I made were focused on trying to dial out the overly bottom heavy tone inherent in my amp. In other words, I was basically hoping to make the top-end stand out a little more. I first replaced the "fixed" tonestack with Bass and Treble control pots, which certainly helped to dial in a better treble response, but unfortunately it didn't quite go far enough since cranking the treble wide open still fell a tad short.

    The next thing I tried was replacing the speaker with one that emphasized the higher frequencies better (a 4ohm Jensen Mod 12-70) and that made a huge improvement, but, I still ended up having to crank the treble up most of the time - depending on what pickup setting I was using.

    Okay, I'm not saying the tone sucked, and on the most part it nailed the vintage vibe I prefer, but I just felt like there was something in the circuitry that could still be adjusted to balance things even further.

    Now, I'd be the first to admit that I'm no guitar amp wizard, so I really needed some help before potentially butchering things up anymore. I know enough about electronics to figure things out, but was a little hazy on where to go next in this tonestack. In other words: I know how to mod, I just didn't know what to mod!

    That's when a member named "clydethecat" over at Strat-Talk came to my rescue after posting a link to a website where you can manipulate various amp tone circuits and show in real-time how the the tone changes on a visual grid:


    After punching numbers and seeing what they did, I came to the conclusion that "R1" had the most overall impact, on the lower frequencies in particular, than anything else.

    "RIN": Changing resistance here only raised or lowered the amplitude across the entire frequency range.

    "C1": This one was pretty interesting, but it mostly seemed to widen or narrow the midrange.

    "C2": This one mostly affected the lowest frequencies as far as I can tell.

    "C3": This one seemed to boost or cut the upper bass / lower mid frequencies.

    "RL" was similar to "RIN"

    "RT", "RB", and "RM" are the Treble, Bass and Mid pots and, since they are adjustable already, any changed settings can be mostly dialed back in one way or another (although their ranges are shown to have changed).

    "R1", on the other hand, appeared to boost or cut in a more specific 10-100hz range, which seemed to be the part that adversely affected my tone the most. The charts below show how the frequencies changed as I adjusted the values for "R1" in that program:

    02  adjust.JPG

    Here's where the "R1" (from the above charts) is located on a Champion 600 (which is actually labeled "R18"):

    03 C600 Tonestack.jpg

    I'd like to add one more thing...

    As mentioned earlier, I installed both Bass and Treble controls. While the Treble control makes a huge difference, the Bass control changes the tone very little from zero to ten. I tried both 250k and 500k pots with absolutely no noticeable effect. I have the pot wired correctly as far as I can tell (to be a variable resistor) but must be missing something. If anyone has any idea why this is I'd sure welcome their insights! Here is how it's currently wired:

    04 Pot Wiring.JPG


    So I decided that changing the R18 resistor would give me the most bang for my buck, but what value should I set it at? After pondering for a while I decided that replacing it with a "trimpot" would provide the most adjustability. Besides, doing trial and error resistor swapping would not only waste a ton of time, but, with very little space to work in, I was concerned about melting stuff I didn't want melted!

    Next thing to consider is what range to go with. After tinkering with the above program it seemed to make sense that the biggest benefit would be somewhere between 200k and 500k. The actual impedance came as somewhat of a surprise (to be explained in greater detail further below).

    Here's the trimpot I ended up getting:

    05 trimpot.jpg

    This is a Piher 500k Adjustable Linear Potentiometer, and it was a little bigger than I thought it was going to be. Luckily I managed to squeeze it in...


    So the first thing I had to do was remove the original resistor from the R18 slot, but instead of desoldering it I just clipped the wires (as close to the main body of the resistor as possible).

    06 Clip Here.jpg

    The reason I did this was because of what happened in that last thread I mentioned earlier. You see, the PC board traces are rather flimsy and basically fell apart when I tried to remove the resistors for my tone pot modifications. So this time I decided to use the old resistor wires as "soldering posts".

    And here's what I was left with:

    07 R18 clipped.jpg

    After figuring out which lugs to hook up I positioned the trimpot to fit best, then I bent one of the lugs to clear capacitor "C3" (and basically rest on it) and covered that one with some black electrical tape (along with the other lug I wasn't going to use)... just to make sure nothing would short out. Then I soldered it to the R18 "posts" and the deal was done!

    08  Trimpot Tab bent Out.jpg

    Now all I had to do was find out how it works...

    The first thing I can tell you that it did in fact shift the tone as you turn the trimpot adjustment; Clockwise = Brighter, Counterclockwise = Darker (Zero k = Dark / 500k = Bright). Easy-Peesy, right? Well, the shift is not entirely predictable. For example, while it does get brighter as the resistance goes up, it also shifts the bass in a way that can thin the sound out and the mids so they become a little too pronounced.

    So it seems that you can easily overdo things if you're not too careful. The mids in particular are what ultimately influenced where to set the trimpot for me. When I max'd the impedance to around the 500k end of the dial, not only did the Bass decrease, but a stark increase in Mids became apparent. The thing I noticed the most was how much more "quackier" my Strat sounded (not surprisingly this was most noticeable in positions 2 & 4). But the lack of bottom end had a tendency to make things sound thin and spindly.

    At this point I tried a different approach... I swapped out the brighter Jensen speaker and installed the much fatter Eminence Texas Heat that I had in it before - with some interesting results...

    The fatter speaker coupled with the middy voicing really let the vowel-like quack shine when the amp was pushed into overdrive, and, while I didn't really spend a lot of time messing with it, I can well imagine how this setup could work well for those who enjoy playing with a lot of dirt. Unfortunately, when cleaning things up, the mids were too overbearing for me, so I switched the Jensen back in and lowered the trimpot impedance... considerably!

    So, where did I end up? Well, as far as I can tell, I currently have it set somewhere around the 150K range, which is more like a subtle tweak than a major change.

    09 Trim Setting.JPG

    But it was enough to shift the tone a bit and now I'm afforded a little more wiggle room for adjustment on the Treble control:

    10 final.jpg

    How does it sound?

    It's definitely different. Not "night and day" different mind you, but certainly not the same. There seems to be a bit of “hair” on the tone’s edge (for lack of a better word), even when playing totally clean - and I mean that in the nicest way possible. I'm assuming a midrange bump thing has something to do with this.

    More importantly, the added brightness is obvious, but not overbearing. I'd say the main thing that defines this new sound is "clarity", since it sounded a little muffled before and now sounds more open.

    There was a tinge more spikiness though, but I wasn’t convinced that it was entirely due to just changing the R18 values, so I did some preamp tube swapping (ending up with a Mesa Boogie STR SPAX7A) and that did the trick!

    Any further treble related issues can be dialed out effectively with the guitar's volume and tone controls. I may try lowering the trimpot to something more like 130k or even 120k and see how it responds to that, but for now I’m quite pleased with the results.

    I'd say the biggest "overall" change is a noticeable shift in the amp's "sweet spot". While it's a little premature to say for sure how usable this extended range is, I seem to be able to get a likable tone over a somewhat wider volume range. You see, before this mod the tone quickly darkened as I lowered the volume, but now that I can make up for this treble rolloff - by increasing the treble control to compensate - and, needless to say, I'm quite stoked about that turn of events!

    Anyway, at least some of the questions about how changing the value on the R18 resistor affects things have been answered. Also, as mentioned at the beginning, this tonestack circuit was used on a lot of Fender amps, so I can only assume that this mod would yield similar results in other amps with the same arrangement.

    I hope someone finds this info useful...

    Last edited: Aug 13, 2020
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