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

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

  1. AxemanVR

    AxemanVR Friend of Leo's 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
  2. AxemanVR

    AxemanVR Friend of Leo's Ad Free Member

    Dec 27, 2011
    Minnesota USA

    So I've been swapping and tweaking on this project again and managed to squeeze some more goodness out of this amp. Two main areas that I focused on involved a "new alnico Speaker" and an "improved Gain structure". Here's what happened:

    You see, since my last entry something significant occurred; I got a fabulous new Fender '57 Custom Deluxe tweed 5E3 amp and it pretty much blew everything away as far as tone goes - including the poor little ChampTone. Suddenly the ChampTone sounded a bit "blaa" compared to the Deluxe, so once again I was determined to find out if anything else could be done to get the little bugger back in my favor.


    The thing is that the ChampTone seemed a bit sterile and tinny now compared to the Deluxe. I decided to go back and swap a few speakers to see if that would make a difference and finally tried a Jensen P12R Alnico speaker I had lying around which gave it a noticeable boost in sweetness and juiciness. * I actually had this speaker in the Deluxe as well but did not like the results. Here's a brief review I did on that at the time:

    1) The Jensen P12R I tried was not a good fit for the Fender 57 Custom Deluxe I own.

    I think part of it has to do with the lower output, which seems to mess with the amp’s sweetspot in a negative way. To be frank, it sounded weak in comparison and basically lost the “umph” that the Eminence has. This also made it sound less lively and a bit more sterile.

    2) Compared to the darker tone of the original Eminence (which caused me to crank the Tone control to 10 or more in most cases) the Jensen’s brightness helped to bring the Tone control down to 8 or less, providing a lot more leeway for adjustment.

    Unfortunately, with the P12R, there was very little need for any treble increase since the brightness was far more than what I’d ever need. There also seemed to be a fairly stiff Midrange that felt somewhat uncomfortable on the most part and was difficult to minimize without causing things to sound muffled.

    The Bass lost its punch as well, again (I suspect) due to the lower overall output of the speaker. So, all in all, the amp ended up sounding a bit neutered with the Jensen.

    3) The final straw was the fact that the edge of the highs also sounded “fizzy” (as others have observed in various reviews).

    The more I cranked the gain the more fizzier it got - especially with my high output humbucker guitar (1991 Gibson Les Paul Studio). The single coils also sounded brittle when cranked (as opposed to the much smoother edge coming out of the Eminence).

    I would never say the Jensen P12R is a bad speaker though, since, in my experience with speakers, what sounds bad in one setup can sound great in something else.

    But, that said, I really can’t say I’d recommend it in a Fender 57 Custom Deluxe.


    So I obviously found a better fit for it here...

    Something else worth noting is that the Jensen P12R is an 8 ohm speaker and the ChampTone's speaker output is 4 ohms, so this would be a "speaker mismatch", although it's usually not considered unsafe going to a higher impedance speaker - with the main side effect usually being that the tubes may run a bit cooler (although I'm not entire sure if that's as much of a factor in a single-ended amp). A speaker mismatch can also affect the tone, so I'm assuming it's doing so in a good way in this particular situation.

    Anyway, due to the bright nature of the Jensen P12R (along with its midrange bump and lack of bottom-end) I turned back to the ChampTone's AA764 circuit to see if there was something else I could do, and discovered something rather interesting...

    The next thing I decided to try was to adjust that R18 trimpot I installed earlier in this thread to see if adjusting it would balance the tone out better, so I started by backed it all the way down (counter-clock-wise) and noticed an immediate boost in gain I hadn't noticed before (since I tweaked it in smaller increments back then). This inspired me to do a little searching and found one reference which confirmed that bypassing this resistor would indeed increase the gain significantly. I was kind of excited at this point, especially since I'd been struggling to figure out how to increase the gain without adversely affecting everything else, so I tweaked the R18 trimpot until I reached a nice zone of "fat bottom-end" and "smooth top-end". The increase in harmonics was obvious - probably due to saturating the tubes better - and suddenly this amp felt livelier and more three-dimensional.

    Not surprisingly this also reduced some headroom, but in a very good way in my opinion - since I like my cleans to have a little attitude anyways. The Jensen P12R speaker still has that fizzy high-end edge thing when pushed (as mentioned in that review) - something I'm hoping will eventually smooth out as it breaks in, otherwise the thick-but-sparkly tone that I dialed in, along with the Jensen's inherent mid-range bump seemingly working in my favor this time, definitely helps to put the ChampTone back in my lineup.

    I'm going to play it this way for a while and will be sure to report back if anything changes...

    Last edited: Jul 12, 2021
  3. AxemanVR

    AxemanVR Friend of Leo's Ad Free Member

    Dec 27, 2011
    Minnesota USA
    I just wanted to briefly mention that I have been pushing the Jensen P12R fairly hard since my last review and am happy to report that the "fizzy" top-end has diminished considerably since then, smoothing out nicely as it continues to break in.

    So, for me, this is no longer an issue.


    I also did some more tweaking to the tone circuit. If you go back to an earlier post where I couldn't get much noticeable change from the Bass pot I added, well, since then I came across an article showing this capacitor attached to it:

    C600 Bass Control.png

    Here's a link to that article, which is packed full of info (to say the least):


    The main problem I found with this mod was finding the 500pF 100v capacitor - I had to special order one from Germany for around $15 (which is ridiculously high for such a simple capacitor) so it must be quite uncommon.

    Anyway, did it work? Well, actually it did!... albeit *slightly*...

    You see, from "zero" to about "3" I can now hear a small but noticeable increase in what I'd call a "low end bump". It is fairly subtle, yet still useful in its small way, and allows for a very narrow bit of tapering off of the bass (or emphasizing the treble) when set below "3" on the dial.

    But even more significantly, adding this capacitor seems to have shifted the tone circuit in a slight but profound way as well; i.e. in a "better" way. While it's difficult to put into exact words, the amp seems to have a smidge of both clarity and richness added to the upper mids that wasn't there before.

    So, while I'm still disappointed that I'm not getting the range of adjustability I was hoping for in the Bass control, I'm actually a little happy I'm getting at least something more than what was there before AND the modification seems to be an overall improvement none the less.

    Now I'm wondering if going back to a 250K pot would provide a wider range of adjustment. I may also experiment with other capacitor values and see what happens. At the very least I'm sure to continue searching for ways to tweak it some more and will, of course, post any results that are beneficial as I come across them...

    Last edited: Sep 8, 2021
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