just bought a fender champion 600. cool mod for anyone interested.

[ode2no1]

http://support.fender.com/schematics/guitar_amplifiers/Champion_600_schematic.pdf

being a tinkerer i started looking up mods for the amp before i even got it out of the box haha...but i plugged into it and thought it sounded cool as is, for what it was anyway. i read a lot of people lifted one leg of r19 to pretty much remove the tone stack from the circuit and tried it, and while the increase in gain was cool i thought it sounded a little dull and got pretty farty it lost any sort of spank on the cleaner spectrum with r19 gone...and to be honest i thought it could have used a tad more high end to begin with, so losing even more highs wasn't doing it for me. aaaaanyway....i discovered a cool alternative to the tone stack removal mod, which was adding a larger cap in parallel to c1. i like .022uF with p90 guitars and .047 with the strat and tele. it basically gives you the gain increase you get from removing the tone stack, but leaves the cleans sounding a bit more lively. i'm going to install a switch to go between stock and these two new cap values. just figured i'd share for anyone who owns the amp and wants a few more flavors....also in case someone sees that this is somehow a terrible idea and that i shouldn't go thru with it haha, but i figure it won't do the amp any harm.

[Guitarboy2828]

Cool, thanks for sharing. I've got one of these lil amps... Been wanting to try do some mods to it. Any way you could post some sound clips?

I'm thinking of adding a tone pot to the panel to try give it a bit more tonal control. Maybe do both your mod and a tone control?

[ode2no1]

i don't have a real way to host clips, but i can record a little something and email it to you showing the difference between the mid boost and stock. i think i'm also going to add a bright cap switch. i like the tone of the amp but with my jazzmaster and sg (both with p-90's) i'm not getting enough sparkle in the neck position.

[Mark Hammer]

The "tonestack" in the 600 is a pretty standard Fender tonestack, but with fixed values subbing for the pots.  Assuming you have the machining capabilities, room on the control panel for installation, and the electrical chops to avoid electrocution, it would not be unreasonable to sub a 250k pot in place of R20/R21.

Although "lifting" R19 is possible, so as to defeat the tonestack, and get a bit of a gain boost to drive V2-B a little harder, treating it in all-or-nothing form is likely to be unsatisfying, as you yourself have found.  Fender amps that have a variable Middle control will have a pot in place of R19.  It would not be at all unreasonable to replace R19 with a 25k pot, to vary how much lower mids you bleed off.  Alternatively, replace R19 with 39k and use a 3-way toggle to patch in parallel resistors to get either stock, more mids, or less mids.  Again, given the size of the control panel, that may be more convenient. 

Given the size of the cab and speaker, attempting to install any sort of Bass control is probably wasted effort.  One might be wiser to simply leave it where it is.

Finally, the 600, like so many amps, uses post-OT negative feedback (via R7) to keep the sound less "ragged".  Normally, this would be part of the "Presence" control.  Consider installing another 3-way toggle to get stock and two varieties of lesser negative feedback by replacing R7 with a larger value (say, 33k-47k) and using the toggle to place two different resistances in parallel to get you 2k2 and some value between 2k2 and the max (e.g., 10k).

[midwayfair]

Weber's 6" signature is a really good upgrade. The speaker in the stock unit is very poor and breaks up far sooner than the amplifier does.

Finally, the 600, like so many amps, uses post-OT negative feedback (via R7) to keep the sound less "ragged".  Normally, this would be part of the "Presence" control.  Consider installing another 3-way toggle to get stock and two varieties of lesser negative feedback by replacing R7 with a larger value (say, 33k-47k) and using the toggle to place two different resistances in parallel to get you 2k2 and some value between 2k2 and the max (e.g., 10k).

Check out the NFB switch in the upper right of my Woodchipper:

https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Woodchipper/Woodchipper.png

Pretty simple to put on a 3-way switch!

Modding my champion 600 was practice for building that amp.

[Mark Hammer]

Q:  Did you like what it did?

I played with the NFB resistor in my tweed Princeton, and even disabled it at one point.  Instant ice-pick through the forehead tone.

[PRR]

What Mr Hammer said. The 600 HAS a full tone-stack, but with fixed resistors where the pots would be.

It "is" an AA-Champ but no user-set tone-knobs.

Lifting the tone-stack makes the amp near-"flat", which as said does sound "a little dull". That's why ALL the Blackfaces (even the AA-Champ) sport this tone-stack (and 2 or 3 knobs on it). Naked string through pickup is mid-range and nasal. The Fender tonestack notches the mids, so the bass and treble come out strong, "Wow!".

Banging pot-holes in the face is a very genuine way to get "better"; but a lot of work for a low-buck crowded PCB amp.

Almost always, the speaker in mass-produced amps is the tweaker's 4th or 5th choice; choices 1 to 3 shot-down by cost. OR-- why did they use an $8 speaker? They ran out of $7 speakers. Usually the speakers sold under the speaker-makers names are better than what the amp-factory demands. Since you are bumping C2 (more treble), look for speaker-claims which suggest a strong top-end.

[Mark Hammer]

If the user is only employing one of the two input jacks, the other hole can be redeployed to use for an additional control.  Nice thing is, that's a reversible mod for retaining whatever cosmetic appeal a one-knob faceplate holds for any subsequent purchaser.

[PRR]

> a reversible mod

Yeah, well, it's a 2-hundred buck amp. Probably $100 on GregsList. It will not be "collectable", or valuable, for 30 years. And they are selling enuff of them, they won't get valuable unless we cut-up some of them soon, leaving fewer "virgins" to collect.

I say Power Drill.

Also, on the Fender tonestack, unless you have specific needs, the only "one knob" mod that makes any sense is a 50K variable under R19, to get part-ways beteen "full tone bending" and "nearly flat". What it really wants is Bass and Treb, two knobs. (The mid knob is less useful. It was added on the big amps because "more knobs" is more "value", left-off the AA-Champ to save a buck and detune the price.)

[ode2no1]

i used to have a 78 fender bronco, which is a vibro champ that was sold with the bronco guitar as a package deal. i removed the negative feedback on that and really didn't like it. seriously bright and fizzy, so i think i'll leave this one alone as far as that goes. i did install a 3 way switch today as i had planned...two levels of midrange boost. i know i changed the treble cap, but it seems that upping the treble cap way out of the range it was designed for shifts the treble range down into mids/upper mids and kind of takes over the tone stack, giving it more of the rounder, gainier no tone stack sound, yet it retains a more EQ'd spanky tone. i read about someone putting a .022 cap over the low input 68k resistor in order to keep things brighter with humbucker guitars. i might give that a shot with some alligator clips tomorrow. adding a treble pot is also in consideration. there's actually quite a bit of room in there...could probably fit 3 or 4 knobs/switches.

as far as using one of the input holes for something else...they're both on a pcb, as well as the volume knob, so that wouldn't be possible.

anyway, fun little amp to mess around with. stock, it is quiet enough to play in my apartment and get some nice tones without bugging anyone. modded, this thing is way too loud to play at home and plugged into my 2x12 vintage 30 cab is seriously loud enough to keep up with a drummer.

[Mark Hammer]

If you need to play at lower volumes, AND you were looking for a little more treble bite, then that would point to a "brite" cap on the volume pot.  Keep in mind that the bright function kicks in most at lowest volumes, and plays less and less of a role as volume gets hiked up.

As for putting any additional caps in parallel with the 250pf treble cap, you can always stick a fixed resistor in series with any additional cap, and tie the resistor to the junction of R21/R20.  So, for instance, if you wanted just a bit more upper mids in addition to the top-end sheen, you could run a 1200pf cap into a 330k resistor going to the R21/R20 junction.  The 1200pf value lets  more upper mid content pass that the 250pf filters out, but since the 330k resistor is a larger value than the 180k, that additional content comes in a bit more subdued than the stuff coming via the 250pf cap.

> a reversible mod

Yeah, well, it's a 2-hundred buck amp. Probably $100 on GregsList. It will not be "collectable", or valuable, for 30 years. And they are selling enuff of them, they won't get valuable unless we cut-up some of them soon, leaving fewer "virgins" to collect.

I say Power Drill.

A hundred bucks is a hundred bucks.  I wasn't thinking so much in terms of collectible value as much as any owners ability to move the thing easily when they need that hundred bucks.  Some folks are turned of by mods, some are turned on.

[Eb7+9]

the classic tone stack doesn't belong in a classic CHAMP-like circuit IMO (fixed or variable) as it attenuates the overall signal levels too much ... I would consider bypassing all that and using the so-called Craig Anderton (inductor based) tone circuit instead // now, you'll get TWEED-like Champ levels, and the opportunity to create a 12-pos discretely variable mid-notch that gives the amp more usable tonal variations than what a classic tone stack can afford ... use a 250k pot instead of 50k

[Mark Hammer]

Hi JC, 

You're probably right about the significant signal loss from the tonestack in that particular topology.  If there was more than one gain stage before the driver stage, that would be one thing, but this thing has a single 12AX7 doing the input and driver stage, making every millivolt of signal precious.

The early Champs and Princetons also had a single 12AX7 .  The tone control there conserves more of the signal than the stack in the Champion, making it easier to send into overdrive, although the Champion fixed stack may provide for a more custom tone.  On the other hand, I suppose if one wants overdrive with a Champion 600, you just need to push the input stage a little harder.

[PRR]

> the classic tone stack doesn't belong in a classic CHAMP-like circuit IMO

True.

I guess the little-amp market was getting flooded by me-too starter-amps, and Fender added the big-amp tone circuit to set the Champ apart from other amps.

There is GREAT charm in the 1-knob and 2-knob Champs.

But there is also a place for a small amp with a Big Amp tone.

A 12AX7+6V6, straight, is "too much" gain for guitar. The first AX7 Champ has un-bypassed cathode resistors in first 2 stages. The 5F2 that Mark shows uses NFB around the output to damp speaker bass resonance and also reduce ripple-buzz (a problem on SE power stages). The final AA-Champ with the lossy tone stack (also the present 600 with fixed-stack) has to do tricks to get significant NFB around the output.

[Eb7+9]

Hi JC,  

You're probably right about the significant signal loss from the tonestack in that particular topology.  If there was more than one gain stage before the driver stage, that would be one thing, but this thing has a single 12AX7 doing the input and driver stage, making every millivolt of signal precious

Hi Mark, long time ...

yes, the way we look at it is this: "how hard do we want to hit that output tube ?" ... we know that the seat of interesting tone and "mojo" (ie., non-linear dynamics) in a tube amp comes from the output stage (ie., preamp circuits on their own, never go that full tone/mojo distance) ... the importance of rightly stimulating the output stage is especially true in a single-ended amp like the Champ

many fans of obscure tube amps know that a little magic can be added to that equation when there is no NFB coming back from the output tranny ... American-made Valco's and National's are especially sought after by small-amp players for their added "mojo"  which I think is simply due to this variation

nothing wrong with tone stacks if we have enough signal left over to drive the crud out of that output tube, if that's what we want ... think Garnet "American Woman" Herzog, a modded Champ without NFB and tone stack ... but if leave the tone stack in there and add a tube stage, now what we're getting into doesn't sound like a Champ anymore, and the Signal/Noise isn't as good // makig the amp a little hissy ... same goes with adding a booster pedal, of course we've all done it

my goal here is to experience an enhanced straight-in-the-amp response ...
a gainier than stock thing
otherwise, yeah // stock works too of course

the approach I take (described in my "unleashedChamp PDF) shows how to get the gain and fidelity of Tweed amps in the BF/SF models ... and go a little beyond, like I did with my '57 Princeton // again, no NFB like the Valco's and Toine pot lifted ... just because I like the rootsy sound of minimalist circuitry in general

I just wanted to mention that added to all that subtraction, I've found that the Inductor-selectableCap-pot combo (from Anderton's article and beyond ...) works really well in this context ... and the reason for that, in my mind anyways, is that setting a triple-control tone stack often results in forming a nice sounding mid-scoop response // so, if that's the goal (except for mid-heavy players) then why use a sucky tone-stack to achieve that profile when we can do the same with a less lossy circuit ?! ... and ALSO make the mid-scoop frequency variable in frequency and depth // so that at zero the response is completely flat, like a Tweed Champ ...

versatile in a tonally useful way, and retaining/exaggerating the original dynamic vibe... it's win-win IMO

and lastly, a few years ago I figured out how to take the hum out completely in the originals (that's in the PDF also) ... there's a trick we all missed

anyhoo, ... all this makes for a perfect closet/recording tube amplifier ...
and SF Champs are still under $300us

[Eb7+9]

Hi JC,  

You're probably right about the significant signal loss from the tonestack in that particular topology.  If there was more than one gain stage before the driver stage, that would be one thing, but this thing has a single 12AX7 doing the input and driver stage, making every millivolt of signal precious

Hi Mark, long time ...

yes, the way we look at it is this: "how hard do we want to hit that output tube ?" ... we know that the seat of interesting tone and "mojo" (ie., non-linear dynamics) in a tube amp comes from the output stage (ie., preamp circuits on their own, never go that full tone/mojo distance) ... the importance of rightly stimulating the output stage is especially true in a single-ended amp like the Champ

I'll provide a little background for the idea I'm proposing and have used on several 600's ...

many fans of obscure tube amps know that a little magic can be added to that equation when there is no NFB coming back from the output tranny ... American-made Valco's and National's are especially sought after by small-amp players for their added "mojo"  which I think is simply due to this variation

nothing wrong with tone stacks if we have enough signal left over to drive the crud out of that output tube, if that's what we want ... think Garnet "American Woman" Herzog, a modded Champ without NFB and tone stack ... but if leave the tone stack in there and add a tube stage, now what we're getting into doesn't sound like a Champ anymore, and the Signal/Noise isn't as good // makig the amp a little hissy ... same goes with adding a booster pedal, of course we've all done it

my goal here is to experience an enhanced straight-in-the-amp response ...
a gainier than stock thing
otherwise, yeah // stock works too of course

the approach I take (described in my "unleashedChamp PDF) shows how to get the gain and fidelity of Tweed amps in the BF/SF models ... and go a little beyond, like I did with my '57 Princeton // again, no NFB like the Valco's and Toine pot lifted ... just because I like the rootsy sound of minimalist circuitry in general

I just wanted to mention that added to all that subtraction, I've found that the Inductor-selectableCap-pot combo (from Anderton's article and beyond ...) works really well in this context ... and the reason for that, in my mind anyways, is that setting a triple-control tone stack often results in forming a nice sounding mid-scoop response // so, if that's the goal (except for mid-heavy players) then why use a sucky tone-stack to achieve that profile when we can do the same with a less lossy circuit ?! ... and ALSO make the mid-scoop frequency variable in frequency and depth // so that at zero the response is completely flat, like a Tweed Champ ...

versatile in a tonally useful way, and retaining/exaggerating the original dynamic vibe... it's win-win IMO

and lastly, a few years ago I figured out how to take the hum out completely in the originals (that's in the PDF also) ... there's a trick we all missed

anyhoo, ... all this makes for a perfect closet/recording tube amplifier ...
and SF Champs are still under $300us