Fender Champion 600 Fat Boost Mod Resistor Values

In the Fat Switch Mod the circuit sees 3 different resistances for the mid resistor in the tone stack: 15K (stock), 30K (Frondelli Mod fat boost value), and 47K (for a little extra boost). The actual resistors on the switch are quite different values. The 47K value is there but the other two are 68K and 22K. Why not the 15K and 30K that the circuit needs to "see" for the mod?



In order to use a simpler switch I approached the mod a bit differently. I decided to replace the standard mid resistor with a 47K one. This sets the max mid resistor value. The fat switch then selects one of two resistors and connects it in parallel with the 47K resistor, lowering the effective resistance. In the center position both of the additional resistors are disconnected so the total resistance remains 47K.

This should be clear from a schematic drawing:



Too find the effective resistance when one of those resistors is switched in use the formula for finding the total resistance of any number of resistors connected in parallel. Incidentally, this is the same formula you would use when connecting speakers in parallel:



Since we only have two resistors connected at any time, it's a bit simpler. All we need is R1 and R2. Here's how the formula for the 47K resistor in parallel with the switched in 68K resistor is solved in detail:



That 28K value is plenty close to the Frondelli Mod value of 30K. If you're wondering how close, take a look at the graph at the bottom of the Fat Switch Mod post. You'll see from comparing the curves for the three fat boost resistor values that that 2K difference doesn't matter much.

Here's the same equation for the 22K resistor in parallel with the 47K one:


Solve that equation and you'll see that the 22K in parallel with the 47K results in 15K - the same effective value as the original R19. So switching in the 22K resistor puts the tone stack back to stock.

Fender Champion 600 Tone Stack Bypass/ Fat Switch Mod

This pic shows the shows a few of the mods I've made to this Champion 600. The two small red switches on the bottom of the chassis are for my Presence Plus and Input Voicing modifications. This particular post concerns the tone stack mod associated with the two bigger switches to the upper right.



This mod takes the coupling cap mod from the Tone Stack Test and hardwires it to a switch so the amp can use the stock tonestack or bypass the tonestack with a single coupling cap to eliminate the mid cut in the stock circuit. The right had switch below handles that job. The left switch acts as a three position "fat" switch.

First....

THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's the finished mod from above:


The combination of resistors on the "fat" switch are selected to combine for three settings:
15K (stock), 30K (Frondelli Mod fat boost value), and 47K (for a little extra boost). The boosts effect the mids most dramatically, but they provide extra gain across the whole spectrum too.

All three resistors connect to the same pin on the right hand switch - the one just to the left of the center pin with the black wire.





The switches I used here each have one more set of contacts than are needed to make the mod. I frequently do this when I'm experimenting so that if I decide to add something to the switch later on I don't need to disassemble the circuit and solder in a new one.

The resistors in the fat boost circuit replace R19 on the pc board. Here's how the wiring runs:



Here's the coupling cap, prepared for installation:


And here it is soldered in place. Since only one end is really fixed and the other will be supporting a wire I put a bead of silicone underneath to make sure it stays in place.


Here's how the wires are run to the switch. Point "A" on the switch runs to point "A" on the board. Same for "B" of course.

Here's the frequency response for the four settings measured at the amp output with a 4 ohm resistive load:




Click on the graph for a high res version. From the bottom to the top the curves are for Stock, Fondelli Mod Fat Boost, Extra Fat Boost and Tone Stack Bypass. The tone stack bypass curve is up about 16dB up at the mid cut frequency!

A final note...if you try this one yourself, be careful to place the switches low enough so they clear the cabinet when you put the chassis back in. It a tight fit.

Fender Champion 600 cathode bypass mod

This post is from the series of mods I've done to this Champion 600. I'm finally getting down to changing the circuit a bit. My main goal is to clean the amp up a bit, especially in the low end. It's probably going to require a speaker replacement eventually, but I'm starting with the cathode bypass caps.

With most 12AX7 circuits, smaller bypass cap values will reduce the low end response by a bit less than 6dB. The frequency at which the response is down by 3dB is, logically, the "3dB down point" or the "knee frequency". Changing cathode bypass caps can be a bit more subtle than changing coupling caps since the low end response is never reduced more than 6dB - only the knee frequency is changed.

There are two cathode bypass caps in the preamp section of the Champion 600. Either or both of them can be changed out to shape the frequency response of the amp.

Here they're indicated on the pc board layout - C3 is for the first stage and C10 is for the second:





A standard tuned 6 string guitar isn't going to produce anything below 80Hz and even the low frequencies it does produce aren't going to reproduce well though the Champion 600's speaker. Below is a chart with values calculated for the 3dB down point with a series of common cap values. Note the stock value of 22uF has a -3dB point of 9Hz. I'm going to start with a 1.5uF in the first stage and a .68uF in the second and work from there.

If electrolytic caps are used (and they probably will be for the larger values unless you've got deep pockets) it's very important to maintain the polarity of the cap or it will explode under use.




This amp came to me with a printed copy of the Frondelli Mod . The mod itself is fine, but a number of the changes detailed there do the opposite of what this amp owner is looking for. He's looking for a cleaner sound with more low end headroom. Even though Frondelli is limiting the low end somewhat, he is also increasing gain for a more saturated sound. If that's what you're interested in, though, you can tweak the bypass cap value to your taste there as well. The blue resistors in the diagram below indicate the ones changed for the 1st stage in the Frondelli mod (R8 and R2). The bypass cap associated with that stage (C3) is indicated in orange.


Jon Frondelli recommends a .68 uF cap here, but you're free to change it to taste. Below is a chart showing calculated -3dB values adjusted for the new plate and cathode resistor values in the Frondelli mod.



If you're interested in how these values are calculated, drop me an email - or visit Randall Aiken's site.