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.