Fender Champion 600 cathode bypass mod Pt 2

When I wrote part 1 of this post I'd been planning to lower the cathode bypass capacitor values in order to clean up the up the bass response. What I ended up doing was removing the cathode bypass capacitors all together in order to get a bit of compression for a fuller low volume clean tone. The effect is a more roundness and a good deal less volume - very nice for a bedroom level clean sound. This meant I needed to find some other ways to control the low end (see the Input Voicing and Presence Plus mods).


For this mod I put in a three pole four position rotary switch that adds the cathode bypass caps in one by one. With the rotary switch the gain of the amp goes up with each setting until it reaches the stock (fully bypassed) position:




This could easily have been three separate switches, but I thought the single control made the operation a bit clearer and didn't clutter up the chassis as much. Since I'm limiting bass response in a couple other ways I ended up keeping the caps at their stock values. The first and second stages could easily have values from part 1 substituted if you have a need for greater bass reduction.

Here is:

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.

And here's the internal view of the switch with the ground wires in place:





This is how the ground wires hook up:




The wire with the black arrow connects to the lower side of the C4 space on the pc board.

The wire with the blue arrow connects to the lower side of the C10 space on the pc board.

The wire with the red arrow connects to the lower side of the C3 space on the pc board.

Here is the rest of the switch wiring. Notice that the leads for the large blue cap (which serves as C4) jump three pins on the switch.


The stage 2 wire jumper two pins. This wire attaches to the shrinkwrapped end of C10 (indicated by the red arrow).



The wire connecting stage 1 to the switch connects to only one pin. It connects to the shrinkwrapped end of C3 (again, indicated by the red arrow).






If you've removed these caps and are reinstalling them, make sure you observe the proper polarity when you put them back in. The indented end of the cap lines up with the indent in the white outline on the pc board.

For convenience of installation, I replaced the original C4 with a axial lead cap of the same value. It's the large blue cap in the photo below. The negative side of the cap connects to the switch and the positive side is connected to the high side of the pc board connection for C4. The negative lead of the cap holds it pretty well in place but there's a dab of silicone underneath just for good measure.



This switch functions kind of like a staged clean master volume. If you're interested in getting distortion out of your preamp circuit, this mod could be rearranged to serve as a sort of dirty master volume instead. If you're interested let me know and I'll post the details.