Did the mod, and I like it a lot. Clips to come today.
Here's what you need:
- Soldering iron
- Electrical tape
- needlenose pliers
- wire
- 4.7 nF capacitor (I used a panasonic film cap) - different values can be used here to sub in different low end center frequencies, with larger values moving the center frequency up. Peavey 6505s and EVH 5150 IIIs use a 6.8nF capacitor for this.
- .1uF capacitor (again, panasonic film cap)
A1M pot (mesa P/N #592739).
Here's how to do it. Note, I did this in such a way that I lost modern mode - but I didn't particularly care about modern mode. This made it easier for me because I could just bypass LDR19 and LDR20.
1. Discharge the amp before you do anything else.
2. Locate R275 and R276 - they're the two 47k resistors to the left of LDR19 and 20 looking at the amp (viewing the amp from the back). R275 is closer to the LDRs.
3. Clip R276 - we're not using it. This does get rid of the increased negative feedback for the clean channel, but it's not a particularly large concern because it really doesn't change the headroom of the clean channel much at all.
4. Lift the end of R275 further from the power tubes.
5. Solder a wire to the lifted end of R275. tape or heatshrink this connection.
6. Remove the red presence pot. Keep the black wire and connect it to the bare wire (the ground line).
6a. If you remove the black wire, you will need to make sure you find another connection to ground for the red channel master and the loop active master - your volume controls will not work properly and the amp will oscillate at low frequencies (motorboat) if you don't do this.
7. Tape off the grey wire - that's the connection to the red channel tonestack, we're not using it.
8. Solder the 4.7nF to lugs 1 and 3 of the 1M pot.
9. Connect lugs 1 and 2 together (with the pot shaft facing away from you and the lugs facing up, the leftmost and center lugs).
10. Solder one leg of the .1uF capacitor to lug 3 of the pot and the other to another wire.
11. Connect the wire to the end of C52 closest to the edge of the PCB. Tape or heatshrink all connections possible.
12. Place 1M pot in the hole for the red presence control.