I got the mods below somewhere off the Internet, and haven't tried them on my Bassmans, so No Guarantee's!
They do look easy and reversible though, so they're worth a try. - Regis
Marshall flavor for the Bass channel
All of these mods are reversible.
First of all DRAIN YOUR AMP OF ALL DANGEROUS VOLTAGE! (READ THE GEO FAQ)
My Fender 1964 Blonde Bassman sounds terrific. However, I found that I rarely if ever used the Bass channel. No highs, too much bass, and general dull is how I would describe the channel. Assuming you are using your Bassman for guitar!
Here is a simple mod I did to make it come alive and sound lots better! Again, thanks to the AMPAGE crowd for helping me with this simple mod.
Replace the 25uF capacitor on the cathode of the first gain stage (in parallel with the 2700 ohm resistor) with a .68uF (50V or higher) cap. I used a Sprague Orange Drop 400V capacitor for mine.
Remove the .002uF capacitor in parallel with the 100K resistor right before the phase inverter section.
What a tone killer! The .002uF drains your highs to ground. While good for 60's bass, bad for your guitar!
Here's what the mods do:
My attempt to give the channel more of the classic Marshall gain stage. The 2.7K/.68uF combo is used in Marshall amps. This mod rolls off more of the bass and emphasizes midrange more like a classic Marshall amp.
Here's an easy one: Adding a midrange control to Fender amps that have no midrange control. (Tube amps)
On the left hand tab of the Bass control (looking at it from the back) is a 6.8k resistor. This is Fender's 'fixed' midrange.
Replace it with a 10Kohm potentiometer, and that's it! If you want a 'boosted' midrange, you may want to try a 25Kohm pot.
The resistor goes from the left hand tab of the pot to ground. When replacing this, run a wire from the left hand tab of the bass pot, to the middle tab of the midrange pot. The left hand tab of the midrange pot should then be wired to ground.