"Soul Control" for Fenders: Craptacular Mod?

[Regis]

I was modding my Bassman 100 to use EL34's and while I had it open I thought I would try the "Soul Control" variable NFB mod. I have had the Resonance control in it and liked it but thought I would try this for the hell of it.
Is it me or does this thing do like... nothing? I can't tell a difference of any kind from one end of the pot to another. Dimed or on 2, no difference.

Here is the schematic I used.

Is the schem wrong, or am I deaf? (not unlikely)

[Rick Erickson]

If you have the resonance control in it then I assume that you have changed the neg. feedback resistor from the standard Fender value of 820 Ohms. If you are in the more Marshall-esque 27K-100K range for feedback resistor then adding an extra 5K via this pot won't do squat. This is not a "universal" modification, it was designed for a stock Fender BF/SF circuit.

RE

[Regis]

Hi Rick,

I guess I wasn't clear... I removed the 1 meg resonance control pot and cap and replaced it with the 5k pot for the SC. The feedback resistor is an 820 ohm.

[Rick Erickson]

Well then I'm clueless, which won't surprise anyone who knows me well. It's probably a subtle effect at best anyway. Listen to the low freq. response. With the control at max resistance the low end should loosen up a bit and sound a bit; I believe the technical term is "flubby".

RE

[Smitty]

I was modding my Bassman 100 to use EL34's and while I had it open I thought I would try the "Soul Control" variable NFB mod. I have had the Resonance control in it and liked it but thought I would try this for the hell of it.
Is it me or does this thing do like... nothing? I can't tell a difference of any kind from one end of the pot to another. Dimed or on 2, no difference.

Here is the schematic I used.

Is the schem wrong, or am I deaf? (not unlikely)

Well the schematic doesn't tell you to break the existing circuit and put the rheostat in series, but you did that, right?

Try bumping up the value of the pot. Also, make sure your feedback divider is 820 ohms and 100 ohms. You should definitely detect an increase in gain/noise with the NFB reduced, even if you can't hear the amp go from tight to loose.

PS Can't run four EL34s in a Bassman 100. Too much heater current.

[Satamax]

Regis, do you have a .010µf in parallel with the 820 res? As in the bassman 135? If yes, that could mess with the soul control. Well, it has worked for me on teh BF bassman, tho, it's far more subtle in a fender than on a sound city for example In the last SC i've messed with, i might gain 1db, or something, it's realy getting louder! I've used a dual ganged pot, one side is a crossline post pi master vol, and the other is a variable NFB, quite funny, you turn the master down, and the volume augments, gets nasty and rough sounding SC on steroids and then decreases