ADJUSTABLE NFB-RESONANCE-PRESENCE on a Marshall

[ViperDoc]

I wonder if this is the right way to apply adjustable NFB and resonance controls to a Marshall. Here's only a basic layout excerpt. Thanks for your help.

SELTAP-NFB-RES-PRES2.png

[Roe]

I don't know about the resonance. but instead of an ordinary nfb resistor, I recommend 27k resistor in series with a 220k pot to the 16ohms secondary or 12k resistor in series with a 100k pot to the 4 ohms secondary or 18k to 150k pot to 8 ohms

[ViperDoc]

The Jube schematic I'm working off of has a fixed 100K NFB resistor off of the 4 ohm tap. I thought it would be cool to vary it up a bit. I'd like to avoid putting the amp into PFB with the control, I'm just not sure what value that would be.

[roberto]

Hi, that layout is wrong.
Assuming the connection to the right is ground, and the one to the left is the PI, you want the resonance in series with the variable NFB pot and fixed NFB resistor.
Only the presence pot goes between the PI and ground.

[ViperDoc]

Thanks. Do you mean NFB pot—Resonance—NFB Resistor—(PI node)—Presence—Ground? I want the resonance control on the front of the amp. Hopefully it’s not as sensitive to long wire runs.

[ViperDoc]

Here's my best interpretation of what you recommended. Feel free to critique/correct as needed. Thank you.

SELTAP-NFBP-RES-NFBR-PRES.png

[roberto]

Draw the three pots close to each other, and the 68k (that is too much, half that is enough, even less) from depth to presence, and connect presence to PI.
Just don't reinvent the wheel: see any layout of an high gain amp.

[ViperDoc]

Reason I'm posting this is I can't seem to find a good nfb-resonance-presence Marshall layout to go by. The only two resonance control diagrams are from RobRob and SDM, both suggesting the resonance goes in series with the nfb pot and resistor like you mentioned. My drawing attempts to show the long lead runs that would exist in my present build, hoping that's not a problem. If so, I could also move the NFB pot to the front of the amp, but some kind of long lead run is inevitable, unless I ditch the variable NFB for the depth control and put it on the back. I'd just really like it on the front if I can.

[roberto]

If you are worried, just use a shielded cable.

[ViperDoc]

Good call.

[ViperDoc]

Could this also work, or does the NFB-RES-NFBR sequence matter? It seems in a fixed NFBR design, the resonance pot would come after, am I right?

SELTAP-NFBP-NFBR-RES-NODE-PRES.png

[ViperDoc]

I just saw this layout from a Ceriatone AHD100 that looks quite a bit like a Friedman. See the layout with the 4 ohm tap leading to the front panel, through Resonance, NFB, Presence and then to the PI and ground.

CERIATONE AH-Deluxe-100W-25-June-2019.jpg