Superlead with variable NFB mod, advise/suggestions

endofall

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I have a 1959 mojotone kit that I recently built and I have been playing with some of the components and the way it’s set up. My favorite for hard rock/metal has been with the NFB resistor as 100k on the 4 ohm tap, 5k presence pot with a .68uf cap. It sounds great but the presence sweep does not honestly have much of an affect at all on the tone. The amp does have a lar/mar master volume. However this setting is a little too much for more mellow tones and found that the 8/16 ohm tap worked better in those situations. After reading about the variable NFB mod I decided to try it with replacing the 100k resistor for 27k and wired a 500k pot in series with it and moved the wire to the 16ohm tap. So the sweep on the NFB control does work and seems ok, however when I turn up the presence control it seems like it’s doing the almost same the same thing that the nfb control is doing and also makes the affect of the NFB control almost non-affective. Should it function like this? Also I am running the ppimv almost all the way open at this point into a fryette ps-100. Thanks
 

william vogel

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The NFB and presence control go hand in hand. The more NFB voltage allowed into the phase inverter the more present the presence control will be heard. The master volume will have a direct effect on the NFB because it’s directly controlling the output voltage. If you like to twist the three knobs it’s going to be difficult to maintain a tone. I’d suggest using the ps100 to control the volume and fine tune the amp carefully then leave the amp alone. Use the ps100 controls after the amp is right and remember that volume level is going to give you the perception of tonal changes. Also remember that some days are better than others with guitar amplifiers and don’t make changes without taking pictures or writing down settings. A diary of settings will put you back to baseline easily and some days that tone you expect cannot be achieved. The Zen moments are farther apart than the okay ones.
 

middy

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As I understand it (probably inaccurately or incompletely) the presence control basically reduces the negative feedback in the highs, so your experience is not surprising.
 

South Park

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When you take the 4 ohm tap of the transformer you reduce the power with a resister you roll off some frequency’s with a cap and adjust with a pot. And return back into the phase splitter input the signal is almost the same as the signal that goes to the 8 ohm tap . The difference in the signals cancels out certain frequency’s . I don’t think Marshall got the combination right on the circuit. I have a SLO 50 clone and the presence works that is the values I would put in on that circuit the the power section is the same
As I understand it (probably inaccurately or incompletely) the presence control basically reduces the negative feedback in the highs, so your experience is not surprising.
 

endofall

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Thanks for the replies everyone and all of that makes sense since I’m using a .68uf on the presence pot that it is almost like a mid drive when I have the NFB level set high and turn the presence up. I just wanted to make sure all of these interactions are correct. The amp does sound great, but I’m finding myself pretty much leaving it set on the lowered nfb setting that would have been what I started with anyways of 100k on the 4 ohm tap. Thanks!
 


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