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  • #1

    NFB probems

    Here is my problem. I built a popular preamp with a marshall power amp. I am trying to add depth to the nfb in addition to the presence. I will give you the schematic as I have it right now. Without the depth resistor and cap the amp sounds great but I want to try the depth circuit because I never built one into my amps and I want to see how it sounds. The problem is I am getting a strange noise when I play a chord hard. I turned the amp up to see if I could hear it better since it is buried in the guitar sound pretty well and it sounds like screaching/hi pitch oscillation that ends as quick as it starts. This amp has no problems except for some hiss, but you tend to get that with 4 gain stages. Actually the hiss is alot quieter with the depth control in.
    Did I do something wrong?
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  • #2
    I dont know why the f##kin pic looks so shitty! but it is just a 800 marshall LTPI with a 47k negfb resistor in series with a 220k//470pf fed to the 4ohm tap

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    • #3
      When you open the picture, click on the green enlarge thing in the lower right. Once it is full size it is perfectly clear.

      Sounds like you have a parasitic oscillation, and that is likely from lead dress. Keep the NFB wires away from any preamp circuits for starters.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Compared to a typical 800 feedback circuit yours is feeding back more of the very high freqs and much less of everything below that. A typical Marshall feedback loop is a 20:1 ratio. You are currently using a 56:1 ratio on all but the very top end. Unless you increase feedback in the circuit a "resonance" control won't do much. You will probably get a tighter bottom end out of the deal too (when the resonance control is turned down).

        Also, C2 should be above the 25k pot in the schem. Otherwise you still get DC voltage on the pot so there is no point to using a peripheral circuit for the presence control. You could just use the normal 5k pot type.

        I'm with Enzo on the parasitic oscillation. Always route NFB and presence control leads away from the preamp and tone controls. Never straight across or under the preamp board. I like to place my NFB lead dropping resistor right on the impedance switch instead of the board to help reduce the amount of radiant field on this lead.

        Chuck
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

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        • #5
          How does the component order in the series 25k pot and C2 matter? Top or bottom, the cap blocks the DC to the pot doesn't it?
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Hmmm. I suppose. If DC can't pass through the circuit it doesn't effect the pot? I don't know, so I assumed that DC voltage on the pot could make it scratchy even without current.

            Chuck

            P.S. Nice to have you back Enzo. Hope you had a good holiday.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz

            Comment

            • #7
              Originally posted by Chuck H View Post
              Hmmm. I suppose. If DC can't pass through the circuit it doesn't effect the pot? I don't know, so I assumed that DC voltage on the pot could make it scratchy even without current.
              Yeah you still tend to get scratchy performance with the cap that way.
              Sounds like you have loop instability, which is not uncommon in guitar amps. Marshall put a 47pF cap between the anodes of the LTP, but this isn't quite enough for all types of OT. The best way to stabilise this is to put a 100pF to 220pF capacitor between grid and cathode of each of the output valves. (You're using two output valves right?).

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              • #8
                Unless there is current flowing through it - ie there is a voltage drop across it - how would the pot know there was DC on it?

                When you turn the control, I can see a momentary charging or discharging current through the cap, and that could cause scratch, but that should happen with either part order.
                Education is what you're left with after you have forgotten what you have learned.

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                • #9
                  Originally posted by Enzo View Post
                  When you turn the control, I can see a momentary charging or discharging current through the cap, and that could cause scratch, but that should happen with either part order.
                  With the cap on the input side of the pot no DC can reach it. Unless the cap is leaking.
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "If you're not interested in opinions and the experience of others, why even start a thread?
                  You can't just expect consent." Helmholtz

                  Comment

                  • #10
                    Well, I spent considerable time on the lead dress and to no avail. So I just put it back the way I had it before which is just a 100k -fb resistor and it works just fine with none of the aforementioned problems. The guy who bought it needed it for a show tonight. Perhaps when I get it back I will take a few shots and post.

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                    • #11
                      DC must have a complete circuit to flow. The cap breaks that circuit wherever you put it.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #12
                        Is it also possible that the OT was disconnected at some point & the primary leads to the tubes were inadvertantly swapped? I've seen where this has been done & the resultant squeal is very subtle as opposed to some that just screech you out of the room...glen

                        Don't know about anyone else, but I really could do without the dodgy language in this forum.

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                        • #13
                          No it wasnt the OT leads to the power tubes. I did that once and wont ever forget it. Also, sorry about the language. Aggrevation getting the better of me.

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