Announcement

Collapse
No announcement yet.

Negative feedback resistor value 5F2-A question

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Negative feedback resistor value 5F2-A question

    Hi folks,
    On the lil Princeton that could build... I want to go with the Allen TO11C output transformer... It will give me 8K to an 8K or 16K secondary for the 6V6 (no 4 ohm). My question is after some researching on that 22K feedback resistor I am a little lost on if I should use that value or 33K? Was the 22K for a 4ohm tap?


    Thanks and appreciate the help,


    Dale

  • #2
    I probably don't need to go into my spiel of "The NFB loop is a voltage dependent circuit... Blah, blah..." So I'll just say yes, the 22k was for the 4 ohm tap. So that's about 4V source voltage through a divider of 22k/1.5k (14.66:1) for a NFB of .272V at the top of the driver stage cathode. If you use the 8 ohm tap you have about 6V source voltage. If you make the divider 33k/1.5k (22:1) you get .272V on top of the driver stage cathode. Bingo I don't know if you just guessed that number because it's a common resistor value or if you had already worked out the math, but how convenient that 33k is a standard value! Use 33k off the 8 ohm secondary tap.
    "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


    • #3
      Originally posted by Chuck H View Post
      I probably don't need to go into my spiel of "The NFB loop is a voltage dependent circuit... Blah, blah..." So I'll just say yes, the 22k was for the 4 ohm tap. So that's about 4V source voltage through a divider of 22k/1.5k (14.66:1) for a NFB of .272V at the top of the driver stage cathode. If you use the 8 ohm tap you have about 6V source voltage. If you make the divider 33k/1.5k (22:1) you get .272V on top of the driver stage cathode. Bingo I don't know if you just guessed that number because it's a common resistor value or if you had already worked out the math, but how convenient that 33k is a standard value! Use 33k off the 8 ohm secondary tap.
      Whoa whoa whoa....not to hijack this guy's thread, but this caught my attention. With my AB763 build, I built it as stock spec'd for an 8 ohm speaker load. But I'm using an OT that allows for a 16 ohm load and I'm using 16 ohm 4x12 cabs. Do I need to alter my negative feedback circuit?

      Comment


      • #4
        Originally posted by Greg_L View Post
        Whoa whoa whoa....not to hijack this guy's thread, but this caught my attention. With my AB763 build, I built it as stock spec'd for an 8 ohm speaker load. But I'm using an OT that allows for a 16 ohm load and I'm using 16 ohm 4x12 cabs. Do I need to alter my negative feedback circuit?
        If your OT has both 8 ohm and 16 ohm taps you would just use the stock values and connect the NFB lead to the 8 ohm tap. Even if that tap isn't allocated to the output jack. If you don't have an 8 ohm tap then you would need to change the NFB resistor if you want the same performance as the stock circuit. And if the NFB resistor is connected to the output jack now and you are using the 16 ohm tap then you do , indeed, have a lot more NFB going than the stock circuit.
        "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


        • #5
          Originally posted by Chuck H View Post
          If your OT has both 8 ohm and 16 ohm taps you would just use the stock values and connect the NFB lead to the 8 ohm tap. Even if that tap isn't allocated to the output jack. If you don't have an 8 ohm tap then you would need to change the NFB resistor if you want the same performance as the stock circuit. And if the NFB resistor is connected to the output jack now and you are using the 16 ohm tap then you do , indeed, have a lot more NFB going than the stock circuit.
          Ahhhh, okay. So my build is set up stock, but I'm using a multi-tap (4, 8, 16) OT, and my negative feedback wire is connected at the speaker jack. It's an 820R on the board, with a 47R leg to ground? This amp will pretty much always be connected to 16 ohm cabs. Should I move the feedback wire from the speaker jack and put it directly to the impedance selector switch?

          Comment


          • #6
            Originally posted by Greg_L View Post
            Should I move the feedback wire from the speaker jack and put it directly to the impedance selector switch?
            Nailed it. Just like that. 8 ohm tap. This may be part of that stringent tone you've mentioned too. The NFB loop around the PI and power amp only without phase compensation is a sort of coarse circuit that you you mostly see in guitar amps only. Being less than perfect, if you introduce too much NFB it can come with an increased ratio of higher, odd order harmonic distortions. You'll probably find the tone rounder and gentler on the ears. And the amp will get a little louder too.
            "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
              Nailed it. Just like that. 8 ohm tap.
              Thanks bud. That could very well be the last piece of my puzzle!

              Comment


              • #8
                I added text above that agrees with this.
                "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


                • #9
                  Originally posted by Chuck H View Post
                  I added text above that agrees with this.
                  Got it, thanks again. So much to think about. I should have known. Seems so obvious now. Hopefully it takes care of my little problem.

                  Comment


                  • #10
                    Thanks Chuck and Hi Greg. I stumbled on the voltage math regarding the feedback resister but I wasn't sure if the 5F2-A schematic was showing us a 4ohm or 8ohm output transformer. Now I know that the 22K value was the originally used resistor for the 4ohm transformer/speaker. Since my output transformer will also have a 16 ohm tap I might use the 16 and a 47K resistor as I read it is a common practice to use the highest tap available even if your not actually plugging into it. (Maybe I will add a selector switch just in case I do want to run a 16ohm speaker).

                    Thanks again for the help and solid advice!

                    Dale

                    Comment


                    • #11
                      Originally posted by tubedood View Post
                      Thanks Chuck and Hi Greg. I stumbled on the voltage math regarding the feedback resister but I wasn't sure if the 5F2-A schematic was showing us a 4ohm or 8ohm output transformer. Now I know that the 22K value was the originally used resistor for the 4ohm transformer/speaker. Since my output transformer will also have a 16 ohm tap I might use the 16 and a 47K resistor as I read it is a common practice to use the highest tap available even if your not actually plugging into it. (Maybe I will add a selector switch just in case I do want to run a 16ohm speaker).

                      Thanks again for the help and solid advice!

                      Dale
                      I sort of covered that above with Greg. If you have multiple taps you'll want to connect the feedback lead to the correct tap at the impedance switch rather than at the output jack. As long as the feedback circuit is connected to the correct secondary at the transformer (8 ohms) the feedback loop will operate correctly no matter which secondary is connected to the output jack.
                      "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


                      • #12
                        Originally posted by Chuck H View Post
                        I sort of covered that above with Greg. If you have multiple taps you'll want to connect the feedback lead to the correct tap at the impedance switch rather than at the output jack. As long as the feedback circuit is connected to the correct secondary at the transformer (8 ohms) the feedback loop will operate correctly no matter which secondary is connected to the output jack.
                        I did this today, and preliminary sound tests show a marked improvement. Doh! I'm such a doofus. Lol. I know this already with Marshalls, I didn't apply it to my Fender build. I just built it as stock and didn't think about the impedance selector. I'm so glad I was just bouncing around reading posts today. It might have taken me years to realize my error. Thanks again, Chuck.

                        Comment


                        • #13
                          And everything in its context. While we start with the stock design, we can change some values to compensate for our using different output taps. But also consider, that as an individual, you might actually prefer more or less NFB than the stock design. SO using a "wrong" tap or resistor might appeal. In other words do what sounds best to your ears, not what pleases the forum. it is just a guitar amp.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            THAT^^^^^^^^^^^^^^^^^^^^^^^^^^^^

                            The ears and hands (feel) have it every time. But I will add that if one is starting with a known design without specific knowledge of what changes will sound like it's probably best to use the stock values (or what is calculated to give the stock result, as in your case) as a starting point for reference.
                            "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


                            • #15
                              Yeah I get that. For me, I've tweaked NFB on Marshalls to my liking, but my AB763 build is my first build ever and I'm not that familiar with Fender amps anyway. I wanted it to be as "standard" as possible...except that I used a multi-tap OT. But I did it too by-the-book. I was hyper-focused on it being correct, and correct for Fender has the NFB wire soldered to the speaker jack. But I'm using a multi-tap OT, so I should have done it like my Marshalls. I didn't even think of that and hopefully it fixes my little issue. It does seem better.

                              Comment

                              Working...
                              X