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  • Fixed Bias LTP with feedback

    Hi, just wanted to dummy check something I'm working on. It's a Hiwatt-like amp, but I don't have the extra triode to DC couple, or use as a voltage reference (regulator?) like you can see here and here, respectively.

    I've breadboarded this and it seems to work, but I just wanna make sure I'm not doing something stupid with how I'm injecting the feedback. This is how I'm doing it. AC coupled with the bias tapped off the B+.

    Click image for larger version

Name:	Fixed Bias LTP with feedback.jpg
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    Does that look okay? It seems to work, I just can't find any other examples of fixed bias like this with feedback.

    Thanks!

  • #2
    About the only thing I could say is that ideally you would have a capacitor in the NFB line to block DC. Because there's a purely resistive path between the speaker and the HV there's a small amount of DC on the speaker (depending on HV source and the speaker load value). It's really tiny and you might not even notice over the usual amp noise.
    "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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    • #3
      Most every other amp with a global nfb loop around its power amp does that though; I guess it’s a matter of magnitude, when it becomes excessive
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Does that look okay?
        Sorry, no. R26 is shorted which puts full B+ on both grids.

        I don't understand why you don't take grid bias from a small additional common cathode resistor (e.g. 470R) like with any Marshall type LTPI.
        - Own Opinions Only -

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        • #5
          Originally posted by pdf64 View Post
          Most every other amp with a global nfb loop around its power amp does that though; I guess it’s a matter of magnitude, when it becomes excessive
          That's a good point, and I never even considered that before now when looking at this circuit!?! I think it's because the usual circuits are tried and true while this one was brought into question.
          "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


          • #6
            There is an extra connection between the top of R26 and the grid of the right triode that should not be there. The bottom of the 220k could be taken to ground. The 22uf doesn't need to be that big , 100nF would do. I have a concern that hum from the power supply could couple into the right had triode's grid so it might be better to split the 1meg upper divder resistor into say 100K ( to supply) and 910K with 10uF cap from the joint to ground to filter the supply a bit.
            Last edited by nickb; 04-16-2020, 04:01 PM.
            Experience is something you get, just after you really needed it.

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            • #7
              Originally posted by Gaz View Post
              Hi, just wanted to dummy check something I'm working on. It's a Hiwatt-like amp, but I don't have the extra triode to DC couple, or use as a voltage reference (regulator?) like you can see here and here, respectively.

              I've breadboarded this and it seems to work, but I just wanna make sure I'm not doing something stupid with how I'm injecting the feedback. This is how I'm doing it. AC coupled with the bias tapped off the B+.

              [ATTACH=CONFIG]58050[/ATTACH]

              Does that look okay? It seems to work, I just can't find any other examples of fixed bias like this with feedback.

              Thanks!
              So - without doing a sim or anything, I suggest you simply remove the CF triode in the HiWatt design. Short the grid signal to the cathode line that feeds the PI. Remove the 100K cathode load - you don't need it. Basically, you'll keep the whole DC bias thing working and simply not have it buffered by the CF. Given the values of the resistors involved and the hi-Z of the PI input, I suspect it will work pretty well - although test or sim would be needed to confirm that.

              Alternatively, if you don't have a triode to use as the CF - why not use an LND150? Problem solved that way too.
              “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
              -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

              Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

              https://sites.google.com/site/stringsandfrets/

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              • #8
                Originally posted by uneumann View Post
                So - without doing a sim or anything, I suggest you simply remove the CF triode in the HiWatt design. Short the grid signal to the cathode line that feeds the PI. Remove the 100K cathode load - you don't need it. Basically, you'll keep the whole DC bias thing working and simply not have it buffered by the CF. Given the values of the resistors involved and the hi-Z of the PI input, I suspect it will work pretty well - although test or sim would be needed to confirm that.

                Alternatively, if you don't have a triode to use as the CF - why not use an LND150? Problem solved that way too.

                Thanks for the thoughts, yes, that's really obvious now for me to see the CF is unnecessary for DC coupling! I would actually do that, but I've since moved the master volume after the tonestack recovery triode to get some more gain, and so I now need to AC couple the last stage to the PI anyway. I hadn't considered using a transistor either, but for the same reason, I am going to try fixed bias off the HT.

                People claim the fixed bias PI is part of the sound, but maybe I should see for myself... Cathode biasing would be less parts.

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                • #9
                  I think the circuit is self (cathode) bias, not fixed bias.
                  ie if cathode current should rise, cathode voltage will increase too, thereby reducing cathode current.
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    Originally posted by nickb View Post
                    There is an extra connection between the top of R26 and the grid of the right triode that should not be there. The bottom of the 220k could be taken to ground. The 22uf doesn't need to be that big , 100nF would do. I have a concern that hum from the power supply could couple into the right had triode's grid so it might be better to split the 1meg upper divder resistor into say 100K ( to supply) and 910K with 10uF cap from the joint to ground to filter the supply a bit.
                    Thanks, Nick. Yes, the wire shorting R26 is an error in the drawing.

                    Here's the original scheme I was working from, from Merlin Blencowe's book:

                    Click image for larger version

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                    I am just trying to add the Hiwatt 10K/2.2k feedback to this. I thought the way I had it hooked up was filtering the HT, but I guess not!

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                    • #11
                      Originally posted by Gaz View Post
                      Thanks, Nick. Yes, the wire shorting R26 is an error in the drawing.

                      Here's the original scheme I was working from, from Merlin Blencowe's book:

                      [ATTACH=CONFIG]58061[/ATTACH]

                      I am just trying to add the Hiwatt 10K/2.2k feedback to this. I thought the way I had it hooked up was filtering the HT, but I guess not!
                      The HT ripple does get a lot of attenuation from the 1Meg feeding into a quite low impedance of 2k or so. It was just a concern - something to look out for. FWIW a significant difference between your/ hiwatt design and Blencoe's is the big tail resistor of 68k.
                      Experience is something you get, just after you really needed it.

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                      • #12
                        Originally posted by nickb View Post
                        The HT ripple does get a lot of attenuation from the 1Meg feeding into a quite low impedance of 2k or so. It was just a concern - something to look out for.
                        Yeah, I didn't notice any noise, but obviously would like to set it up ideally if possible. Does this look better?

                        Click image for larger version

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                        Originally posted by nickb View Post
                        FWIW a significant difference between your/ hiwatt design and Blencowe's is the big tail resistor of 68k.
                        Yes, mine is with 12AT7 instead of 12AX7, and I with the value in my scheme I have the bias voltage right at about 2V like the orginal Hiwatt.

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                        • #13
                          Does this look better?
                          Sorry, no. C18 is AC shorting the right side grid to ground, thus disabling NFB.
                          - Own Opinions Only -

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                          • #14
                            ^^^^This. Delete C18.
                            Experience is something you get, just after you really needed it.

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                            • #15
                              Originally posted by Helmholtz View Post
                              Sorry, no. C18 is AC shorting the right side grid to ground, thus disabling NFB.
                              Originally posted by nickb View Post
                              ^^^^This. Delete C18.
                              Ok, thanks, I just thought C18 was needed for filtering the HT like in Merlin's schem, but of course I see now what you mean with the NFB being disabled.

                              Is grounding the 220K really much different than my original scheme?

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