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  • Think The asymmetry is not a fact of how the circuit work, but more how the tube is linear or not. As time the signal swing more asymmetrical due to its characteristic, operation point, how much it swing relative to the input , etc...you.ll get those differences. Use the tube in its linear region and will become more symmetrical. Just for checking: invert one of the channel of you scope if have this possibility- the traces will show identical...or very close.
    Last edited by catalin gramada; 06-30-2021, 06:31 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    • Originally posted by catalin gramada View Post
      Think The asymmetry is not a fact of how the circuit work, but more how the tube is linear or not. As time the signal swing more asymmetrical due to its characteristic, operation point, how much it swing relative to the input , etc...you.ll get those differences. Use the tube in its linear region and will become more symmetrical. Just for checking: invert one of the channel of you scope if have this possibility- the traces will show identical...or very close.
      Yes that makes good sense, thank you.

      Comment


      • The 6SN7 couldn't afford 6dB NFB loss, so I tacked this together with 6SL7. I'd like to stick with an octal tube for this project.
        Seems to work ok, balance is much better with the separate cathode resistors. However, it hums badly as if the instrument is unplugged.
        I suspect it has to do with the floating OT common, or does something else look incorrect?
        Attached Files

        Comment


        • Originally posted by catalin gramada View Post
          That simple. In original circuit you have 240 ohm shunt nfb resistor in series with first cathode. Because the cathode is not full bypassed but partial you will loose some of you precious gain rising in same time the internal tube resistance. You need to make the shunt resistor as small as practical, but take a look at circuit: the series nfb resistor is also the bias resistor for those cathode: the closed path from cathode run also from: cathode- series nfb- OT secondary-signal ground which is in parallel with cathode- shunt nfb- bias resistor- signal ground. You don't need to bias the tube through series nfb resistor and OT secondary winding. This is a unwanted consequence of the circuit as time you want to keep common OT tied to the ground. But there is no real reason to do that. (..allright there are some but not subject of our analysis). Close the nfb loop over nfb series/shunt resistor and bias the tube just from bias resistor not through series nfb in parallel with bias resistor-. The common OT wire should not touch the ground, you need isolated speaker jacks.
          And btw be careful when you do the measurements with your scope, sometimes may create a loop path through ground probe clip- earthing wire..and depend what you measure you may involuntary run some currents in circuit through this path....that's can happen...sometimes..
          The "floating" OT secondary will not harm anything.
          Any reason not to keep the OT ground connection and put the shunt resistor beneath the cathode R/C? Like this?
          The floating OT caused a lot of hum in my amp...
          Attached Files

          Comment


          • Originally posted by hylaphone View Post

            Any reason not to keep the OT ground connection and put the shunt resistor beneath the cathode R/C? Like this?
            The floating OT caused a lot of hum in my amp...
            No. You can. As time you use individual cathode resistors you version is the way to go for. I figured the shunt nfb resistor in the top just to keeping stuck with original circuit with shared cathode resistor. Using individuals you drawing is correct electrical way. Normal should not be any difference keeping the shunt resistor in ground or in top but I did a lot of experiences and found it did (simple way to check).
            Put a 10% value trimmer resistor (say 10k/1w) on the plates supply to can easy adjust DC offset between tubes.
            Btw. The floating OT should have nothing with you hum issue. Pretty sure some is not correctly isolated or close an accidental loop by measuring instrument you used.
            Last edited by catalin gramada; 07-10-2021, 02:58 PM.
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

            Comment


            • There is a very good material regard paraphase circuit: High-Fidelity Circuit Design, Norman Crowhurst and George Cooper, 1957, 296 pages - Courtesy of John Atwood. Sorry cannot upload but can be downloaded by PMillett site. Read from pag.111 and further.

              http://www.tubebooks.org/technical_books_online.htm

              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

              Comment


              • Originally posted by catalin gramada View Post
                There is a very good material regard paraphase circuit: High-Fidelity Circuit Design, Norman Crowhurst and George Cooper, 1957, 296 pages - Courtesy of John Atwood. Sorry cannot upload but can be downloaded by PMillett site. Read from pag.111 and further.

                http://www.tubebooks.org/technical_books_online.htm
                Excellent thank you

                Comment


                • If you want run OT "float" do the following check : check if any short between output secondary and OT Iron (thinking any internal OT screen is linked together with OT common and then to OT lamination- very improbable but possible so please check) , bolt isolated jacks to chassis and check for any short from jack ground to chassis, wire the secondary OT to output jacks then check again for any short between common secondary and chassis, solder the series nfb resistor directly to the jack, then run a twisted pair of wires to the shunt resistor. Solder 100nF or whatever cap to common OT at the output jack to serve as ground for you scope probe ( tie the scope probe alligator to the OT common wire through this cap for measurements) . You should not get any hum...because is no source of hum involved in the circuit.
                  Last edited by catalin gramada; 07-10-2021, 06:38 PM.
                  "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                  Comment


                  • Like that

                    Attached Files
                    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                    Comment


                    • We cannot help you with the hum problem as time you did not show how you amp is wired. A wiring layout or photo may help...
                      "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                      Comment


                      • Originally posted by catalin gramada View Post
                        If you want run OT "float" do the following check : check if any short between output secondary and OT Iron (thinking any internal OT screen is linked together with OT common and then to OT lamination- very improbable but possible so please check) , bolt isolated jacks to chassis and check for any short from jack ground to chassis, wire the secondary OT to output jacks then check again for any short between common secondary and chassis, solder the series nfb resistor directly to the jack, then run a twisted pair of wires to the shunt resistor. Solder 100nF or whatever cap to common OT at the output jack to serve as ground for you scope probe ( tie the scope probe alligator to the OT common wire through this cap for measurements) . You should not get any hum...because is no source of hum involved in the circuit.
                        Thank you, I will explore this in time. What advantage does the floating version offer over my last schematic, with the shunt resistor to ground beneath the cathode c/r?

                        As shown in that schematic, it's fairly well balanced until about 5kHz. The inverting side loses amplitude at higher frequency, -6dB at 10kHz. Again, bypassing the 470K in the divider with a small capacitor helps somewhat, but I'd like to understand what is actually happening. Is this some LPF with the grid capacitance?

                        Comment


                        • Originally posted by catalin gramada View Post
                          We cannot help you with the hum problem as time you did not show how you amp is wired. A wiring layout or photo may help...
                          Yes, I'll post something if I experiment more with the floating version.

                          Comment


                          • I don't see any difference between two circuits as time cathode resistor(s) are bypassed so mostly original version take advantage from economy of components...As time you use individual cathode resistors the shunt nfb resistor in the bottom to ground rules no need to use a "floating" secondary, but even so the corect wiring as time you use the global nfb is exactly how it was presented in my sketch no matter if common OT is grounded or "floating" = meant the common secondary should be grounded quite where you grounded the cathode of the stage where nfb is applied not ground at the speaker jack. So you keep the OT common isolated, the speaker jack isolated and run the nfb twisted wires pair (send and return) directly over 10 ohm resistor. Put the 10 ohm in the bottom to ground and...voila! You have a grounded secondary. Then will be very easy to choose and see any diferences just moving the 10ohm resistor -together with nfb wires from OT- in top or bottom. Anything else should be keeping isolated.
                            Last edited by catalin gramada; 07-10-2021, 11:43 PM.
                            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                            Comment


                            • You circuit should look quite like that and is your option to choose where you install the 10ohm resistor: in top to keep secondary "floated" or in bottom to keep secondary grounded. Think common wire as signal nfb return then as ground reference...or...think optimise the circuit path first then hit where you choose to ground it.
                              Sorry, due my limitation English skills cannot be more clear like that...
                              Attached Files
                              Last edited by catalin gramada; 07-11-2021, 12:24 AM.
                              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                              Comment


                              • Originally posted by catalin gramada View Post
                                You circuit should look quite like that and is your option to choose where you install the 10ohm resistor: in top to keep secondary "floated" or in bottom to keep secondary grounded. Think common wire as signal nfb return then as ground reference...or...think optimise the circuit path first then hit where you choose to ground it.
                                Sorry, due my limitation English skills cannot be more clear like that...
                                I understand you well, and thanks for spending so much time with my questions.

                                Do you have any further thoughts on the imbalance I'm seeing at higher frequencies ?

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