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  • #31
    Originally posted by g1 View Post
    Because PI is not perfect.
    Does not need to be complete half cycle, that was just an example. Can you guarantee no signal can possibly get to point 'X' ?
    Ultimately, and if PI is not perfect.
    You have a perfectly paired 12AX7. All components in PI are 1%.
    Both 12AX7 anodes and both g1 6L6s are short-circuited to each other (see schematic).
    How much signal will appear at point 'X'? 10, 20, 30% or <1%

    You don't need a simulation. Practical check requires only 10cm of wire and 5 minutes.
    Let's forget the philosophizing these 32 posts. How to explain that some certain famous brands that use CLMV in their guitar amps (#12).

    It's All Over Now

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    • #32
      Originally posted by vintagekiki View Post
      That's right colleague. Output from PI at point ? (X) will be zero1) and graphic form at point ? (X) will be horizontal line, because equal half-periods are canceled.

      When the output from PI is zero1), there is no drive at the g1 output tubes, through power tubes only idle (bias) current will flow.

      2) I don't understand how you found the negative half-cycle at point X, when PI gives a symmetrical output voltage ???
      Originally posted by Helmholtz View Post
      ...PI triodes will work at their limit to supply short circuit currents...
      I'm having a tinker with my poor old Sound City 50 Plus (modding it to be similar to a JTM50), so as it's on the bench I thought I'd investigate this. When a Type 3 master vol is set very low (below about 5k), the common mode thing does seem to be real; with the LTP fully overdriven, a 24.6Vp-p common mode signal passes to the power valve control grids. Red trace is channel A, the 1st/inverting/82k LTP output, blue trace is the (100k) inverting output.

      If the master volume is turned back up and a current meter connected across it (ie replicating the master volume set to min), the current is 1.426mA, no change to the waveform etc.
      So far I've not got the EL34 fitted. The amp's HT voltage during the testing at the reservoir cap node is 420V, at the LTP HT node it's 350V.
      Click image for larger version

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      Attached Files
      Last edited by pdf64; 01-08-2021, 03:27 PM.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #33
        Originally posted by pdf64 View Post
        If the master volume is turned back up and a current meter connected across it (ie replicating the master volume set to min), the current is 1.426mA, no change to the waveform etc.
        That's about 2mA peak and less than I expected. But I should have considered that a triode can't deliver more than its saturation current, which depends on Vak.
        I wonder what was the cathode voltage during the measurement. It might have increased, especially during current peaks, because of in phase/common mode currents, thus lowering effective Vak.
        Common mode tube currents will not show in your measurement but will make the cathode voltage swing.

        Unfortunately I can't read the scales in your low resolution scope picture.
        - Own Opinions Only -

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        • #34
          Originally posted by Helmholtz View Post

          That's about 2mA peak and less than I expected. But I should have considered that a triode can't deliver more than its saturation current, which depends on Vak.
          I wonder what was the cathode voltage during the measurement. It might have increased, especially during current peaks, because of in phase/common mode currents, thus lowering effective Vak.
          Common mode tube currents will not show in your measurement but will make the cathode voltage swing.

          Unfortunately I can't read the scales in your low resolution scope picture.
          Oops, sorry about the pic resolution, should be lots better now.
          EDIT Re cathode current, the VDC across the 470ohm resistor at idle is 1.16VDC, fully overdriven it rises to 1.185V master vol at max, 1.38V with the master vol at min.
          The corresponding voltages between cathode and 0V common are 36.6VDC, 36.9V and 42.5V (!).
          Last edited by pdf64; 01-08-2021, 06:42 PM.
          My band:- http://www.youtube.com/user/RedwingBand

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          • #35
            I don't understand what you meant with measuring and simulations to say (prove).
            To remind you, the question was like in # 14

            https://music-electronics-forum.com/forum/music-electronics/920070-master-volume-and-treble-loss?p=920246#post920246

            What do you get if g1 power tubes mutually shorted.
            It only takes 2 wires to move from simulations to practice.

            Look at practical solutions
            http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23SM_scena/VOX/Vox_AC30CC2_AC30CC2X%20(2005)%20SM.pdf
            page 6

            http://dealers.korgusa.com/svcfiles/TB18C1.pdf
            page 3
            It's All Over Now

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            • #36
              Originally posted by pdf64 View Post
              Oops, sorry about the pic resolution, should be lots better now.
              Re cathode current, the VDC across the 470ohm resistor at idle is 1.1VDC, fully overdriven it rises to 2.95V master vol at max, 3.5V with the master vol at min.
              The corresponding voltages between cathode and 0V common are37VDC, 36.8V and 42.2V (!).
              Thanks!

              I'm not sure how to interprete your measurements.

              Assuming all values are (averaged) DC, the increase from 1.1V to 3.5V across the 470R would correspond to more than tripling the plate currents. From 1.17mA to 3.7mA per tube.
              But as the same current must run through the total cathode resistance, the cathode voltage should increase by the same factor?

              What are the peak-to peak signal voltages at the PI cathode?
              Last edited by Helmholtz; 01-08-2021, 05:45 PM.
              - Own Opinions Only -

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              • #37
                Originally posted by vintagekiki View Post
                I don't understand what you meant with measuring and simulations to say (prove).
                To remind you, the question was like in # 14

                https://music-electronics-forum.com/forum/music-electronics/920070-master-volume-and-treble-loss?p=920246#post920246

                What do you get if g1 power tubes mutually shorted.
                It only takes 2 wires to move from simulations to practice.

                Look at practical solutions
                http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23SM_scena/VOX/Vox_AC30CC2_AC30CC2X%20(2005)%20SM.pdf
                page 6

                http://dealers.korgusa.com/svcfiles/TB18C1.pdf
                page 3
                The pic I posted is a screenshot of my scope traces taken from the amp mentioned, my scope being a USB picoscope. It's real, not a simulation
                With EL34 now in circuit and the type 3 master vol turned to min, their (dc average) cathode current rises about 50%, eg from 32mA to 45, when an input signal is applied, as per the scope trace pic previously posted.
                So the common mode effect does seem to be, in this case, real.
                I'm pretty sure the finding is reproducible.
                Here are traces of the signals at the EL43 1 ohm cathode resistors (I can't find my grounding clip for the red probe ).

                Click image for larger version  Name:	Both EL34 cathode current full overdrive min master vol.png Views:	0 Size:	107.8 KB ID:	922073
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                • #38
                  Originally posted by vintagekiki
                  It only takes 2 wires to move from simulations to practice.
                  Actually only one.

                  So, why don't you show your own measurements?
                  Or don't you have a scope?
                  Last edited by Helmholtz; 01-08-2021, 05:48 PM.
                  - Own Opinions Only -

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                  • #39
                    Originally posted by Helmholtz View Post
                    ...Assuming all values are (averaged) DC...
                    Yes, but many apologies, due to a somewhat messy tagstrip layout, my previous readings were between the cathode and the non-inverting grid
                    Please see amended readings above.

                    Originally posted by Helmholtz View Post
                    ...What are the peak-to peak signal voltages at the PI cathode?
                    Looks to be about 5Vp-p. The red trace is at the cathodes, the blue is at other end of their 470ohm bias resistor.
                    The HT VDC at the LTP supply node (via a 10k dropper) drops about 2.5V between idle and full overdrive into the short (type 3 at min).

                    Click image for larger version

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                    • #40
                      Originally posted by pdf64 View Post
                      The pic I posted is a screenshot of my scope traces taken from the amp mentioned, my scope being a USB picoscope. It's real, not a simulation
                      With EL34 now in circuit and the type 3 master vol turned to min, their (dc average) cathode current rises about 50%, eg from 32mA to 45, when an input signal is applied, as per the scope trace pic previously posted.
                      So the common mode effect does seem to be, in this case, real.
                      I'm pretty sure the finding is reproducible.
                      Here are traces of the signals at the EL43 1 ohm cathode resistors (I can't find my grounding clip for the red probe).
                      The communication problem during machine translation arises due to different language syntaxs. sorry.
                      The question is what do you get if g1 power tubes mutually shorted. (CLVM potentiometer at min).
                      No current or voltage values are required, a logical conclusion is required.

                      By the way the oscillogram you specify is for type 3 master vol and "Both EL34 cathode current full overdrive min master vol.png"
                      An honest question. Have you ever practically tried CLVM? It only takes 2 wires ... ...

                      It's All Over Now

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                      • #41
                        Originally posted by Helmholtz View Post

                        Actually only one.

                        So, why don't you show your own measurements?
                        Or don't you have a scope?
                        It is illusory to comment on something where you have expressed an opinion in advance.
                        Originally posted by Helmholtz View Post
                        I don't like this CLMV so I don't use it.
                        I have
                        Click image for larger version

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                        It's All Over Now

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                        • #42
                          Originally posted by vintagekiki View Post
                          It is illusory to comment on something where you have expressed an opinion in advance.
                          ]
                          That's not an answer to my questions and I could return your statement to you. The truth is not in opinions but in real measurement results.

                          I have
                          Well, I have too.
                          Last edited by Helmholtz; 01-08-2021, 08:26 PM.
                          - Own Opinions Only -

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                          • #43
                            Originally posted by vintagekiki View Post
                            ...The question is what do you get if g1 power tubes mutually shorted. (CLVM potentiometer at min).
                            No current or voltage values are required, a logical conclusion is required.
                            I don't understand, what parameter are you enquiring about?

                            [/QUOTE]...By the way the oscillogram you specify is for type 3 master vol and "Both EL34 cathode current full overdrive min master vol.png"
                            An honest question. Have you ever practically tried CLVM? It only takes 2 wires ... ...[/QUOTE]
                            The Type 3 master vol pot is in a real amp, it is being tried, that is what the measurements and scope traces are in regard of.
                            The 1M type 3 control pot has been in the amp for several years, however after initially fitting it, I didn't use it much, so then removed the wiring to it. So the redundant pot has been sitting there for a few years. I wired it back up yesterday for the purposes of the tests I've been reporting,


                            My band:- http://www.youtube.com/user/RedwingBand

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                            • #44
                              Originally posted by Helmholtz View Post
                              That's not an answer to my questions and I could return your statement to you. The truth is not in opinions but in real measurement results.
                              Who mentioned the measurements.
                              Why get into a story when the answer is known in advance.
                              Originally posted by Helmholtz View Post
                              I don't like this CLMV so I don't use it.
                              You will agree there is no point in conducting a vain dialogue.


                              It's All Over Now

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                              • #45
                                Originally posted by pdf64 View Post
                                I don't understand, what parameter are you enquiring about?
                                The Type 3 master vol pot is in a real amp, it is being tried, that is what the measurements and scope traces are in regard of.
                                The 1M type 3 control pot has been in the amp for several years, however after initially fitting it, I didn't use it much, so then removed the wiring to it. So the redundant pot has been sitting there for a few years. I wired it back up yesterday for the purposes of the tests I've been reporting,
                                The story about here debated is about Cross line master volume (CLMV)
                                Please see post # 37
                                It's All Over Now

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