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  • Boogie Simulclass biasing

    I've got an Electra Dyne here I'm installing a quad of new JJ 6L6s into. It Manual says it runs 2 tubes in Class A and 2 in AB. I don't think that is truly accurate, but the reality is there is less neg grid voltage on the two outside tubes, so of course they are running hotter than the two inner ones. Something like 40mA and 20 mA respectively. I've already installed a trimpot to adjust the neg voltage. Also, the amp has a half power switch that looks like it lifts a pair of 6L6's cathodes from ground.

    So my question is, at what operating point do I adjust the bias. I was going to run the outer two around 45ma (500V plates) and let the chips fall where they may for the "AB" pair.
    edyne_schem.pdf
    Attached Files

  • #2
    45mA with 500v on the anode is 20W dissipation. A full class A on the outer pair V8 & V11, due to the pair of pull up resistors on the grids. V9 & V10 will probably sit around 75% dissipation and around 40mA.
    A tad on the hot side for my liking.
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #3
      I just checked, and the way I have it set, V8 and V11 are pulling 40mA each, and V9 and V10 are pulling 23mA each.
      Is that reasonable?

      Edit: I mean, is what I'm trying to do here bias 1 pair not TOOO hot, and the other not TOO cold?

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      • #4
        That looks about right.
        Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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        • #5
          The 2.2M resistors between the outer tube grids and ground constitute voltage dividers together with the 440k series resistance, thus lowering the grid bias value by 10% compared to the inner pair.

          It is hard to tell (without measuring) f the outer pair could run in full class A without knowing the plate impedance.
          It's possible with high idle current but would require a large load impedance at that high plate voltage.
          Last edited by Helmholtz; 11-24-2021, 03:09 PM.
          - Own Opinions Only -

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          • #6
            Thanks guys! Just to add, the high gain channel sounds really bad and fizzy on these amps. A 1n across the Rp of V2a R8, and a 1n across 470K R9 really woke it up and made it sing.

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            • #7
              If you get chance, it would be interesting to know what the OT primary impedances, or winding ratios, of the inner and outer pairs were
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                Indirectly answering you: that can be easily calculated using the datasheet curves.

                Solve separate for each "Class" section to have plate swing as much as possible, from +V idle to plate saturation, which will NOT be zero but around 60-80V .

                Each section will have an optimum load impedance coming from peak to Peak voltage swing and related peak to peak current swing, that will define both optimum load and peak power output.

                You also want to make lowest saturation tubes the "outer" ones, the inner ones by definition will stop further away from ground.

                In my view, a useless design, nothing to be gained from it, but of course itīs "unique" and so "patentable", an important consideration for MB.

                It also includes the magical "Class A" buzzword.
                Juan Manuel Fahey

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                • #9
                  It’s just that I recall a report that when investigated, the leads to the inner and outer anode came from the same point, the start or end of the winding. Rather than the inner leads being taken from taps along the primary winding.
                  ie thereby making the whole simul class thing something of a nonsense, which existed only as a patent rather than being implemented in real world amps.
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    For full class A operation with an idle current of 45mA to 50mA, plate load per tube should be at least 8k. Meaning Zaa = 16k for the pair.

                    A simple method to find out if it's actually full class A would be measuring the supply current for the pair in "class A" mode.
                    If the supply current at full output is significantly higher than at idle, it's not (full) class A.

                    - Own Opinions Only -

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                    • #11
                      thereby making the whole simul class thing something of a nonsense, which existed only as a patent rather than being implemented in real world amps
                      That would be VERY funny.

                      And of course entirely possible.

                      If not mistaken, guess they even patented fixed bias or something.

                      And a host other "discoveries"
                      Juan Manuel Fahey

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                      • #12
                        Originally posted by pdf64 View Post
                        It’s just that I recall a report that when investigated, the leads to the inner and outer anode came from the same point, the start or end of the winding. Rather than the inner leads being taken from taps along the primary winding.
                        ie thereby making the whole simul class thing something of a nonsense, which existed only as a patent rather than being implemented in real world amps.
                        I recall that as well. I think it was for early simul-class. And thus invalidated the whoe patent thing.
                        I believe someone on one of the other forums had done an autopsy on the OT.
                        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                        • #13
                          Just did a Simu Class Satellite Same set up with an ultra linear output trans.

                          If I remember right, it's so you can run 6L6's in the hot sockets and EL34's in the cold.

                          Simu Class!
                          Get it?
                          Ha, ha.

                          But most just run all 6L6's.

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                          • #14
                            Below is the original simul class OT data (part # 562003). Later the stack thickness was reduced and later (mark IV B for example, part # 562004R-1) they made it without the additional taps.
                            The simul class schematic is usually wrong because the 6L6 pair connects to the outer OT taps.

                            Iron EI96x47mm, laminations 0,35мм, 2 laminations per row

                            Winding №1- 2 layers (37 + 37 turns), wire thickness, 1mm
                            Winding №2 2 layers (94 + 110 turns), wire thickness, 0,3mm
                            Winding №3 7 layers (118 turns), wire thickness, 0,3mm
                            Winding №4 1 layer (43 turns), wire thickness, 0,9mm
                            Winding №5 7 layers (118 turns), wire thickness , 0,3mm
                            Winding №6 2 layers (94 + 110 turns), wire thickness, 0,3mm
                            Winding №7- 2 layers (37 + 37 turns), wire thickness, 1mm

                            Total – Primary turns 1652-2060 / Sec 4 Ohm – 74 turns, 8 Ohm – 117 turns
                            All wires’ diameters include the wire enamel ( insulation).
                            For a single layer insulation the copper only diameter will be as follows:
                            0,28mm/0,85mm/0,95mm
                            For a double layer insulation the copper only diameter will be as follows:
                            0,25mm/0,8mm/0,9mm
                            Turns ratio for the 4 Ohm secondary 1652/74=22,3 2060/74=27,8
                            Reflected primary impedance: 22,3^2=1989, assuming COP of 90% ~2210 Ohms
                            27,8^2=3091 Ohms, assuming COP of 90% ~3435 Ohms

                            Part #562004

                            Primary:
                            A1 6L6 Blue
                            A1 EL34 Blue/White
                            B+ Red
                            A2 EL34 Brown/White
                            A2 6L6 Brown

                            Secondary:
                            0 Ohm Black
                            4 Ohms Green
                            8 Ohms Yellow




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                            • #15
                              Sorry, what is COP?

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