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Bassman AB165, bass control is hardly working

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  • #16
    Thanks G1,

    Just getting back to the amp now.

    Yeah - ground from stack to chassis is good.
    I also changed out pots - Bass, treble, and even volume.
    I've tried to jumper past the hook up wires going directly from the tone pots to the tone/coupling caps - no difference.
    The bass control on the bass channel "does" have a small effect, especially with the deep switch engaged. But, it is so subtle it is almost imperceptible. The normal channel's bass control is very effective.

    The amp needs a recap job - I'm thinking about going ahead and taking care of this to see if has any effect on my bass channel bass control issue, but I kind of doubt it.
    But, its a mystery I have to solve or I won't be able to have a good nights sleep! :-)

    Comment


    • #17
      How about on the bass pot, there is a 8.2K from right (from front) lug to ground? The pot has good mechanical ground connection (not loose or corroded where meeting star washer/chassis) ?
      There is also a bit of wiring under the board in that circuitry, so those connections need to be verified as well.

      There is a layout on pg.2 of attachment. Dotted lines show where wires go under board.
      Attached Files
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      Comment


      • #18
        I've got one in good working order if you need anything I can measure with a meter... Only changes are 220k feedback resistors @ 6L6s removed & a Twin Reverb OT. Lemme know if I can help.

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

        Comment


        • #19
          Can You post a picture how you tone stack is wired ?
          "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

          Comment


          • #20
            Tomorrow, sure. I'll see what I can do with just my phone.

            Justin
            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

            Comment


            • #21
              The 8.2K resister on the bass pot is supposed to set the midrange in the circuit, I believe. Mine measures correctly (I removed the resister to check). I also substituted another working pot with no difference. The ground connection for both the original and the substitute were good.
              I did verify the wiring connections running under the board. I also tried bypassing them with jumper wires (because I can't get under the board unless I unsolder a bunch of wires).

              Justin - I wish I knew what to ask you to measure...

              catalin - My tone stack is wired just as in the layout.

              Comment


              • #22
                It´s very possible from the first moment that the circuit works as it is.
                The use of the 0.1 capacitor in the mid control shifts the center frequency down, being very close to the dominant bass frequency. The result is that the effectiveness of bass control should be limited to a few decibels and in a very narrow frequency range.

                EDIT: I include a graph that shows more or less that effect regardless of the large bypass capacitor in the anode resistor. First channel with the DEEP activated (if I remember correctly it is usually with the selector down, not up as in the brightness). With the selector up (DEEP OFF) it should be even worse.

                Click image for larger version

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                Last edited by Pedro Vecino; 02-10-2020, 01:32 PM.

                Comment


                • #23
                  Philington,

                  WHAT model of Bassman are YOU working with?

                  Justin
                  "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                  "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                  "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                  Comment


                  • #24
                    Originally posted by Justin Thomas View Post
                    Philington,

                    WHAT model of Bassman are YOU working with?

                    Justin
                    First channel in Bassman AB165.

                    Comment


                    • #25
                      According to Justin in post #5 (and what I've read elsewhere), both bass controls (normal and bass channels) should be very effective.
                      I wrote this on post #16 above - "The bass control on the bass channel "does" have a small effect, especially with the deep switch engaged. But, it is so subtle it is almost imperceptible. The normal channel's bass control is very effective."

                      I am working with a 1967 AB165 Bassman, which appears to be stock - except the bias cap looks like it might have been replaced.
                      It appears just like the schematic/layout posted by "g1" post #17.

                      Comment


                      • #26
                        Okay. Gimme a few minutes to get mine aarr & I'll see if I can put up pictures from my phone.

                        Justin
                        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                        Comment


                        • #27
                          Well, my memory failed me. I had modded mine in the Bass Instrument channel. But this was a recent addition, which I'd done to cut some of the lows out for guitar use since I use the Normal channel for bass. But, I do remember even before modding that the Bass controls were bot MORE than minimally effective. I'm not sure how effective the Deep switvh was, as I never use it. So, lemme see if it'll let me post pics...

                          I'll mod mine back to stock & try it out then. Might take a few days to get back to you as my cab is elsewhwre, but, science!
                          Attached Files
                          "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                          "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                          "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                          Comment


                          • #28
                            The only other thing I can think is to check continuity if you haven't already from the tone stack wires to the respective caps. They aren't completely under the board but it doesn't take much to have a break. Otherwise I'll check mine out ASAP, now that I modded it back to stock .1s everywhere in that channel.

                            Justin
                            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

                            Comment


                            • #29
                              The cap values are different between the normal and bass channel.
                              It is easy to check what Pedro mentioned in post #22, swap the 2 caps that are different between channels.


                              Click image for larger version

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Views:	1
Size:	68.2 KB
ID:	856704
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment


                              • #30
                                I don't believe there is anything wrong with the .1 caps on the bass channel. That is the original design.
                                I substituted two new .1 caps coming off the bass pot - no difference. I substituted .047 caps - no difference.

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