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Bassman rebuild/hotrod

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  • Bassman rebuild/hotrod

    A friend brought me this 1970 Bassman to make operational again. He wants some modifications to add versatility.



    So, down the rabbit hole I went and came up with a preliminary design.
    I would like to submit it for review by the panel of experts.
    I don't see any mechanism for attaching files, how do I post a schematic for your viewing pleasure?

  • #2
    Open the reply box and type your message. Click on GO ADVANCED in the bottom right.

    Scroll down to attachments.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      Open the reply box and type your message. Click on GO ADVANCED in the bottom right.

      Scroll down to attachments.
      It says I may not post attachments. Gotta jump through hoops to post pictures, too.
      What's up with all that?

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      • #4
        Originally posted by clarkleach View Post
        It says I may not post attachments. Gotta jump through hoops to post pictures, too.
        What's up with all that?
        Prevents drive by spammers . A couple more posts will likely cure it. Do you have a chassis number on it ?
        Nosaj

        Nosaj
        When I die I want to go quietly in my sleep like my grandpa did. Not kicking and screaming like the passengers in his car.

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        • #5
          Originally posted by nosaj View Post
          Prevents drive by spammers . A couple more posts will likely cure it. Do you have a chassis number on it ?
          Nosaj

          Nosaj
          I didn't come here to lurk. What the heck am I supposed to post?
          Yes, it has a chassis number, but I don't know the relevance.
          It's a 1970 AA270 currently. It's in very rough shape and I'm gonna basically gut it and rebuild, probably point-to-point with the exception of the doghouse, I may buy a board for that.
          I just want to post the schematic so people can maybe point out any problems.

          Comment


          • #6
            Any new member gets a probationary status for the first few posts. We do have any number of visitors who are just spammers. In fact we get bots that try to post stuff here. A couple posts under your belt to let the administration know you are for real and it will all fall into place.


            It won't show your changes, but I attach the AA270 for reference. Hopefully my file will stick.
            Attached Files
            Education is what you're left with after you have forgotten what you have learned.

            Comment


            • #7
              Originally posted by Enzo View Post
              Any new member gets a probationary status for the first few posts. We do have any number of visitors who are just spammers. In fact we get bots that try to post stuff here. A couple posts under your belt to let the administration know you are for real and it will all fall into place.


              It won't show your changes, but I attach the AA270 for reference. Hopefully my file will stick.
              It stuck...
              This will be only loosely based on the AA270/AB165.

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              • #8
                Can you tell us a bit about what kind of things the owner wants to achieve in terms of added versatility?
                "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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                • #9
                  Originally posted by g1 View Post
                  Can you tell us a bit about what kind of things the owner wants to achieve in terms of added versatility?
                  It would be easier to just show you my schematic. He didn't make any specific request other than to suggest fixing the retarded NFB loop and making the bias voltage actually adjustable. Both of these things I planned on doing anyway.
                  And he requested a master volume. Other than that he just wants me to "do my thing"... whatever that is.

                  Other tweaks include:
                  1. Output stage biasing switchable between fixed and cathode bias.
                  2. Push-pull master/Vox-style cut
                  3. Add "RAW" switch to normal channel tone stack.
                  4. Remove 2 input jacks, add switch to normal channel to change input impedance/level.
                  5. Add switch for internal bridging of channels.
                  6. Total redesign of "Bass" channel with "Marshally" input, first stage cold-clipping or Fendery (switchable), use the unused triode of V2 as cathode follower to drive the tone stack, variable slope resistor

                  That's about it... :-)

                  Comment


                  • #10
                    Not sure I am familiar with retarded NFB. I know about retarded tempo.

                    1. Output stage biasing switchable between fixed and cathode bias.
                    In my experience, having done some of that, it winds up being a feature that once they play a while, one way is the favorite and the switch never gets flipped again. I was smart enough to put the switch out of the way on the chassis surface on most of them.



                    Your experience may differ.


                    And I don't mean your address, but it would be helpful in your profile to note what part of the world you are in.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      Originally posted by Enzo View Post
                      Not sure I am familiar with retarded NFB. I know about retarded tempo.



                      In my experience, having done some of that, it winds up being a feature that once they play a while, one way is the favorite and the switch never gets flipped again. I was smart enough to put the switch out of the way on the chassis surface on most of them.



                      Your experience may differ.


                      And I don't mean your address, but it would be helpful in your profile to note what part of the world you are in.
                      Both the AB165 and AA270 share a similar NFB circuit. Maybe stupid is a better term. I'm changing it to the AA864 style.
                      I was planning to put the bias selection switch where the "Ground" switch is.
                      I would add my location if I had permission to access my profile. Lapeer, MI btw

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                      • #12
                        Given the 6L6 feedback is removed from my AB165, I'd call it basically the same as yours. I think it gives a non-Master Marshall a run for its money. Buckers on 5 = more than AC/DC crunch & it gets better from there. I'd say just start with trimming a few caps to cut out the extra lows.

                        If you're gonna "fix" the negative feedback, make sure you switch the plate wires or the OT secondary.

                        Honestly, in their continual attempts to make the Bassman a better bass amp (which it IS), they kinda screwed up & made one of the greatest crunch machines ever...

                        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 -

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                        • #13
                          Ah, so you are in the "thumb". Something that means zero to anyone not from the Great Lakes area.

                          Stupid, retarded, whatever, in my own clumsy way I was kinda asking what is stupid or retarded about it? The idea of NFB is to take the output signal and inject some of it back into the signal path out of phase. There are many ways to do it.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            Originally posted by Enzo View Post
                            Ah, so you are in the "thumb". Something that means zero to anyone not from the Great Lakes area.

                            Stupid, retarded, whatever, in my own clumsy way I was kinda asking what is stupid or retarded about it? The idea of NFB is to take the output signal and inject some of it back into the signal path out of phase. There are many ways to do it.
                            It's just a weird way of doing it. Apparently Fender thought so, too. They never did it that way before or again.
                            "Thumb" didn't mean anything to me until I moved here a year ago. Originally from the deep woods of Maine where the men are men, the women are ugly, and the sheep are nervous.

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                            • #15
                              Yeah, we have that whole mitten thing. I came from Washington DC, and thought it kinda weird when I first got here.

                              When it comes to circuits, they work or they don't. AS to the specific models, the AA864 and AA165 didn't use it but the AB165 did. That was 1965. AC568 still using it, 1968. AA270 still using it, 1970. AA371 used it, slightly changed, 1971. SO really, the early ones used the more conventional arrangement, but the one you don't like was used for at least six years and several model changes.
                              Education is what you're left with after you have forgotten what you have learned.

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