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Marshall 2103 Cold Maximum Bias Mystery

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  • #31
    Originally posted by greengriff View Post
    Thanks for the detailed input everyone. I'm busy all day today but hopefully I'll get some time tomorrow to run through all the suggestions and report back. I've ordered 240k and 270k resistors anyway (the one I connected with crocodile clips temporarily was a 250k guitar volume pot so not exactly viable as a permanent solution ;-)

    @Chuck thanks for the tips re: soldering in the piggyback resistor. It's going to be a tricky job as there's not a lot of the original leads sticking out of the boards, what there is is partially covered with plastic that's bled over from the resistor body, and I don't have particularly steady hands!
    Totally confused by this - Chuck was referring to the bias supply for a 50W model, which is derived from the HT winding. Whereas your 100W ampís bias supply is derived from a dedicated winding. The resistor value between winding and rectifier diode is 10x different for the different designs.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #32
      I think he's now talking about just piggy backing another resistor (soldering in parallel on the exposed resistor leads) across the 56k resistor. I've seen this done A LOT.
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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      • #33
        Originally posted by greengriff View Post
        It's going to be a tricky job as there's not a lot of the original leads sticking out of the boards, what there is is partially covered with plastic that's bled over from the resistor body,..
        ??? I've never seen plastic bleeding off a resistor body so this is confusing (and concerning) me.

        EDIT: I think you must mean that the enamel the resistor body is covered in is also partly covering the leads? It took me several mental images to put that together. But yes, I've seen that before.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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        • #34
          ...
          Attached Files
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

          Comment


          • #35
            Originally posted by Chuck H View Post
            ??? I've never seen plastic bleeding off a resistor body so this is confusing (and concerning) me.

            EDIT: I think you must mean that the enamel the resistor body is covered in is also partly covering the leads? It took me several mental images to put that together. But yes, I've seen that before.
            Yes that's exactly what I mean. My poor explanation.

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            • #36
              Originally posted by pdf64 View Post
              Totally confused by this - Chuck was referring to the bias supply for a 50W model, which is derived from the HT winding. Whereas your 100W ampís bias supply is derived from a dedicated winding. The resistor value between winding and rectifier diode is 10x different for the different designs.
              I think half the problem is the way this forum sorts the threads. To summarise: I intend to piggyback whichever one of those 240k or 270k resistors allows me to get closest to a value of 47k onto the 56k resistor currently on the PCB. 47k is the value fitted in that spot in the 2203x reissue and it will allow me to bias the amp properly. As it is now with the 56k resistor there and the trim pot maxed out I can only get to around 16ma per valve. At 47(ish)k I could get into the mid 30s ma no problem. (I achieved this temporarily by crocodile clipping a 250k guitar volume pot that I had laying around to the 56k resistor).

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              • #37
                I think half the problem is the way this forum sorts the threads.
                The forum offers different thread display modes. I use and suggest the linear display mode. To check, go to Settings\My settings\General settings and scroll down to display modes.
                - Own Opinions Only -

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                • #38
                  Originally posted by pdf64 View Post
                  What are the voltages at the screen grids pin 4? And control grids pin 5 (to avoid oscillation it may be bet to take those measurements with the ECC83 in the phase splitter socket removed)?
                  And the bias supply output, at the node of the 220k bias supply output and grid leak resistors?
                  Maybe its just a matched set of weak EL34s, any others to try in there?
                  Pin 4: 466v
                  Pin 5: -39v
                  Bias supply output: -48v

                  Do these sound about right?

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                  • #39
                    That applies to all 4, and anodes still at 475Vdc?
                    If so that all looks fine
                    My band:- http://www.youtube.com/user/RedwingBand

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                    • #40
                      Originally posted by pdf64 View Post
                      That applies to all 4, and anodes still at 475Vdc?
                      If so that all looks fine
                      That's reassuring, thank you. Yes anode voltage is still 475vdc. I didn't check all 4 sets of pins. Do you think it's worth doing that? Cheers!

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                      • #41
                        I didn't check all 4 sets of pins. Do you think it's worth doing that?
                        Never think up reasons not to check something. It takes a very few seconds to go down the row of tubes checking voltage.


                        Any time I am in a tube amp, I go down the power tubes checking for B+ on both plate and screen pins and for bias on the grid (or cathode).
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #42
                          Originally posted by Enzo View Post
                          Never think up reasons not to check something. It takes a very few seconds to go down the row of tubes checking voltage.


                          Any time I am in a tube amp, I go down the power tubes checking for B+ on both plate and screen pins and for bias on the grid (or cathode).
                          Makes sense I guess. Aren't the plate voltage connections for all 4 power valves daisy chained off one wire? I'm not looking at the amp now so I can't be sure.

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                          • #43
                            Even if something all comes from the same source, checking at each tube verifies all connections like sockets, etc.
                            "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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                            • #44
                              Most important is that all 4 tubes' idle currents are close. A different voltage at one tube would show in a different idle current.
                              Last edited by Helmholtz; 06-07-2020, 11:36 PM.
                              - Own Opinions Only -

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