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MusicMan HD130. Tube PI. Upward ramping Bias

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  • MusicMan HD130. Tube PI. Upward ramping Bias

    After months of psychological counselling following the rampant Marshall bias issues of last year, my heart sank when I see this MusicMan HD130 2575 with the same problem.

    This amp has the tube PI in what looks to be a Cathodyne configuration. Oddly, the 10 ohm Cathode resistors are missing, with the cathodes connected directly to ground. As it appears that these are only used to monitor cathode current, I have left the resistors off and monitor the individual currents with my bias probe.

    The bias voltage at the junction of the two 330K resistors is relatively stable at around -42 volts. Its what happening on the pin 5's of all the tubes that is wild. Over the course of a few minutes, the voltage on the pin 5's ramps up from -42 to -22, with the cathode current going from 12mA to a staggering 70mA - with a plate voltage of 700V!! Meanwhile the bias supply is relatively stable at -42 to -40. I thought leaky coupling caps so I replaced the 0.047uF caps.

    I pulled out the still good USA Sylvania's and tried a fresh set of JJ EL34L's. Same. Same. I'm stumped as to what else it could be. There doesn't seem to be anything else in the circuit that would effect the bias voltage at pin 5 that doesn't effect the actual bias supply. The Bias supply is stable.

    Have you seen anything like this before? Any suggestions on where to go from here are much appreciated. Meanwhile, I will try to find that psychologist's phone number. I can recommend him for therapy following Triple Rectifiers, 410C's, SVT's, and more! Only problem for you is that he's in Sydney. :-)

    Music Man 2475-130_and_2275-130.pdf

  • #2
    Maybe leaky (carbonized) tube sockets. Do all pin 5s behave the same?
    - Own Opinions Only -

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    • #3
      Originally posted by Helmholtz View Post
      Maybe leaky (carbonized) tube sockets. Do all pin 5s behave the same?
      i think so too,if it goes less negative and the tubes are good,probably there is leaking voltage from pin 4,some dust,try cleaning with alcohol,monitor the screen voltage too

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      • #4
        . As it appears that these are only used to monitor cathode current, I have left the resistors off
        Wrong.
        They are not to "monitor" cathode current but to *set* it.

        And forget "voltage bias" ; grid voltage is set **strongly** positive so tube on its own passes all the current it can and then some, and cathode transistors decide how much.

        You remove the sensing resistors, drive transistors have no clue on actual tube current and it drifts anywhere it likes.

        Usual "tube amp knowledge" does not apply much here, because this game uses very different rules.
        Juan Manuel Fahey

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        • #5
          Yup... They used a system that can squeeze every last bit of life out of the power tubes; Peaveys of the era did the same. I'm not familiar with them so I pass on them if people ask. Plus my meter only goes to 630... I know when I'm beat.

          That said, their system is quite good at doing its intended job.

          Anybody care to explain it to us?

          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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          • #6
            Are the output tubes fixed biased? If there are just check for a defective tube measuring any voltage over each grid leaks resistors. Check also with the standby sw off to see if any voltage is present over grid leaks resistors. I suspect one of the power tube is broked
            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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            • #7
              This is the 2575 model. It is a conventionally grid one fixed bias driven design. The schematic is almost unreadable but it looks like the two 10 ohm resistors might be connected in parallel at the node marked "y". The note "1/2VAC" hints that the purpose is to monitor AC operation but they handy for checking DC bias current too, provided the tubes are matched. I'd leave in place. Not that that is the problem.

              I suggest you monitor the g1 voltage with the EL34's out.
              Experience is something you get, just after you really needed it.

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              • #8
                Thank you for the suggestion. I have given the bases a good scrub and the sockets, too. No luck yet

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                • #9
                  Its fixed!! There was a couple of things, and I'm not sure which one was the cause in the end:

                  1. I measured the resistance of each of the grid resistors. All were up a little from the 1K5, to say 1K6, but one had drifted up to 2K2. I pulled them all out and replaced with 1K5 metal film.

                  2. With no tubes in place, but with my bias probe connected I was dropping 3v across the 330K resistors (from -43 to -40V), where I would expect no voltage drop with no tubes installed. I appears that my bias probes were conducting. That's unexpected.

                  3. Because of this, I stopped using the Bias Probes and installed a 1ohm from Cathode to Ground on each tube (remember, someone had removed the 10ohm resistors at some stage in the history of the amp).

                  I powered up the amp, and it seems rock solid on the bias. I set the bias around 20mA +/- at 695v. So in the end, because I did so many things between tests, I'm not 100% sure if it was leaking coupling caps, crappy grid stoppers or the missing cathode resistors. Either way, my mouth is smiling and my ears are ringing.

                  So how do you recommend I charge the customer? If I went straight to it and did those tasks, I would have taken 2 hours with caps and resistors. With the testing and searching it would be double that. How do you charge for this? Do you charge for the testing time, or just the time on the actual repair?

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                  • #10
                    Sorry, I had thought you had the "normal" MusicMan which is cathode driven, now I searched for that spcific model and found it´s a "conventional" Tube driven one, so yes, you can have leaking coupling caps, etc.

                    Tubes are abused like on the other one, so I guess it will be very hard to get modern over the counter tubes to work there reliably.
                    Oh well, glad you solved it

                    EDIT: to each his own but in this case I would straight charge the "real" 2 hour repair time.
                    Anything extra, assign it to improving your knowledge so next time it takes "just" 2 hours.

                    In fact, next time same problem "should" be made within 1 hour
                    Juan Manuel Fahey

                    Comment


                    • #11
                      Thank you for your advice on how to bill for such repairs, Juan. I can knock out most repairs in good time, but sometimes I will spend a day, sometimes more, on a single job which I can't charge a customer for. I write that time off as my apprenticeship, although at my age I am closer to retiring than being a young apprentice.

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                      • #12
                        Originally posted by christarak View Post
                        Its fixed!! There was a couple of things, and I'm not sure which one was the cause in the end:

                        1. I measured the resistance of each of the grid resistors. All were up a little from the 1K5, to say 1K6, but one had drifted up to 2K2. I pulled them all out and replaced with 1K5 metal film.
                        A drift to 2.2k won't cause the problem you saw. OTOH that was just a one sample data point, the resistor might be open at other times.

                        2. With no tubes in place, but with my bias probe connected I was dropping 3v across the 330K resistors (from -43 to -40V), where I would expect no voltage drop with no tubes installed. I appears that my bias probes were conducting. That's unexpected.
                        No, it should be expected. Your meter has a finite input resistance and that current causes a drop across the 330k.

                        3. Because of this, I stopped using the Bias Probes and installed a 1ohm from Cathode to Ground on each tube (remember, someone had removed the 10ohm resistors at some stage in the history of the amp).

                        I powered up the amp, and it seems rock solid on the bias. I set the bias around 20mA +/- at 695v. So in the end, because I did so many things between tests, I'm not 100% sure if it was leaking coupling caps, crappy grid stoppers or the missing cathode resistors. Either way, my mouth is smiling and my ears are ringing.
                        Glad it's done but if you don't know why, what do you think you learned?. What would you do differently next time?
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #13
                          Thanks Nick. This self-apprenticeship is very hard and frustrating at times. Imagine if I had learned from people like you! I never suspected my meter (Fluke 87V) to offer sufficient load to cause a voltage drop. By the end of the day, I was frazzled and just wanted to get the job finished. But I hear what you're saying. With 3 strategies tried between tests I don't know for sure which one was the final villain in this case. In the future I would change-test-change-test. Having read you comments again, I am now left wondering if I have really solved the problem.

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                          • #14
                            Originally posted by christarak View Post
                            I write that time off as my apprenticeship, although at my age I am closer to retiring than being a young apprentice.
                            Oh, in this job you can NEVER stop learning,or you are left behind like yesterday´s paper.

                            I am legally retired since December, yet I will keep the shop open until physically impossible.

                            Worst case, will close it but stay in a couple Forums, at least to keep the mind working.

                            Sure beats playing cards and Bocce, and feeding corn and bits of bread to the pigeons at closest public square.
                            Juan Manuel Fahey

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                            • #15
                              I wonder if a tube is just running away?
                              The reason I ask this is the vast difference in idle dissipation between the grid drive and cathode drive versions of this amp.
                              Both versions have around 700V plate, 350V screen.
                              Grid drive version idles (spec) at 25mA per tube (17.5W or 70%)
                              Cathode drive version idles at 3mA per tube (2.1W or 8%)
                              I don't think the big difference would just be due to cathode drive? Maybe they were finding that the tubes didn't like the high voltage, and I think it would be an even bigger problem with modern tubes?
                              I sure wouldn't feel comfortable running the grid drive version that hot.
                              Attached Files
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


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