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Music Man HD-150 2275 Gain Stages, Voltage Chart, blown speaker sound, volume loss, switches

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  • #16
    Glad to hear you found it. I realized my numbers wouldn't mean much when I saw it was a 150 you had, not a 130.

    As far as the idle current imbalance, did you check that the 3.9ohm resistors are both on spec.? And which driver transistor type did you use? Were they both new from the same batch?
    "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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    • #17
      Originally posted by g1 View Post
      Glad to hear you found it. I realized my numbers wouldn't mean much when I saw it was a 150 you had, not a 130.

      As far as the idle current imbalance, did you check that the 3.9ohm resistors are both on spec.? And which driver transistor type did you use? Were they both new from the same batch?
      Different beasts eh? It's the thought that counts, I appreciate you chiming in. Yeah, I stuck new 3.9's at some point, used 2N6488s, right now it has SK3188A's new from the same batch. Could just be dumb luck that I'm plugging in higher numbers in one side I suppose (transistor, tubes, resistors). Swapped all the 470s, A/B'd the 1458 with another. I mean, it's within the bulletin's tolerance, just thought I could get it closer.

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      • #18
        If you really want to get to the bottom of it, you could swap the transistors and see if the higher current side stays with one transistor, or if the difference is elsewhere in the circuit.
        "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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        • #19
          Any any difference in the voltage across the 3.9 ohms resistors is primarily due to transistors differences. If a 3.9 was out of tolerance, the tube were different etc the idle current would be different but the voltage would be about same. It would affected to only to small extent by the Early effect of the transistors.

          The negative feedback is very heavy in this design which is why it operates virtually in class B. For that reason a small difference of bias current is of no concern.

          If you really want it the same you will need to split the bias on to two bias trimmers.
          Experience is something you get, just after you really needed it.

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          • #20
            A few breakthroughs today...

            I just couldn't sleep well knowing I had juggled almost the entire driver board and single tubes and could never quite get a great pairing. Do you guys get like that (rhetorical, I think I know the answer)? Today I pulled those transistors (and swapped 1k shunts for 10k's, filter caps, a few other electrolytics). When I stuck them on my meter the SK3188A's measured 77, 47hfe. Then I measured the original 2N6488's (I assume stock) they measured 75, 24hfe. Well...

            I went through my batch and found a 81hfe to go with the 77 (SK3188A). I realize these numbers have a curve (and have read your prior posts more than once, great insight). Now the reading at the emitter/3.9ohm is 25.0mv, 25.6mv.

            Also, plate voltage is up to +470v low power, +707v high power, and seems to have a little more punch. A quick output check was around 144 RMS, 200 peak @8ohms

            I'm still not crazy about the master volume taper, may change to 10k dual down the road (per your suggestion). Most of the volume is at the tail end. I'm assuming this is by design.

            I did re-measure signals through the amp and will repost that. If it looks good, great, or if we need to make changes we will.

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            • #21
              Updated numbers

              Here are the signal voltages now (in blue):

              https://i.imgur.com/vYsX6PH.jpg

              Seem about right?

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              • #22
                Follow Up

                Cap job- I finally rounded up the needed parts to update the small caps, large caps have been done too. The signals are marginally stronger throughout the amp. The electrolytic coupling caps should be improved. The signal coming in and out of IC8 is now 871mv:970mv (it was 824mv:280mv), big difference.

                Here's a schematic/voltage chart with where this amp stands now, with new electrolytics, many updated resistors, different/matched driver transistors, master volume load changed from 1k to 10k (x2), misc odds and ends and hours and days .

                Click image for larger version

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                https://i.imgur.com/tDoqi7M.jpg

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                • #23
                  Originally posted by antrecords View Post
                  Here are the signal voltages now (in blue):

                  https://i.imgur.com/vYsX6PH.jpg

                  Seem about right?
                  Sorry. I didn't get a chance to follow up on this. The one I had is gone.
                  Experience is something you get, just after you really needed it.

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                  • #24
                    No problem, I really appreciate all your insight. Invaluable. It would've been so much easier if it were smoked or dead, it was definitely mostly working (sans imp switch). But upon opening this box I feel like there were all these little things that had a cumulative negative impact on the signal. It's been an excellent learning curve on a (somewhat) rare amp with a fairly unique design.

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