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

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

    Hello and thanks for looking. This amp just might be designed like no other I've seen. I've been reading the schematics, studying up on old posts, read "lessons"...I've also read/posted to the MM forum which seems like a great resource. I'd like to contribute a schem/voltage chart to the forum for future reference/people going down the rabbit hole I've been stuck in for a while. Here's the first pass:



    The complaints on this amp were blown speaker sound and volume loss. I feel like I've shored a lot of that up. Going rounds I was thinking filters, tubes, OT's...but the related issues I found were intermittent impedance selector, scratchy deep switch with frequency vibration, maybe actual speaker rub though I haven't seen them yet.

    I've tested, A/B'd or updated way too many components, in a good learning curve way. The dc voltages now look pretty correct (replaced 22v zener). I'm still going to do the filters, some electrolytics.

    The problem is the volume doesn't get going until the Master is on say 8-10. I think this is a part of the original complaint but I'm not quite sure. It'll still clock around 130w RMS, 175w cranked. Seems pretty good for old filters and existing tubes. But it's much lower volume from say 1-7 on the Master. All the taper is at the tail end. Is it broke? Is it design? I feel like I'm in some gray area.

    This is where some expertise on these models can be of assistance. How does the schem/voltage chart look? Anything jump out as funny business?

    The input is 680mv, volume on 6.5, master on 7.5, AC in red, DC in green. Anything circled has been tested, A/B'd or replaced.

    More gray area, I've juggled the tubes, done one at a time in different positions, old tubes, new tubes, I don't ever feel I can get the emitter/3.9ohm voltage to quite balance (25mv, the other side is always higher, say 10-15mv). I've rebuilt most of the driver board trying to get this balanced. It does sit in the 25-45mv guideline. I'm saying I can put the same single tube in socket 1 or socket 4, it reads different. And I've also addressed all the circled components. Am I overlooking something?

    Any feedback or similar experience on this one much appreciated.

  • #2
    For your uneven idle currents, check the voltage at pin4 of all the power tubes. Uneven screen voltages will cause uneven idle voltages at those emitter resistors.
    You are getting way too little out of the preamp I think.
    I have a 2275-65 on the go, but I won't have a chance to check for awhile, maybe by tomorrow. I can see what your settings should give at the preamp out.
    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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    • #3
      I have a 2275-150 here at the moment so if you want to cross check anything specific let me know.

      I think the behavior of the master is by design. A 100K reverse-audio pot has been used and it's shunted by 8.5k ( 10k in parallel with 56k). If you wanted change it you should replace the 100K pot with a dual 10K log one and remove the two 10k resistors R62 and R63.
      Last edited by nickb; 06-05-2019, 08:11 PM.
      Experience is something you get, just after you really needed it.

      Comment


      • #4
        Thanks nickb, this is the gray area I'm not so sure about. The pots definitely seem a little slow on the dial. I'm also wondering if there's low preamp gain as g1 mentioned. With volume and master around 6-7 each is yours very loud? This one's definitely loud mv on 10.

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        • #5
          I suggest you feed 200mVpp 1Khz sine wave in the normal channel, set the tone controls mid way and measure the signal at the top of the master. I'll do the same (tomorrow now) and we can compare.
          Experience is something you get, just after you really needed it.

          Comment


          • #6
            Right, that would be great. Did you by chance see the link above? I've tried to mark the sine wave voltages from beginning to end if that cuts to the chase.

            Comment


            • #7
              Originally posted by antrecords View Post
              Right, that would be great. Did you by chance see the link above? I've tried to mark the sine wave voltages from beginning to end if that cuts to the chase.
              I confess I did see it but I missed the "5" next to the tone controls and 6.5 next to the volume. 690mV is too big an input though. Can you redo with 200mVpp (70mVRMS). I'll use the same settings and see what I get.
              Experience is something you get, just after you really needed it.

              Comment


              • #8
                Sure, let me comb through again and we'll see how it's looking. Thanks for doing that.

                Comment


                • #9
                  With 204mv on the input jack I get 770mv coming out of the master volume

                  200mv input
                  tone controls on 5
                  volume on 6.5
                  master volume on 7.5
                  grids and screens on the output tubes are 21.6v x4, 349v x4

                  Here's an updated schem with detailed voltages:
                  https://i.imgur.com/EUTpvBq.jpg

                  Comment


                  • #10
                    I had to reduce the input to 100mvpp (35mVRMS) to prevent clipping. It looks like you might be confusing Vpp and Vrms measurements on your sketch. Here is what I see (NORMAL) channel.

                    Click image for larger version

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                    Grids 21.7V
                    screens 362V ( different wall voltage )
                    cathodes 69.6V

                    You should redo to avoid clipping using 35mV RMS in. Are you using a true RMS meter or a scope?
                    Last edited by nickb; 06-06-2019, 07:16 AM.
                    Experience is something you get, just after you really needed it.

                    Comment


                    • #11
                      Much appreciated nickb. Yes those levels were way too hot, below is 35mvRMS. I think I was output testing, trying to get something going. I just use a Fluke true RMS, it usually matches Fender schematics pretty well etc. I have an old Soar scope I use, more for visual inspection.

                      CH 1 Normal

                      IC-1
                      pin3 32mv
                      pin1 163mv
                      pin5 37mv
                      pin7 236mv

                      IC-3
                      pin2 1.1mv
                      pin1 2.6mv

                      Master Volume (7.5)
                      in 2.6v
                      out 400mv

                      Channel 2 Reverb

                      IC-2
                      pin3 32mv
                      pin1 167mv
                      pin5 37mv
                      pin7 236mv

                      IC-3
                      pin2 1.4mv
                      pin1 2.6v

                      Master Volume (7.5)
                      in 2.9v
                      out 227mv

                      IC-8

                      pin6 .13mv
                      pin7 246mv
                      pin2 1.7v
                      pin1 245mv

                      Comment


                      • #12
                        The input to the master is about half in your case and the output is substantially less. Does it look like the MV pot might have been changed? Seems that either it is bad or the load is much lower than it should be. Some resistance measurements around the MV will give you more clues.
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #13
                          I just took the pot out and it's 100k (from '79 I might add). I measure almost 1k to ground at the master vol out, and it appears R62 and R63 are 1k's (my schem says 10k)...

                          Click image for larger version

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                          • #14
                            Yup. They sure look like 1K to me too. The easy solution is just to clip thru one lead of each of the 1K. I think that will give it a much better feel anyway. I don't see that they serve any useful purpose as the opamps are JEFT input types with very low bias currents.

                            You also have the option to change them to 10K or do what I suggested earlier to get a conventional log response.
                            Experience is something you get, just after you really needed it.

                            Comment


                            • #15
                              I think step one I'll snip or raise the resistors R62 and R63, and see if that opens things up a little. These amps would not sound the same side by side/same settings. That's exactly what I meant by grey area, how's it supposed to sound (at that setting)? Talk about going in circles lol. I'll check these voltages again after, maybe you can tell me if they look more like yours. Thanks.

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