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musicman rp-65 bias

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  • musicman rp-65 bias

    Anyone know how to adjust the bias on a Musicman 65? Looks like they control it through 2 transistors in the cathodes of the 6l6's.

    At idle, the centertap of the OT measures increadibly low like 4ma, however the amp behaves rather normally.

    I'm thinking this arrangement somehow tracks the bias through it's power range, so you don't need to cook the tubes at lower power levels...not sure.

    I see it is basically set up as cathode bias with a lower positive voltage on the grids than the cathode equating to about -40vdc between g-1 & K.

    That sounds about right, but I don't quite know where (how) they want you to adjust this 'tracking' circuit', if that is indeed what it is.

    thanx, g

  • #2
    If you're working on a Music Man this will be your best resource:

    You need to identify the amp by "chassis number", probably stamped on the serial number plate if I remember correctly. Possibly 2165-RP or 2165-RD.

    Somewhere in that site there is just about everything you'd ever want to know about those unusual amps. If not in the documentation then in the forums.

    Have fun!


    • #3
      I believe it's 25 mv's across the 3.9 ohm emitter resistor Glen. Of course you can increase it to your liking but yeah your right that it's with respect to the Cathode but I wouldn't say it's Cathode bias but more of a Cathode injection or current source. Very low current flow at quiescent but supplies more as it's asked for it or tracking as you say. The Musicman sight is pretty good but I've seen some differences of opinions mainly on the HD130 that were a little off and the sight is under construction so the official pages of correct bias are missing but I'm sure someone like Enzo has the original Music Man aka Ernie Ball biasing procedure. I've seen it and remember 25mvs. FWIW everyone of these I get have the original silver Mallory's under the hood and blue type on the main board and they have to be 40 + years old and some still sound pretty good but starting to hum and do freaky noises.


      • #4
        Looks like Amp Kat is right on the money (good memory!).

        I did find this through the forums, though parts of the site do appear to be under construction:

        Based on my (not all that recent) experience you should also check the values of the zener dropping resistors for the low voltage supplies. I seem to remember having to replace those in many of the amps I saw due to large drift in value. Ran across a few out-of-spec zeners too.

        Edit: Aha - found the good stuff:
        Last edited by Mark Black; 03-29-2007, 07:42 PM.


        • #5
          BEAU-TI-FUL! Just what I needed. That is a strange ckt. Thanx too for the history regarding those dropping resistors. Always nice not to have to learn via a rework.
          It still amazes me how many manufacturers use so much current & generate so much heat instead of just using a simple 3 legged regulator...even these days.



          • #6
            The reason these are so cold it the amp is running pretty much in class B. They idle near cutoff. They idle at about 6ma.

            The adjustment is 25mv across the 3.9 ohm emitter resistor. If they are not equal, set the lower one to 25mv as long as the higher one does not go over 55mv.

            Note: use only 1458 op amp to drive this. Do not use 4558 or others.
            Education is what you're left with after you have forgotten what you have learned.


            • #7
              Thanx all the info from the Musicman link that Amp kat supplied. Interesting design...I'll bet those tubes last forever idling in class B. Do you know if there was any inherent problems with this design? Seems like a good idea at first blush...g


              • #8
                Welcome to the world of Class B output circuits Glen and pity the tech who tries to adjust the idle bias current as if it was a class AB1. I don't know if you would call it an inherent problem but performance will degrade faster as tubes age and anything else happens to change the bias setting. This is more of a problem for Hi Fi amps because the cross over distortion can quickly increase. On the other hand, you can get more power, like up to 70Wrms out of a pair of 6L6s. That Musicman site is a lifesaver.


                • #9
                  The circuit in question is a cascode. The transistor is a gain stage and output tubes are working in grounded grid. This circuit offers high bandwidth and low distortion.


                  • #10
                    OOOO, grounded grid...I can relate that back to my schooling on common base ckts. As I recall low input impedance, but as you mentioned increased bandwidth. Interesting...g


                    • #11

                      Here's a bit on cascode circuits. You get the best of both worlds: high input impedeance and increased bandwidth. But seeing as this isn't a concern for guitar amps, Musicman surely did it to squeeze more power out of two tubes like Mr. Phillips was saying.


                      • #12
                        thought that arrangement would give a lower input impedance...better go back & refresh the memory....thanx, g


                        • #13
                          A common base circuit by it's lonesome has a low input impedeance so your memory is still good


                          • #14
                            Originally posted by Ptron View Post

                            Here's a bit on cascode circuits. You get the best of both worlds: high input impedeance and increased bandwidth. But seeing as this isn't a concern for guitar amps, Musicman surely did it to squeeze more power out of two tubes like Mr. Phillips was saying.
                            Nelson Pass did some work with cascode power amps. Here's his white paper on that work.


                            • #15
                              I always liked that Music Man cascode circuit, from a tech geek perspective at least. (I'm not so sure how it would sound.) I think they did it for simplicity and economy.

                              Like the old saying goes, "If you want high current out of tubes, or high voltage out of transistors, be prepared to pay for it." That means that solid state circuits, when designed for minimum cost, end up having a low characteristic impedance compared to tube circuits. (If you look at a tone stack in a solid-state amp, the pots should be about 1/10 of the resistance, and the capacitors 10 times bigger, than they would be in a tube amp.)

                              The cathode is the lowest impedance point in a tube, and the collector is the highest impedance point of a transistor, so if you're going to connect transistors to tubes, the Music Man cascode circuit is kind of the natural path of least resistance. The resulting composite device has the high gain of a transistor and the high voltage handling capability (and hopefully some of the sweet sounding distortion) of a tube.
                              "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


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