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Calling all Tube Techs -- Crazy NAT Single Ended Triode amplifier

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  • Calling all Tube Techs -- Crazy NAT Single Ended Triode amplifier

    I am an electronics Tech with far more experience in solid state
    and digital circuits, than with vacuum tube equipment.
    Customer brought in an
    NAT model # "Single" It is a high-end SET Stereo Integrated amp.

    I gave up on it recently. Each channel uses a single #805 Triode
    driven by a 6H1N dual section tube.
    I have NEVER seen any tube amp with negative 530 volts on the GRID
    of the #805 tubes and the PLATE (which shows negative 20 volts) connected thru the
    output transformer PRIMARY to CHASSIS/CIRCUIT GROUND.
    The filament supply is a only a bridge rectifier ...unfiltered and providing 10 volts.

    On google images I don't see ANY schematics showing this scheme
    or that any manufacturer has ever done this.
    I have always seen a high POSITIVE voltage on the plates of tube amps.
    I don't know if a tube amp can actually work with the plates of
    the output tubes referenced to ground.

    Both channels only produce about 3 watts RMS into 8 ohms.
    This is a TWIN AMP / completely discrete design.
    The only channel commonality is the line cord, the power switch and power transformer.
    Manufacturer (in Serbia) promotes this as a 50 watts per channel amp.

    There is a huge amount of sine wave signal on each output tube grid but
    very weak at the plate. I don't see any feedback loop problem.
    My opinion is the amp was made this way.....a mistaken design.
    I have some dis-jointed schematics of this amp but they
    are too simplified and hugely unhelpful.

    The amp does play thru speakers ....so the customer did not know
    the output was so weak. He only brought it to me to replace
    frayed anode wires and a couple of broken RCA jacks at the CD input.

    I would like to refer him to someone local in South Florida with more tube
    experience than I have and that he can bring this 110 pound amp to.
    Customer will spend good money to repair, since this amp is worth
    something like 20,000 dollars.
    I can't imagine him shipping this extremely heavy thing anywhere.

    Thank you for your time ! Anyone in South Florida interested in tackling this
    please contact me at : roy-lakes@comcast.net

    =============================================

  • #2
    Hi Roy, we already have this post on your other thread. We are all the same people reading these sections. I suggest we stick to one thread about the amp.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      He originally posted in Tubes, but that one got moved to Other Amps at about the same time I suggested he might have been better served posting in this board, hence the two threads. And, I believe he is new to forum posting in general.
      It's weird, because it WAS working fine.....

      Comment


      • #4
        Fair enough.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Problem Solved

          Thank you Enzo....Yes I did post here when Randall suggested maybe I had posted in the wrong place the first time.
          Sorry... I am new to this. Never been on a forum before.

          Also I wanted to let everyone know I solved the problem.
          I have never worked on car amps or any amplifier where my scope and audio generator couldn't be grounded to the AC Line.
          Because unlike the original NAT SINGLE model that has 600 volts positive on the #100TH Tube plates....
          the NAT Single mkll references the PLATE of each #805 output tube to the chassis ground thru the output transformer.
          I suddenly realized that my AC Power Line GROUNDED test equipment might be causing an interaction when connecting to the NAT Amplifier.

          Lifting all the GROUNDS on the AC Power Cords of my scope and audio generator I could then measure about 32 watts RMS / channel at 8 ohms.
          (or 45 watts PEAK / Channel) The RCA Receiving Tube Manual data sheet indicates this is normal output, given the Negative 550 volts applied across the #805 output tube. The 550 Negative volts is correct per the schematic and that is what I measured. What thru me off was the PLATE
          of the #805 tubes referenced to ground thru the output transformer primary, something I had never seen before in any tube amp.
          The input jacks, chassis and black speaker output posts are also star grounded at the same point as the output transformer primary.
          When Enzo so wisely indicated that this odd connection of the SET output tubes would actually work I suddenly realized what was happening.

          Since I am new to forum posting I am wondering if I need to close or end the thread and I don't know how to do that, or just leave it be,
          now that the problem with the amplifier is solved. Thank you all.

          Comment


          • #6
            So glad you figured it out, I would have probably missed that.

            No need to close a thread, it automatically falls away into the searchable archives as it ages with no additional input. Your good work may help someone else in the future.
            It's weird, because it WAS working fine.....

            Comment


            • #7
              This is what was mentioned in the other thread, and why it is better to have only one thread going for a particular topic.
              Glad you got it figured out.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                Link to other thread: https://music-electronics-forum.com/...ad.php?t=49678
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment

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