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Traynor YBA-1 Odd Output

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  • Traynor YBA-1 Odd Output

    I have a YBA-1 on the bench (a later '74-ish model), with a brand new Hammond OT (the one they make as a YBA-1 replacement). The amp has an oddly distorted output (see attached). The signal on both input grids looks clean, and I tried replacing the output tubes with no change. I carefully checked the socket solder joints, everything looks OK. Any suggestions?

    At low levels, it looks clipped on the positive side, at the signal gets hotter, the 'divot' in the top of the waveform gets deeper.

    The voltages on all pins look normal.
    Attached Files
    Last edited by mhuss; 02-25-2020, 02:44 PM. Reason: more info

  • #2
    You may be seeing 'positive feedback'.

    Try lifting R33 in the feedback path.
    If the signal then is reduced at the output, reverse the OT primaries.

    Comment


    • #3
      Thanks for the suggestion, I just tried that, no difference (ignoring the extra gain ). I would expect if the primary was reversed it'd be howling.

      Comment


      • #4
        It doesn't have those funny brown sandbox 'protection' devices across the OT primaries does it?
        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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        • #5
          No, just power tube pin 3 connected directly to the primary (x2).

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          • #6
            i would check the OT out of circuit for mismatched wiring

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            • #7
              What are plate and screen voltages both measured directly at the sockets?
              - Own Opinions Only -

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              • #8
                And check the grid and screen resistors, if you haven't.
                --
                I build and repair guitar amps
                http://amps.monkeymatic.com

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                • #9
                  Resistor problems will show from actual voltages.

                  The observed signal asymmetry could be caused by:

                  - a non-conducting power tube (including open connection to one side of the OT primary)
                  - an open screen resistor.
                  Last edited by Helmholtz; 02-25-2020, 04:21 PM.
                  - Own Opinions Only -

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                  • #10
                    OK, I have officially lost it. Alex and Helmholtz are both pointing in the right direction.

                    The primary connections are blue, red, brown. With the primary disconnected:
                    bu-bn = 54 ohms
                    bn-rd = 54 ohms
                    rd-bu = 780 ohms!!!
                    How is this even possible? Has Ohm's law been repealed?
                    I measured and remeasured several times, both polarities. Same result. What the heck is going on?

                    EDIT: When all else fails, read the directions. On this particular transformer, red is the end and brown is center. So much for "following the original color code as much as possible."

                    I connected it with brown to B+ and red to the tube and now it's working OK.

                    I still don't get those resistance measurements, though...
                    Last edited by mhuss; 02-26-2020, 12:43 AM.

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                    • #11
                      Originally posted by mhuss View Post
                      EDIT: When all else fails, read the directions. On this particular transformer, red is the end and brown is center. So much for "following the original color code as much as possible."
                      Why would they do that!?!

                      Ok, but they put it in the literature. But why would they do that!?! What could possibly have been wrong with "red = B+"???
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "If you're not interested in opinions and the experience of others, why even start a thread?
                      You can't just expect consent." Helmholtz

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                      • #12
                        Eh, probably some junior engineer with no history/experience. They let go of the old guy because he was costing too much.

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                        • #13
                          Agreed. Yes it's in the lit, but what about convention? I don't want them wiring my house........ special notes ("green is hot") in the breaker panel and all.
                          "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                          • #14
                            Originally posted by The Dude View Post
                            Agreed. Yes it's in the lit, but what about convention? I don't want them wiring my house........ special notes ("green is hot") in the breaker panel and all.
                            ..."Except where we haven't rewired. We don't have complete notes on where that is"...
                            Last edited by Chuck H; 02-26-2020, 09:39 PM.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "If you're not interested in opinions and the experience of others, why even start a thread?
                            You can't just expect consent." Helmholtz

                            Comment


                            • #15
                              For future reference, the Red and Brown are swapped and Blue stays where it is (for proper phase with the NFB) on a Hammond 1750G. Other variants (like the 1750RA) have the colors correct.

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