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Gibson GA-5T Skylark Phase Inverter Transformer?

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  • Gibson GA-5T Skylark Phase Inverter Transformer?

    Howdy y'all!
    Any ideas as to a good phase inverter replacement transformer for the GA-5T amplifier? Got one here with a missing transformer so I've got little to go off of. Hoping someone has found a suitable replacement and can offer their experience.

  • #2
    Schematic Enclosed
    Attached Files

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    • #3
      Hammond have a selection https://www.hammfg.com/electronics/t...24?referer=968


      Rather hard to say when the right one would be but if I were to hazzard a guess, I'd go for the 124A, given the limited choice.

      I don't see the DC current limit rating in the datasheet. Strange as that is rather important.

      A concern I have is the secondary is essentially open circuit (at least at low frequencies) and means the LF response will suffer. I'd be prepared to add a couple of 100K resistors from each output tube grid a to ground to provide a load to overcome this.
      Experience is something you get, just after you really needed it.

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      • #4
        https://www.tubesandmore.com/product...terstage-10-ma

        I've used this transformer for exactly that purpose, also in other similar amps.
        Enjoy. Every. Sandwich.

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        • #5
          Unfortunately, Hammond states for their 124 series:
          "Should not be used for single ended applications. They have no gap for DC bias current present in SE mode."
          - Own Opinions Only -

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          • #6
            Originally posted by Helmholtz View Post
            Unfortunately, Hammond states for their 124 series:
            "Should not be used for single ended applications. They have no gap for DC bias current present in SE mode."
            Well spotted, but it begs the question on how they intended these to be used. Capacitively coupled on the primary do you think? Being rated at 5W that would make for a very inefficient driver. Odd.
            Last edited by nickb; 09-07-2018, 06:26 PM.
            Experience is something you get, just after you really needed it.

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            • #7
              Originally posted by Leo_Gnardo View Post
              https://www.tubesandmore.com/product...terstage-10-ma

              I've used this transformer for exactly that purpose, also in other similar amps.
              Fantastic! Appreciate you sharing your experience with this! I would imagine suitable for the Music Master Bass amplifier as well?

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              • #8
                Originally posted by nickb View Post
                Well spotted, but it begs the question on how they intended these to be used. Capacitively coupled on the primary do you think? Being rated at 5W that would make for a very inefficient driver. Odd.
                Now, this is the big question, indeed. But judging from the symmetrical (center tapped) primary and the relatively high power rating of 5W, it looks like they are intended as an OT of a small power PP driver amp (being able to drive some grid current) for a high power output stage. As such it doesn't replace a phase splitter but provides isolation and a voltage gain of maybe 3. I actually have no clue, where this would be required.

                Edit: I just saw that the versions A to C don't have center-tapped primaries. So these would have to be used capacitor-coupled - not very efficient.
                Last edited by Helmholtz; 09-07-2018, 08:40 PM.
                - Own Opinions Only -

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                • #9
                  I'm unclear on how this PI works. I see V2 drives the primary of the transformer, but how does it split and invert the signal. Does the way it is drawn with the primary at the top have anything to do with it?
                  It's weird, because it WAS working fine.....

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                  • #10
                    Originally posted by Randall View Post
                    I'm unclear on how this PI works. I see V2 drives the primary of the transformer, but how does it split and invert the signal. Does the way it is drawn with the primary at the top have anything to do with it?
                    Would it help if you thought of it like an power transformer that drives a full wave rectifier? The 'ends' of the center tapped secondary are in anti-phase.
                    Experience is something you get, just after you really needed it.

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                    • #11
                      I checked with Hammond on the 124A. It has the identical electrical spec as the Stancor A53C, i.e. max 10mA DC.
                      Experience is something you get, just after you really needed it.

                      Comment


                      • #12
                        Originally posted by Randall View Post
                        I'm unclear on how this PI works. I see V2 drives the primary of the transformer, but how does it split and invert the signal. Does the way it is drawn with the primary at the top have anything to do with it?
                        It works like an output transformer in reverse. An OT takes two anti-phase signals applied to the ends of its centre tapped primary and converts them to single ended to drive the speaker. The driver transformer takes a single ended signal on its primary and converts it to two anti-phase signals at the ends of its center tapped secondary.

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