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    MK3 Driver Interconnect

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    jwb474

    Posts : 27
    Join date : 2014-07-13

    MK3 Driver Interconnect

    Post by jwb474 on Thu Oct 11, 2018 7:31 am

    I am getting ready to start the MK3 build. Studying the instructions I notice a note that says "Output wires must cross to opposite tubes as shown". My question is why is this? Could you not just reverse locations of the output tubes? I have V14 revision drawings.

    stewdan

    Posts : 180
    Join date : 2010-03-07
    Age : 80
    Location : Houston Texas

    Re: MK3 Driver Interconnect

    Post by stewdan on Thu Oct 11, 2018 9:33 am

    JWB ---- when I built my VTA Mk3's several years ago, I did not criss-cross the outputs to the tubes and I had one BIG HORRIBLE SQUEAL coming out of my amps.

    When I reversed the outputs, I had no squeal. Roy would have to answer your question as to why the criss-crossed wires.

    Hope that helps.

    jwb474

    Posts : 27
    Join date : 2014-07-13

    Re: MK3 Driver Interconnect

    Post by jwb474 on Thu Oct 11, 2018 10:25 am

    stewdan wrote:JWB ---- when I built my VTA Mk3's several years ago, I did not criss-cross the outputs to the tubes and I had one BIG HORRIBLE SQUEAL coming out of my amps.

    When I reversed the outputs, I had no squeal.  Roy would have to answer your question as to why the criss-crossed wires.

    Hope that helps.

    It may very well have given me a clue. I and going to bet it has to do with the relative phasing of the NNF signal from the output transformer and which tube is conducting. I believe you formed an oscillator by having an in phase positive feedback instead of negative phase to stabilize circuit.

    stewdan

    Posts : 180
    Join date : 2010-03-07
    Age : 80
    Location : Houston Texas

    Re: MK3 Driver Interconnect

    Post by stewdan on Thu Oct 11, 2018 7:49 pm

    JWB --- Somewhere in my electronic past I took a college course dealing with tube electronics and have managed to forget most of what I learned.
    But I think you are right about the oscilator!

    jwb474

    Posts : 27
    Join date : 2014-07-13

    Re: MK3 Driver Interconnect

    Post by jwb474 on Fri Oct 12, 2018 5:16 am

    I finally got industrious and looked up the MK3 Output transformer spec. The winding phasing is the BLU-WHT wire is primary Start and the BLK (Com) wire is secondary start. The NFB signal comes from the YEL (16 ohm) secondary which is Finish. The BLU-WHT wire connects to the top tube (V1) plate. When the BLU-WHT wire is going positive then the NFB signal is going Negative which is what you want. When the BLU wire of the transformer ( V2 Plate) is going positive then the NFB signal is also going positive which is what you don't want. So you have to make sure that the grid drive connections comes from the properly phase driver outputs.

    Before I start the new builds I have some clean up work to do on my ST70. Over the years of constant mods it has gotten very nasty. At that time I will scope the signals to either verify what we now think or disprove it and start over with new theories.

    But, now I think that I should be able to swap locations of V1 and V2 as long as I keep the transformer connection with the stated tube.

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