The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    Trouble-Shooting my New ST-70 (VTA PCB): Voltages at Pin 5

    Doctor Hugocat

    Posts : 92
    Join date : 2015-08-19
    Location : Los Angeles CA

    Trouble-Shooting my New ST-70 (VTA PCB): Voltages at Pin 5

    Post by Doctor Hugocat on Sun Dec 11, 2016 6:53 pm

    Hey Guys -

    I recently posted a topic about my newly-rebuilt ST070 (with a VTA PCB) giving horrible problems at first startup:

    Well, acting on the excellent advice of a couple of members here (Thanks again)! I double checked my wiring and found no problems. I then looked through the Curcio trouble shooting guide and did the following:

    • Replaced the rectfier with a solid-state one from eBay
      powered up with NO tubes or rectifier: powers up fine.
      Powered up WITH solid-state rectifier: power up fine.

    I then measured the voltages at pin 5 of the power tube sockets (no power tubes) and got this:

    Rear right -43vdc
    Front right -42.2vdc
    Rear left -43vdc
    Front left -43vdc

    This seems within spec, right? Doesn't this rule out the quad cap and power trans?

    So what now? I don't have any way of testing the power tubes and the Curcio guide suggests power tubes could be at fault. SO.....what would my next step be? All the Dynaco manual test points seem to be WITH tubes: and since I can't trust the power tubes, what to do next??


    Posts : 944
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Trouble-Shooting my New ST-70 (VTA PCB): Voltages at Pin 5

    Post by peterh on Mon Dec 12, 2016 3:41 am

    As i wrote on audiokarma :
    I would start with a new set of el34, and a new GZ34.
    Tubes are expendable and once flashed may easily flash again.

    But first, double check that the bias resistors ( 10 ohm?) are all healthy, and that the bias
    pots are turned fully in the direction of most negative voltage for all 4 power tubes. This is
    a starting point when installing new power tubes !
    Also power up with shorting plugs on inputs and a proper loading on the output ( speakers or 8 ohm
    power resistor)

      Current date/time is Thu May 24, 2018 9:22 pm