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    ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

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    Doctor Hugocat

    Posts : 89
    Join date : 2015-08-19
    Location : Los Angeles CA

    ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

    Post by Doctor Hugocat on Fri Aug 19, 2016 5:54 pm

    Hi Guys –

    I’ve been rebuilding my ST-70 with a VTA board. I’ve had a couple months out of the project due to house move etc, and I’m just taking it up again. I’ve been using the attached color wiring diagram as a guide.

    I’ve just noticed a place where I deviated from the diagram, and wonder whether I should redo it.

    It’s the wiring for the bias test sockets where they solder to the tube sockets (purple wires on the diagram). On the diagram:

    V2 socket: 10ohm resistor solders to pin 1, jumps to pin 8 and test wire solders to pin 8
    V3 socket: 10ohm resistor solders to pin 8, jumps to pin 1 and test wire solders to pin 1
    V6 socket: 10ohm resistor solders to pin 8, jumps to pin 1 and test wire solders to pin 8
    V7 socket: 10ohm resistor solders to pin 1, jumps to pin 8 and test wire solder to pin 8

    When I soldered in the 10ohm resistor, I soldered it in to pin 8 on ALL sockets and jumped it to pin 1. I haven’t yet attached the test wires. It seemed to me at the time it made no difference and this looked neater(!). Now of course, since I’m a newbie at this game, I’m doubting this. I’m trying to lower the likelihood of an explosion as far as I can.

    Any help would be appreciated!! Thanks, and let me know if this post isn’t clear Very Happy

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    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

    Post by j beede on Fri Aug 19, 2016 6:51 pm

    In your diagram pins #1 and #8 on each socket are shorted together--they are the electrically the same node. You should have four such nodes, one for each output tube socket.

    You need to have a 10 Ohm resistor going from each socket's pin 1/pin 8 shared (jumpered) node to ground for all four output tube sockets. That will require four 10 Ohm resistors to implement.

    The English language is less than ideal for describing circuit connectivity. That's why schematics and netlists have persisted!

    Good luck, j

    Doctor Hugocat

    Posts : 89
    Join date : 2015-08-19
    Location : Los Angeles CA

    Re: ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

    Post by Doctor Hugocat on Fri Aug 19, 2016 11:14 pm

    Thanks J! That's what I thought originally: that once the terminals were jumpered, they formed a single connection. Glad to have it confirmed. Probably a silly questions, but I prefer to keep the magic smoke inside the amp for as long as possible. Laughing Laughing

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

    Post by j beede on Sat Aug 20, 2016 12:27 am

    One nice thing about DIY tube amps... when you do let the smoke out it's often a (cheap) resistor that got hot. Fix the wiring error, replace the failed cap, and a fifty cent resistor puts you back in business!

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    Re: ST-70 Recuild with VTA board: have I made a mistake with the bias wiring? Thanks!

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