The Dynaco Tube Audio Forum

Would you like to react to this message? Create an account in a few clicks or log in to continue.
The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


    VTA ST70 wiring for bias seems wrong

    avatar
    rollinlikemarcbolan

    Posts : 2
    Join date : 2020-07-20

    VTA ST70 wiring for bias seems wrong Empty VTA ST70 wiring for bias seems wrong

    Post by rollinlikemarcbolan on Mon Jul 20, 2020 10:47 am

    Hi all, brand new member.  I've had Dynaco equipment for years, but I've always hired technicians for repair and upkeep.  I've decided to learn some rudimentary electronics during the "Covid break"...   I appreciate your patience, as I'm only taking my first steps into this world!

    I traded a pair of Dynakit MKiii mono blocks for a nice VTA ST70.   It worked fine except for occasionally blowing fuses more and more frequently.  After some digging around online, I decided to replace the rectifier tube with a SS one from Weber.  I fired it up and it worked.

    After adding the SS rectifier, I wanted to bias the tubes. I noticed that only one bias pin on the front socket was reading any voltage.  Everything I see says there should be two, one for each tube... from my understanding, one of the luxuries of the updated board is so that you can bias each tube individually!  

    When adjusting both pots on one channel, I see the voltage goes up and down.  It seemed like both tubes are hooked up to one pin on the front socket.

    I opened up the amp, and sure enough, the front bias socket only has one wire heading to the tubes, which appear to be connected.  Has anyone seen this done before?  I really would rather have it set up the "right" way... but I'm afraid i'm not willing to experiment on this valuable piece of equipment.  Any advice is much appreciated.

    I've attached a photo, hopefully it makes sense.  Thanks a bunch.

    EDIT: I'm not allowed to post a photo for a few days. I'll try again soon.


    Last edited by rollinlikemarcbolan on Mon Jul 20, 2020 10:51 am; edited 1 time in total (Reason for editing : image not displayed)
    peterh
    peterh

    Posts : 1434
    Join date : 2012-12-25
    Location : gothenburg, sweden

    VTA ST70 wiring for bias seems wrong Empty Re: VTA ST70 wiring for bias seems wrong

    Post by peterh on Mon Jul 20, 2020 11:19 am

    There should be two wires to each front socket. Both are connected to the cathode of each tube.
    The original builder must have forgotten this detail.
    Do connect the second one on each side to a free outlet pin, i think pin 4 ( opposite the working )is
    recommended in the build instructions.
    And it't vital that both are connected since you have no clue of the back tubes bias without this.
    avatar
    rollinlikemarcbolan

    Posts : 2
    Join date : 2020-07-20

    VTA ST70 wiring for bias seems wrong Empty Re: VTA ST70 wiring for bias seems wrong

    Post by rollinlikemarcbolan on Mon Jul 20, 2020 12:00 pm

    peterh wrote:There should be two wires to each front socket. Both are connected to the cathode of each tube.
    The original builder must have forgotten this detail.
    Do connect the second one on each side to a free outlet pin, i think pin 4 ( opposite the working )is
    recommended in the build instructions.
    And it't vital that both are connected since you have no clue of the back tubes bias without this.

    It looks like whoever wired this board did this intentionally, as both tubes per channel are wired together for one bias. As soon as I'm allowed to post photos, I'll show what I mean.

    Thanks for your reply
    tubes4hifi
    tubes4hifi
    Admin

    Posts : 1665
    Join date : 2008-11-30

    VTA ST70 wiring for bias seems wrong Empty Re: VTA ST70 wiring for bias seems wrong

    Post by tubes4hifi on Thu Jul 23, 2020 9:50 pm

    you can run a wire from each output tube to the front test sockets. That probably wasn't done.
    Refer to the VTA wiring diagram here . . . . www.tubes4hifi.com/VTA70wiringdiagram3.jpg
    There should be a 10 ohm resistor on each output tube cathode to ground so that you can measure the bias.
    Set for 40ma per tube which is 400mv, without music playing. Adjusting any one tube will have a slight effect on the others,
    but going back and forth 2-3 times and they should all be in the 390-410 range.

    Sponsored content

    VTA ST70 wiring for bias seems wrong Empty Re: VTA ST70 wiring for bias seems wrong

    Post by Sponsored content


      Current date/time is Wed Oct 21, 2020 11:50 pm