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    how to bias my tubes on a ST70 without any biasing points?

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    triotary

    Posts : 3
    Join date : 2013-06-24

    how to bias my tubes on a ST70 without any biasing points?

    Post by triotary on Mon Jun 24, 2013 4:06 am

    Hi Guys,

    I'm new to this forum. Recently got myself a rebuilt ST70 with the older VTA driver board (non CCS) and TriodeElectronics Power Supply board. I need to bias my EL34 tubes, however I'm facing a problem.
    My ST70 does not come with the dual front biasing socket. Where am I suppose probe my positive pin of my multimeter to measure the DC voltage?


    Many thanks in advance!

    triotary

    Posts : 3
    Join date : 2013-06-24

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by triotary on Mon Jun 24, 2013 4:09 am

    I did read up a few diagrams and instructions, from my understanding, am I suppose to probe on pin #8 of each output tube?

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by Bob Latino on Mon Jun 24, 2013 6:39 am


    Hi,
     
      If you don't have any place to measure bias on the outside of the chassis then you have to open the amp up. Measure the bias from pin 8 to chassis ground on each output tube. Use the 0 to 2 volt DC scale on your meter. For EL34 output tubes set each tube to .400 DC. That is POINT 4 volts DC which is 4/10 of a volt.
     
    Bob

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by peterh on Mon Jun 24, 2013 6:59 am

    triotary wrote:I did read up a few diagrams and instructions, from my understanding, am I suppose to probe on pin #8 of each output tube?
    Yes. But you will have to figure out q few things :
    - is each channel using a common catode resistor ?
    - how large is that resistor

    Depending on your findings you will have to calculate the voltage across the resistor.
    Dynaco stock used a 15.6 ohm resistor common to both tubes in each channel, the
    recommended voltage is 1.56V which will give 50mA / tube.

    More modern gear might have a different value, and the current wizdom
    is to use 40mA / tube as a startingpoint.

    Finally: if you don't have any measuring connections outside i would recomment that you create one.

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by Bob Latino on Mon Jun 24, 2013 7:05 am


    The original poster mentioned that this amp had a VTA driver board. I can check with Roy but AFAIK the VTA driver board always used a fixed (adjustable) bias system with a 10 ohm bias resistor on each output tube.
     
    Bob

    peterh

    Posts : 680
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by peterh on Mon Jun 24, 2013 7:49 am

    Bob Latino wrote:
    The original poster mentioned that this amp had a VTA driver board. I can check with Roy but AFAIK the VTA driver board always used a fixed (adjustable) bias system with a 10 ohm bias resistor on each output tube.
     
    Bob
    the VTA driver boaard has no 10ohm resistor, they are mounted on the tube sockets.

    Bob Latino
    Admin

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    Join date : 2008-11-26
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    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by Bob Latino on Mon Jun 24, 2013 7:53 am


    Peter,
     
      I mentioned in the post that the VTA driver board had a 10 ohm bias resistor "on each output tube".  I never mentioned or inplied that the 10 ohm bias resistor was on the driver board ..
     
    Bob

    tubes4hifi
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    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by tubes4hifi on Mon Jun 24, 2013 11:34 am

    gee guys, let Triotary figure it out.   resistance from pin 8 on each output tube to ground.   If it's a VTA mod, it's 10 ohms.
    You want 40ma of bias.   Ohm's Law, anybody heard of that?   0.40vdc across each resistor

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by sKiZo on Mon Jun 24, 2013 11:58 am

    oooooOOOOOooooooohhhhhhmmmmMMMmmm ...

    Sounds like time for a circuit mod. OP's gotta get inside anyway, right?

    Easy enough to add bias test points - I found some nice ones at Weber's online store when I went in to get my Copper Cap. Anybody got the exact step by step for that specific model to make it happen? Make for a nice sticky too ...

    Weber's website is kinda hard to navigate ... just stumbled across these myself, so here's a link:
    https://taweber.powweb.com/store/plugord.htm

    They've got all sorts of neat stuff buried here and there ... bit of a treasure hunt though.

    tubes4hifi
    Admin

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    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by tubes4hifi on Mon Jun 24, 2013 10:50 pm

    instructions for tip jacks - connect the center hot lead to pin 8 (one for each output tube)
    connect the outer ground lead to chassis ground
    there 'ya go, step-by-step

    sKiZo

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    Join date : 2013-04-01
    Location : Michigan USA

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by sKiZo on Tue Jun 25, 2013 1:14 pm

    I was hoping for something simpler ... got any pictures? tongue

    triotary

    Posts : 3
    Join date : 2013-06-24

    Re: how to bias my tubes on a ST70 without any biasing points?

    Post by triotary on Tue Jun 25, 2013 10:54 pm

    Thank you all! Really appreciated! Yes the Amp comes with an older version of VTA board without the CCS. It did fired up well and managed to drive a pair of 15" tannoy berkeley very well, with the help of a Marantz 7C. Now I will go ahead and bias the tubes from pin 8 of each EL34 output tubes.

    Yes, i was also hoping that I could bias the tubes while probing some points of the VTA board ^_^"

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