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    Please assist me bias a new to me ST-70!

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    hrudu90

    Posts : 26
    Join date : 2011-01-02

    Please assist me bias a new to me ST-70!

    Post by hrudu90 on Wed Jul 18, 2012 1:00 am

    I just bought a used factory built VTA ST-70; finally, finally!
    It is the version with ECC81 driver tubes.
    Unfortunately it didn't come with any documentation.
    Now I try to bias the amp for el34 and 6P3S-E tubes.

    I installed all tubes, connected amp to preamp and connected speakers to amp,
    turned on the whole system and made sure no signals go the the amp.
    Then I measure the bias of the front left tube by placing the black probe
    on the chassis and the red probe in the 'Biasset 1.56v' hole
    and adjust the adjuster until the reading is about .4 V.
    For the back left tube I placed the red probe in the hole on the opposite side of the socket.
    Then I did the same thing on the right side.
    I did go back and check all reading. they are all about 04 V now.
    The system was on from the beginning until I was 'done'.

    I am not sure if it was the right way to bias this amp.

    Oh, And if someone has a copy of the user manual please email me.
    I would appreciate your helps. Thank you

    ~~~


    Last edited by hrudu90 on Sun Jul 22, 2012 2:31 am; edited 2 times in total

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Please assist me bias a new to me ST-70!

    Post by Bob Latino on Wed Jul 18, 2012 7:33 am

    Hi,

    Yes - You did set the bias correctly ... On the VTA ST-70 amps you set the bias at .400 VDC for all four output tubes with either EL34 or the Russian 6P3S-E output tubes.

    NOTE > I tried to send you an Email with the VTA ST-70 users manual on the attached file and the Email bounced back to me. If you can provide me with a valid Email address, I can try to send the manual to you again ..

    Bob



    hrudu90

    Posts : 26
    Join date : 2011-01-02

    Re: Please assist me bias a new to me ST-70!

    Post by hrudu90 on Wed Jul 18, 2012 10:27 am

    Thank you, Bob

    Someone told me to install and bias only 2 left tubes first turn off amp.
    Install 2 right tubes, turn on amp and bias the right tubes.
    I did put all tubes in the amp turn it on and bias.
    I hope it works this way too.

    I tried to modify my email but it did not work.
    Still the same but now .com instead of .net.

    Again thank you Bob

    ~~~

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Please assist me bias a new to me ST-70!

    Post by GP49 on Wed Jul 18, 2012 11:12 am

    Your way, doing both channels at the same time, is probably better. You may notice that the adjustments interact; when you adjust one, the other channel shifts, as the load on the bias circuitry is causing its voltage to shift slightly. So once you adjust one channel, then the other, you should go back to the first one and check it, then readjust if necessary. Re-check the two channels back and forth.

    hrudu90

    Posts : 26
    Join date : 2011-01-02

    Re: Please assist me bias a new to me ST-70!

    Post by hrudu90 on Thu Jul 19, 2012 12:12 am

    I am wondering if it would be better to bias an amp with out a preamp plugged in?
    This way no signals will ever get through the amp.

    ~~~

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Please assist me bias a new to me ST-70!

    Post by Bob Latino on Thu Jul 19, 2012 5:15 am

    Hi,

    It doesn't matter whether the preamp is plugged in or on. The key thing is that there should be no signal running through the amp when you set the bias. The amp should be at "idle". You should also have your speakers connected to give the two output transformers a proper "load".

    Bob

    hrudu90

    Posts : 26
    Join date : 2011-01-02

    Re: Please assist me bias a new to me ST-70!

    Post by hrudu90 on Sun Jul 22, 2012 2:33 am

    Bob,
    Can I replace the center 12at7 center tube with a 12au7?
    Gin

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Please assist me bias a new to me ST-70!

    Post by Bob Latino on Sun Jul 22, 2012 7:39 am

    Yes - You could try a 12AU7 in place of the 12AT7 in the center position. The gain of the amp will drop very slightly. Understand though that the resistor set on your amp was optimized for 12AT7 driver tubes.

    Bob

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