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    What board and version?

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    hrudu90

    Posts : 26
    Join date : 2011-01-02

    What board and version?

    Post by hrudu90 on Thu Mar 26, 2015 11:06 pm

    I finally open up my new to me VTA ST-70:



    I am wondering what version do I have?


    And what are the blue pots for?

    Thanks


    ~~~

    MontanaWay

    Posts : 767
    Join date : 2014-02-06
    Age : 58
    Location : Cameron, Montana

    Re: What board and version?

    Post by MontanaWay on Fri Mar 27, 2015 10:04 am

    This looks like an earlier version of the VTA pcb, perhaps the low gain version? Not too sure what those two trimmers do on that version, I think they adjust the constant current source, as the later version VTA circuits use the LM334 for the CCS.

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: What board and version?

    Post by Bob Latino on Fri Mar 27, 2015 10:31 am

    Your VTA ST-70 amp is a fairly early version built in 2007-2008. The reason I can tell is that you have Micanol (mustard colored) sockets which eventually became unavailable so I switched to Celanex (black) sockets. You also have yellow backing boards for the input jacks and output binding posts. The newer backing boards are black in color. Your amp also has a C-354 choke (200 milliamp rated). I switched to the C24-X chokes because they are 240 milliamp rated. Also - in later amps the Russian PIO cap leads have a black tubing covering the leads because they could short to the body of the cap which is metal and is conductive.

    Those two blue trimmers on the BOTTOM of the driver board set the phase splitter part of the driver circuit. Once set you just leave them alone and do not touch them. The reason they are put on the bottom is that you will not have easy access to them and try to readjust them. They are set at the proper place when you build the board. As MontanaWay has mentioned the newer ST-70/ST-120 boards have CCS - a constant current source for the phase splitter part of the driver circuit - SO - no 10K AC balance control is needed.

    Bob

    hrudu90

    Posts : 26
    Join date : 2011-01-02

    Re: What board and version?

    Post by hrudu90 on Fri Mar 27, 2015 9:57 pm

    Thank you Bob!
    Which resistors would you recommend metal or carbon?
    I would like to convert it to a low gain version.

    BTW, all I need to do is to replace some 12 resistors?
    Please see pic.

     photo Standardandlowergainboards_zpsco11nih0.jpg

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: What board and version?

    Post by tubes4hifi on Fri Mar 27, 2015 10:10 pm

    yes, you only need to replace the parts with the X next to them. Metal films are usually better, but carbon films have a mellower laid back sound

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