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    Auto Biasing ST70

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    terry37932

    Posts : 22
    Join date : 2011-05-18
    Location : Knoxville

    Auto Biasing ST70

    Post by terry37932 on Tue Nov 15, 2011 11:06 am

    Not wanting to sound too uneducated, there is someone on Ebay (250932051473) selling an auto biasing ST70, is this actually possible?

    Terry

    jhoak

    Posts : 46
    Join date : 2009-10-16
    Age : 58
    Location : Central Florida

    Re: Auto Biasing ST70

    Post by jhoak on Tue Nov 15, 2011 11:44 am

    He probably has this (or something like it) installed.

    http://www.dynaco-doctor.com/BCS_3.htm

    I briefly considered getting one for my first VTA/ST-120. After talking to Bob about it I decided that checking the bias is so easy and done so rarely that it wasn't worth the $$ to me.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Auto Biasing ST70

    Post by tubes4hifi on Tue Nov 15, 2011 1:25 pm

    sure, it's simple if you want to complicate things. It's like a servo. The easy way to do it is use an adjustable current source.
    The easiest and most reliable way is to turn a pot once in a while, trusting yourself instead of some "gimzo" circuit.
    Ooh, now looking at the photo, that's not technically "auto-bias", that's good old 1930s "self-bias", a resistor and a capacitor,
    the same way an ST35 is biased. It works fine as long as the tubes don't age too much, once they do, either the tubes or the resistor
    will burn out. Grab it for under $300 though for the chassis and transformers.

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