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    ST70 with new VTA Board (12AU7) upgrade question/problem?

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    tgrier
    tgrier

    Posts : 4
    Join date : 2019-01-05
    Age : 60
    Location : NE PA

    ST70 with new VTA Board (12AU7) upgrade question/problem?

    Post by tgrier on Sat Jan 05, 2019 3:22 pm

    I am new to forum and this is my first post. Hope I am in right area for a question...

    I just completed  VTA replacement in my old ST70. The only things not replaced were transformers and hardware. I pulled bias test points to jacks in old preamp sockets and used the Mono/Stereo switch space for ground jack for testing bias (I made from mounting plates from some aluminum sheet I have). Replaced the GZ34 with SS Rectifier and added CL80 current limiter. Was having problem with rectifier popping fuses after warm up so went solid state and seems to working fine. Bias is stable at ~.4v.

    Anyway it sounds VERY quiet and clean! Great Board/design! Might try Golden Lion GZ34 in future. Might also try some KT77 and roll some other tubes.

    The PROBLEM/Question is that I now have to put the volume on my Marantz 3600 a quarter turn higher to get same volume as before with a more of less stock ST70. I am using JBL 4312A's as speakers. Is this typical after this type of upgrade/resto? I am concerned it is indicative of a problem.

    Thoughts?



    Thanks, Tony
    pichacker
    pichacker

    Posts : 77
    Join date : 2016-08-01
    Age : 54
    Location : Near to London - UK

    Re: ST70 with new VTA Board (12AU7) upgrade question/problem?

    Post by pichacker on Mon Jan 07, 2019 4:20 am

    Tony,

    I believe from reading previous posts that the VTA board has slighly less gain than the original derivative. If both your channels are at equal levels and sound clean without any hum or distortion then i'd say you don't have any issues.

    Regarding the rectifier, I know this is an emotive point, but it appears that the 5AR4/GZ34 is the weakest link in the original design and I too have replaced with SS rectification in my ST120. There are plus and minuses to doing this which I am sure are well documented.

    Sit back and enjoy.

    Steve
    tgrier
    tgrier

    Posts : 4
    Join date : 2019-01-05
    Age : 60
    Location : NE PA

    Re: ST70 with new VTA Board (12AU7) upgrade question/problem?

    Post by tgrier on Tue Jan 08, 2019 7:01 pm

    Thanks that is reassuring, I am listening but can't sit back! It's driving me mad I have to delve further...I have some GZ34's coming in I'll see if that changes anything...I will also check all voltages...I will report back if I find anything...Thanks, Tony
    tgrier
    tgrier

    Posts : 4
    Join date : 2019-01-05
    Age : 60
    Location : NE PA

    Re: ST70 with new VTA Board (12AU7) upgrade question/problem?

    Post by tgrier on Sat Jan 12, 2019 2:24 pm

    Just an FYI, after going from SS rectifier to Gold Lion GZ34 I did have to raise bias 0.1v and feels like I did get some more gain but still have to go about 1/4 turn higher on the pre. A small loss for such a nice sounding board/upgrade. Thanks, Tony

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    Re: ST70 with new VTA Board (12AU7) upgrade question/problem?

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