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    grounding ST70/120

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    grounding ST70/120

    Post by Guest on Wed Apr 16, 2014 12:26 pm

    Is here any benefit in grounding the ST70 or ST120 to house ground, ie, using a 3 core mains cable instead of the two core, and running ground to the chassis.

    peterh

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    Re: grounding ST70/120

    Post by peterh on Wed Apr 16, 2014 12:28 pm

    MontanaWay wrote:Is here any benefit in grounding the ST70 or ST120 to house ground, ie, using a 3 core mains cable instead of the two core, and running ground to the chassis.
    Yes, the risk of fatal electrocution is reduced. Also many sources of hum will be reduced ( some sources will increased; those that has another route to ground, cable-tv is one such)


    corndog71

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    Re: grounding ST70/120

    Post by corndog71 on Wed Apr 16, 2014 12:29 pm

    I had to with my ST120 otherwise I got a major hum.

    Guest
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    Re: grounding ST70/120

    Post by Guest on Wed Apr 16, 2014 12:32 pm

    corndog71 wrote:I had to with my ST120 otherwise I got a major hum.

    so I wonder how they got away with not having it grounded.......I know house ground is a relatively 'recent' addition to house wiring, when compared to the age of the ST70's.....

    Guest
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    Re: grounding ST70/120

    Post by Guest on Wed Apr 16, 2014 12:35 pm

    I have a very low level hum, makes no difference in volume change or with or without input connected.
    I have checked wiring, capacitor, tubes, cannot seem to find the cause so far...

    Bob Latino
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    Re: grounding ST70/120

    Post by Bob Latino on Wed Apr 16, 2014 1:57 pm

    The VTA ST-70 and ST-120 are perfectly safe with a 2 wire cord. If properly built there is no voltage on the chassis. My metal toaster has a 2 wire cord ... 350,000 original Dynaco ST-70's were wired with a 2 wire cords. If you do use a 3 wire cord and you ground the third wire to the chassis, you do run into the possibility of a "ground loop" (and increased hum) IF another component in your stereo system ALSO has a 3 wire cord. Another thing - some older houses with older wiring may not have the third wire grounded properly because over the years the wiring has been "updated" by a previous owner himself and not by an electrician. IOW that third ground wire on your wall outlet may not be properly grounded?

    My advice is if you want > TRY a 3 wire cord and if the 3 wire cord appears to cause more hum than a two wire cord, then use a 3 to 2 wire "cheater plug" or just remove the 3rd chassis ground wire inside the chassis.

    Bob

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    Re: grounding ST70/120

    Post by Guest on Wed Apr 16, 2014 2:41 pm

    well I changed it over to a 3 cord, as well as a DPDT power switch with a cap filter across it, no extra hum, but that very low level hum is still present, guess I keep digging.
    Since our house is only 18 months old, the ground wiring will not be an issue here.
    I was not so much worried about the safety issue, just trying to see if I can get rid of that hum, thanks Bob.

    nmchiefsfan

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    Re: grounding ST70/120

    Post by nmchiefsfan on Wed Apr 16, 2014 11:28 pm

    I had a very low level hum when I completed my Latino ST-120 and found that it was the center tube on the driver board.  I have the older board with the higher gain 12AT7's.  If that is the board you are using, swapping tubes around may be benificial.   Heck, even if you are using the low gain board juggling the tubes around could help.  With me, none of the tubes supplied with the amp were quiet enough for the center position.  I had some extra's as spares for my guitar amps and they were dead quiet hum wise but quite microphonic.  I ended up buying a NOS tested and "guarenteed quiet" ECC81 and have been enjoying hum free music for a couple of years now.  (knock knock).

    sKiZo

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    Re: grounding ST70/120

    Post by sKiZo on Thu Apr 17, 2014 12:13 am

    MontanaWay wrote:well I changed it over to a 3 cord, as well as a DPDT power switch with a cap filter across it, no extra hum, but that very low level hum is still present, guess I keep digging.
    Since our house is only 18 months old, the ground wiring will not be an issue here.
    I was not so much worried about the safety issue, just trying to see if I can get rid of that hum, thanks Bob.

    I played around with the idea of adding a separate ground plane for a three wire setup when I built my ST120.



    All "chassis" grounds would have been to the copper plate, and that would have been completely isolated from the chassis plate itself. Then somebody was kind enough to point out that there'd be a good chance of the extra plate acting as an antenna, drawing all sorts of stray RF. Noise could have been more than just a simple low level hummmmm, so I left it out.

    Makes a nice back scratcher though ...

    PS ... are you sure your hum is electrical? If so, you should hear it from the speakers. A mechanical hum from the transformers is common. I do get just a touch of transformer hum, but the speakers are dead quiet on standby with just a very slight hiss if I hold my ear right up to a tweet or high mid.

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    Re: grounding ST70/120

    Post by Guest on Thu Apr 17, 2014 12:41 am

    sKiZo wrote:
    MontanaWay wrote:well I changed it over to a 3 cord, as well as a DPDT power switch with a cap filter across it, no extra hum, but that very low level hum is still present, guess I keep digging.
    Since our house is only 18 months old, the ground wiring will not be an issue here.
    I was not so much worried about the safety issue, just trying to see if I can get rid of that hum, thanks Bob.

    I played around with the idea of adding a separate ground plane for a three wire setup when I built my ST120.



    All "chassis" grounds would have been to the copper plate, and that would have been completely isolated from the chassis plate itself. Then somebody was kind enough to point out that there'd be a good chance of the extra plate acting as an antenna, drawing all sorts of stray RF. Noise could have been more than just a simple low level hummmmm, so I left it out.

    Makes a nice back scratcher though ...

    PS ... are you sure your hum is electrical? If so, you should hear it from the speakers. A mechanical hum from the transformers is common. I do get just a touch of transformer hum, but the speakers are dead quiet on standby with just a very slight hiss if I hold my ear right up to a tweet or high mid.

    yes I do hear the hum through the speakers, both of them. The hum starts as soon as the TDR kicks in. Of course once music starts playing, one cannot hear them hum at all. As I've said, it is low level but noticeable none the less.
    I still only have a limited supply of tubes that I can roll with, I will try some changes tomorrow and see what happens.

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