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    New AC plug wiring for ST-70

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    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    New AC plug wiring for ST-70

    Post by cc43fan on Wed Mar 07, 2012 8:15 am

    If I installed a new AC plug (the type with one wider blade), which side (fuse or switch) would that wider blade get wired to? Would it even matter?

    Thanks.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New AC plug wiring for ST-70

    Post by Bob Latino on Wed Mar 07, 2012 9:36 am

    Hi,

    On a two wire cord, such as on a Dynaco tube amp, it really doesn't matter. The white neutral wire inside the AC outlet is supposed to go to the "wider blade" on the outlet itself. Some say that this wire should go to the switch and the black wire side should go to the fuse - BUT - I have connected those power cords up either way and it makes no difference in the sound of the amp or the noise level of the amp. On the original Dynaco amps the plug on the AC cord was not polarized (with one wide blade and one smaller blade) and Dynaco gave no instructions about what wire goes where when you installed the power cord.

    Bob

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: New AC plug wiring for ST-70

    Post by cc43fan on Wed Mar 07, 2012 9:42 am

    Bob Latino wrote:Hi,

    On a two wire cord, such as on a Dynaco tube amp, it really doesn't matter. The white neutral wire inside the AC outlet is supposed to go to the "wider blade" on the outlet itself. Some say that this wire should go to the switch and the black wire side should go to the fuse - BUT - I have connected those power cords up either way and it makes no difference in the sound of the amp or the noise level of the amp. On the original Dynaco amps the plug on the AC cord was not polarized (with one wide blade and one smaller blade) and Dynaco gave no instructions about what wire goes where when you installed the power cord.

    Bob

    Hi Bob,
    Thanks for the fast response!!

    GP49

    Posts : 718
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: New AC plug wiring for ST-70

    Post by GP49 on Wed Mar 07, 2012 2:40 pm

    Bob Latino wrote:Hi,
    On a two wire cord, such as on a Dynaco tube amp, it really doesn't matter. The white neutral wire inside the AC outlet is supposed to go to the "wider blade" on the outlet itself. Some say that this wire should go to the switch and the black wire side should go to the fuse - BUT - I have connected those power cords up either way and it makes no difference in the sound of the amp or the noise level of the amp. On the original Dynaco amps the plug on the AC cord was not polarized (with one wide blade and one smaller blade) and Dynaco gave no instructions about what wire goes where when you installed the power cord.
    ]

    Which is often an advantage, because one orientation of the line cord may result in lower hum than the other.

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: New AC plug wiring for ST-70

    Post by heyraz on Thu Mar 08, 2012 4:08 am

    It will make no sonic difference to the amp (because the transformer primary has no "polarity" and is isolated from chassis) but I think convention dictates the black ("hot") wire of the polarized power cord should first go to Fuse, then to switch, then to transformer. The other primary from the transformer is now considered neutral and is connected to the "white/neutral" conductor from the power cord. If you use a 3 prong grounding plug, the "safety" green ground wire should be fastened to the chassis as close to the entry point as possible. The safety ground should not be attached to any screw that could loosen, such as any screw passing through anything plastic which could compress and loosen. If you go with a 2 prong polarized plug, the "narrow" tang is considered "black/hot". I would strongly recommend against using a non polarized plug. You can consider this the first upgrade to your ST-70.
    Personally, I think the benefit of having a safety ground outweighs the risk of developing ground loop noise. One way to minimize ground loop potential is to make sure all components are plugged into the same circuit, keeping all plug polarities the same. Another thing you can do to minimize hum riding into the amp over the ground conductor of the input's interconnect cable is to connect the ground side of each input jack to chassis through a 0.01uf ceramic capacitor to a chassis mounted solder tag. Any high frequency AC will be drawn to the chassis rather than the circuit. The input jack's normal ground conductor to the driver board ground input remains the same, the capacitor is an addition.
    PS-Worst Case Scenario of not switching the hot wire and not having safety ground would be assuming the amplifier is turned "off" (because neutral wire is switched open) and short develops between hot wire and chassis. If no safety ground is present, chassis will be energized with no low impedance path back to circuit panel to trip breaker. Amplifier will be "off", yet chassis will be hot. Chassis fuse will be useless as it is not in circuit. All you need for a shock is to contact the "hot" chassis and a nearby ground.
    See attached photo examples of input ground capacitors to solder tags and green ground wire to solder tag at power switch. Also attached is photo of how I put an IEC "tail" on the back of the amp instead of a 2 wire power cord. Middle photo shows how I fixed a grommet to the power cord to keep it from pulling out. I used a short piece of high tensile wire twisted around the waist of the grommet till it gripped the power cord firmly. Excess wire was clipped flush with internal grommet.
    Rich
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