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    3 prong power cord on st70?

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    buschfsu

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    3 prong power cord on st70?

    Post by buschfsu on Sun Mar 26, 2017 4:13 pm

    This is always recommended on guitar amps. Does the same mod apply here? I would think so. Just picked up a carcass with iron and am very excited to start restoring!
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    Peter W.

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    Re: 3 prong power cord on st70?

    Post by Peter W. on Sun Mar 26, 2017 4:38 pm

    No no no no no no, and did I mention NO?

    Hum loops are the issue, and unless the amp has been designed for a ground, just do not.

    buschfsu

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    Re: 3 prong power cord on st70?

    Post by buschfsu on Sun Mar 26, 2017 4:40 pm

    yikes! glad i asked ok ,thanks

    on fender amps i was told that 3 prongs protect the chasis from being energized, is there a risk of that in this amp?
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    peterh

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    Re: 3 prong power cord on st70?

    Post by peterh on Sun Mar 26, 2017 4:59 pm

    Yes Yes Yes, use 3 prong power cords and make sure the wall outlet has a working ground connector

    Hum might occur if only some of the equip is ungrounded. Make all equip have a real ground connection.

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    j beede

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    Re: 3 prong power cord on st70?

    Post by j beede on Sun Mar 26, 2017 5:06 pm

    Regarding Dynaco, grounding, safety, and hum:

    -A floating chassis represents a safety hazard
    -Every Dynaco product I have come across uses the chassis as a common, floating ground
    -Every Dynaco product I have come across uses a two prong power plug
    -Every Dynaco amplifier I have worked on hums when fitted with a 3-prong plug
    -The ST-70 and MkIII (at least) have high voltage nodes exposed when the cage is removed

    Bottom line, if safety (in the UL, OSHA, etc. sense) is important than hum suppression to you, a more recent or more electrically responsible design may make more sense.
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    Peter W.

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    Re: 3 prong power cord on st70?

    Post by Peter W. on Sun Mar 26, 2017 5:08 pm

    peterh wrote:Yes Yes Yes, use 3 prong power cords and make sure the wall outlet has a working ground connector

    Hum might occur if only some of the equip is ungrounded. Make all equip have a real ground connection.


    You are in Sweden. In the US, altering a UL listed item violates all sorts of codes, voids the listing, and *will* often cause hum loops. Keep in mind that vintage dynaco uses the chassis as the common rail. So, again, emphatically, NO.
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    j beede

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    Re: 3 prong power cord on st70?

    Post by j beede on Sun Mar 26, 2017 5:10 pm

    peterh wrote:Yes Yes Yes, use 3 prong power cords and make sure the wall outlet has a working ground connector

    Hum might occur if only some of the equip is ungrounded. Make all equip have a real ground connection.


    From a safety point of view peterh's advice is very good. However, grounding multiple components will almost certainly induce ground loops (hum). It is an easy experiment to run and you should do it.
    ...j
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    peterh

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    Re: 3 prong power cord on st70?

    Post by peterh on Mon Mar 27, 2017 2:41 am

    j beede wrote:
    peterh wrote:Yes Yes Yes, use 3 prong power cords and make sure the wall outlet has a working ground connector

    Hum might occur if only some of the equip is ungrounded. Make all equip have a real ground connection.


    From a safety point of view peterh's advice is very good. However, grounding multiple components will almost certainly induce ground loops (hum). It is an easy experiment to run and you should do it.
    ...j
    It will NOT create hum loops if all chassies are grounded to the same ground.
    Signal ground should not carry current, and should not be used to connect
    several floating chassies to the same potential.
    Properly grounding chassies will also reduce the hum picked up by low voltage deviced s.a. magnetic cartridges.

    Installing 3 prong power cords will give the user the possibility to try
    and choose either method; using grounded outlets or using a non-grounded
    strip connector.
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    pichacker

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    Re: 3 prong power cord on st70?

    Post by pichacker on Mon Mar 27, 2017 4:03 am

    We do have a slightly different consideration over here in Europe. Our line voltage is 220-240V so we have always been wary of having a floating chassis. Grabbing a handful of 120V gives a fair tingle, 240V hurts a lot!!!

    My ST120 has a grounded chassis and true, hum loops can be an issue, especially with the stray magnetic field from the mains transformer inducing small currents into the signal leads.

    I was contemplating having my amp ground floating from the chassis ground by a few ohms and maybe a couple of diodes.... trouble is this would require mounting the filter cap on an insulated board and returning the cathode resistors to a common point. Maybe if I build another i'll experiment.

    It's a kit so in Europe has no "ratings" as such that we can violate.
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    peterh

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    Re: 3 prong power cord on st70?

    Post by peterh on Mon Mar 27, 2017 5:24 am

    pichacker wrote:We do have a slightly different consideration over here in Europe. Our line voltage is 220-240V so we have always been wary of having a floating chassis. Grabbing a handful of 120V gives a fair tingle, 240V hurts a lot!!!

    My ST120 has a grounded chassis and true, hum loops can be an issue, especially with the stray magnetic field from the mains transformer inducing small currents into the signal leads.

    I was contemplating having my amp ground floating from the chassis ground by a few ohms and maybe a couple of diodes.... trouble is this would require mounting the filter cap on an insulated board and returning the cathode resistors to a common point. Maybe if I build another i'll experiment.

    It's a kit so in Europe has no "ratings" as such that we can violate.
    The important point to get hum-free is to have all units properly grounded.
    This will prevent humcurrent to pass through the interconnecting grounds.
    Lifting signal ground from chassies with a few ohm's is beneficial
    if possibly to implement.
    Then issues like cable routes will start to affect.

    buschfsu

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    Join date : 2017-03-25

    Re: 3 prong power cord on st70?

    Post by buschfsu on Mon Mar 27, 2017 5:38 am

    Well my head is spinning but i know that my turntable is grounded to my outlaw receiver that uses a 3 prong. I guess ill have to try both. Is that why old stuff has a ground lift switch?
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    peterh

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    Re: 3 prong power cord on st70?

    Post by peterh on Mon Mar 27, 2017 7:00 am

    buschfsu wrote:Well my head is spinning but i know that my turntable is grounded to my outlaw receiver that uses a 3 prong. I guess ill have to try both. Is that why old stuff has a ground lift switch?
    Yes, i think that is the reason for a "ground lift".

    Installing 3 prong mains cable enables the user to choose by plugging
    into either grounded outlet or an ungrounded. But the grounded
    outlet is the safest combination.
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    Peter W.

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    Re: 3 prong power cord on st70?

    Post by Peter W. on Mon Mar 27, 2017 7:48 am

    OK - I will get a little bit into this so that our Euro friends and our US friends can see what this is about.

    HUM can come from several sources - induced fields from transformers, signal crosstalk, wires carrying AC too closed to wires carrying signal, and so forth. Hum can also be exacerbated by having different ground fields within equipment.

    Getting *everything* to the same ground is not trivial and for several reasons. The most common being that the system *Neutral* and the system *Ground" are not necessarily the same. Back in the panel, yes. But there is often many meters/feet between the panel and the receptacle in use. Under the most modern NEC code extant here in the US, grounds and neutrals must use the same gauge conductor end-to-end. This was not necessarily so. Back in the day, (up to the 1970s), a separate ground-conductor could be one gauge less than the primary conductors. BX shielding could be used as a ground well into the 1980s. And receptacles could use a metal box as a ground if the box were grounded.

    Our house was built in 1890, with substantial improvements in 1928. Our previous two were built in 1895 and 1898 respectively. This is just an aside.

    So, there is no guarantee that the ground at any given receptacle is 'as good as' the neutral. In point of fact, in many cases, it is not. Any system, thereby, that uses a chassis ground as the third wire will have that chassis *ABOVE GROUND* as it relates to the neutral and the rest of the system. This is exactly the condition one wants to avoid. And, very often why adding the third wire can be a bad idea.

    Now, writing for myself, as we have a houseful of pets, two active and very curious cats and two active dogs, and as they are permitted in all parts and places in the house - and as often as not we have two, up to four grandkids in the house similarly permitted, I am a bit of a bug on basic safety. The guns have no firing pins available (much safer than a lock, and one is black-powder), and nothing is run with anything hot or 'hot' exposed to paws or small fingers. Sharp tools are out of reach, locked down or similar. So, every item available for touch has had a leak-test. I can be sure that there is no 'floating power' available on the exterior of any equipment in play.

    EDIT: This has been niggling at me and niggling at me. So, a few more words:

    Guys and Gals: If you are well-and-truly worried about stray voltages on the outside of any equipment you might have, there are ways of dealing with it. However, you really need to be certain of the need, and absolutely certain about *what* you are using for a ground, *where* it is and *exactly how* it relates to the system neutral.

    a) It is not enough that all the equipment be connected to the same receptacle as, if the ground in that receptacle is compromised in any way (if it is 'above' the neutral) the cure is worse than the disease.
    b) Any external ground chosen must be *better than* the system neutral and/or the system ground. Only in this way will you be sure that stray stuff actually bleeds off.

    What you can do at home:

    You will need a very good VOM capable of accurate measurements of MV and MA.
    You will need a 10,000 ohm resistor.
    You will need a very good ground.
    '
    Install the item-under-test (here-and-after ITEM) and turn it on. Run signal through it if possible.
    Between the ground and the ITEM, measure MA and MV. Multiplying these together should give you the number of watts involved.
    Install the 10,000 ohm resistor in series with either probe and remeasure. If the aggregate in this case exceeds 1/10 watt, you have a problem. If it exceeds 1/4 watt, you have a dangerous problem - and *SOMETHING IS WRONG* inside that item. Fix that before worrying about any third wire.

    NOTE: It should be 0MV and 0MA, within the limits of the VOM in any case. Nothing mains-connected escapes from my bench until it passes the above test.

    I am a great fan of polarized plugs so as to make sure that the 'hot' is on the switch, by the way. But I am not that much of a fan of grounding equipment that was not designed for it. There are other ways of making sure that there are no shock hazards - such as the one noted above.

    Lastly, electric guitars as connected to tube amplifiers which are often knocked about, poorly maintained, with wonky and leaky caps (deliberately chosen that way in some cases) and held by people often sweating a great deal, and often using metal picks, dressed in peculiar ways, imbibing strange substances, that often spill are an entirely different matter than a simple tube amplifier sitting on a shelf in the typical listening area. The fundamentals of user-safety are very different. I would expect that instrument amps would be grounded as a matter of course for that, alone.

    By the way, turntables are grounded for an entirely different reason than electrical fields - and more properly should be grounded to one of the headshells of the phono-amp input jacks. AR, Harmon-Kardon and a few other US manufacturers make a point of this in their operating manuals. AR turntables were factory-supplied this way - back in the day.


    Last edited by Peter W. on Mon Mar 27, 2017 2:31 pm; edited 1 time in total
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    arledgsc

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    Re: 3 prong power cord on st70?

    Post by arledgsc on Mon Mar 27, 2017 11:47 am

    In my youth I can't tell you how many times I was shocked by my old Fender Bassman amp.  They didn't come with a line voltage reversal switch for nothing.  For ultimate safety ground the chassis. You can always decide later to lift and isolate ground if needed but at your own peril. But if you are restoring the amp for maximum value think about keeping it stock.  

    To minimize ground noise plug all your equipment into the same AC wall socket.  Connecting one piece to a given AC wall socket and others to another is inviting ground loops as the actually common ground point will be back at you breaker switch load center.  And ungrounded equipment connections are actually sharing a common ground point via the audio interconnect.  Differences in ground potential cause current on the interconnects and create noise.  I have seen ground potential differences enough to fry oscilloscope leads when one piece is grounded at the plug (the o'scope) and the other piece floating its ground.  

    And adding an C14 power socket allows you to change power cords.

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