Grounding, again...

Discussion in 'Amp Tech Center' started by moosie, Mar 13, 2019.

  1. peteb

    peteb Friend of Leo's

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    if the high voltage finds a path to ground.



    Your plan is to DC float the circuit, but not AC float it. Right? The cap?


    All I can do is compare to 2 prong DC floated chassis on the old amps which I am very familiar with. Somehow the, the DC on the chassis stays within a few volts of earth ground level, unless of course the high voltage shorts to the chassis which is a rare occurrence.




    This only happens when the 2 prong Plug is oriented wrong and the chassis is hot with wall voltage.





    I wouldn’t worry about the voltage exceeding the B+, personally. 400 volts is bad enough. Higher voltages can even be safer. It burns the flesh so the electrical connection is lessened. But again, if you are not grounded, the voltage could be 500 or a 1,000 and you won’t feel it at all.




    That reminds me of an interesting experiment that I have either done or nearly done. Make sure you are wearing shoes. Open up the amp circuit and turn the amp on. You can do this standing on concrete, but if you want to be really safe, stand on a non conducting surface. Then, with one finger, touch the plate of the power tube. Nothing will happen.


    Variation: with one finger or one hand, touch the power tube plate and the chassis at the same time. This is also safe because the electricity travels thru your hand which is no big deal.
     
    Last edited: Mar 15, 2019
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    WTF?
     
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  3. peteb

    peteb Friend of Leo's

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    yeah, I know it is a little extreme.



    Haven't you been there, done that? Not on purpose of course.






    the main point here of course is that if you are not grounded, you cant get shocked.
     
  4. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Which is exactly my point. By ground you mean earth, and a floating circuit by definition has no path to ground.

    Interesting thought experiment... Is @dsutton24 some kind of EE? Calling.... :lol:
     
  5. peteb

    peteb Friend of Leo's

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    This is not a planned event, except the fuse is planned to fail when it called upon to fail.



    What I mean is that if and when the high voltage finds a path to ground, probably caused by some fault, then the fuse will fail.
     
  6. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Right, but in the case of the floating circuit and a tube plate/cathode short, which is what I thought we were talking about, no path to actual earth ground will be found. @clintj says the fuse will still blow because of the current. I get that in concept, but can't quite picture it in this situation.
     
  7. peteb

    peteb Friend of Leo's

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    how about picturing the primary of the PT plugged into the wall, and the secondary shorted out, shorted to itself, which is basically what happens when the tube shorts, the current goes back to the PT without doing any work.
     
  8. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Yeah, but it doesn't flow back on the same circuit as the fuse, which is on the other side of the PT. I "know" that doesn't matter, but it's what I seem to be stuck on. That and picturing the resulting voltage levels on the circuit after the smoke clears.
     
  9. clintj

    clintj Friend of Leo's

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    The whole amp is one big loop of current, as you've probably read a few times. Normally the bias point of the power tubes sets a quiescent current for that whole subsection of amp, but when the tube shorts all bets are off. Current becomes limited by the strength of the PT secondary and rectifier tube. Even if the circuit floats, the rectifier circuit will still strive to maintain B+ voltage, relative to whatever ground is set as.

    The rectifier/reservoir/CT loop will still attempt to maintain reservoir voltage ~350V above whatever the CT reference voltage is (and by extension the power tube cathode ground point), basically. This provides motive force to sustain the tube arc until something decides to be a fuse.

    Sent from my Pixel 2 using Tapatalk
     
  10. moosie

    moosie Doctor of Teleocity Silver Supporter

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    That helps. Thanks.
     
  11. clintj

    clintj Friend of Leo's

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    This may help. Transformer power in will always approximately equal power out. Low voltage/high current on the primary equals high voltage/low current on the secondary.

    What this implies is that if voltages are steady on each side of a transformer, then currents are proportional. A rise in current demand or power on the secondary side equals a rise in current demand or power on the primary. And, a large rise in current demand on the secondary side will cause a large rise on the primary which the fuse will see.

    Lastly, to use a real world example. A PT that converts 120 from the wall to produce 720 on the secondary is a 6:1 ratio. This ratio works on voltage and current. 120V/2A primary equals 720V/0.33A secondary.

    So, if the secondary sees an additional 1A load, the primary sees a 6A load.

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  12. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Well, that's just too friggin' obvious. Dang. Of course!

    My broken mental image had the two sides of the PT (for *this* thought exercise) kinda disconnected. In reality, it's more like gears on a bike. Fixed ratio. Pedal faster, wheel turns faster. Turn wheel very fast, pedals break ankles. Or something like that. :)
     
  13. peteb

    peteb Friend of Leo's

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    Consider the OT. The power tubes work into it. The OT presents the load the Power tubes work into. The load the power tubes see is not present unless the speaker load is coupled to the OT secondary. The OT reflects the load of the speaker onto the power tubes.


    Similarly, the wall power works into the PT and the PT works into the load of the power section. If there is no load on the PT secondary, then the primary has no load either. The primary is in a shorted situation. The hot is connected to the neutral, but without a load, it is a direct short, blowing the fuse.
     
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  14. peteb

    peteb Friend of Leo's

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    Thanks Moosie,

    If that’s acceptable, then why is ‘shorted’ preferred over ‘open’ on the secondary of an OT? The better option is reversed for PTs versus OTs. And what’s a light bulb limiter for?


    On a PT, a short on the secondary causes a short on the primary, blowing a fuse. The light bulb limiter makes it so the primary is no longer a short circuit.



    On the OT, a short on the secondary is preferred over an open. The open is bad because the transformer reflects back on the tube and the tube can’t operate backwards, or something like that. But what happens when the OT has a shunted or shorted secondary? The primary should go into a short circuit too, but it doesn’t. My thought is that a shorted tube is overly shorted but it is not a complete short circuit, the tube still has some internal resistance, I’m thinking the plate is not in physical contact with the cathode, the electrons still have to fly thru space. The remaining internal resistance of the tube acts in the same role as the light bulb limiter, it makes it so the primary is not a short circuit.
     
  15. peteb

    peteb Friend of Leo's

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    The subject was floating circuitry and it’s safety.


    A good comparison for floating circuitry is the old two prong design. I can’t explain why, but for some reason, something keeps the chassis that’s floating to DC, grounded, I don’t know if that’s the word, but DC floating chassis stays at zero volts DC and it might be the cap.


    But that makes no sense.


    Look at the coupling cap, it is of the same size and construction and it keeps the plate voltsge off of the grid.








    It has been confirmred. I touched the live high voltage plate pin, no problem. I don’t even have shoes on.
     
  16. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Found this article from Hartley Peavey that you may find interesting. (attached PDF)

    On page 6:
    If (for whatever reason) the SECONDARY of the [output] transformer has no “load,” the magnetic energy created by the primary (and “stored” in the core) has NO place to GO! As the alternating signal in the primary goes down, the magnetic field built up in the core “collapses,” thus inducing an extremely high voltage “spike” back into the transformer’s primary windings. Because this energy is substantial, and because the primary coils have many turns of wire, an extremely high voltage “transient” (often 3000 volts or more) can be developed. The primary of the output transformer is directly connected to the tubes and tube sockets of the amp, and neither the tubes nor the sockets (and associated wiring) are designed for that kind of voltage. These electrical “spikes” can (and often do) cause “arcing” inside the transformer and/or the tube elements, especially between the terminals on the tube sockets and the associated wiring.

    The "primary coils have many turns" statement alludes to the rest of the article, where he describes how a main problem with tube amp design is how to match tubes (high voltage low current devices) with speakers (low voltage high current). I know it's obvious, but the OT is a stepdown, with many fewer coils on the secondary side. When 'flowing' from primary to secondary, high voltage is made low (with the corresponding increase in current). But if forced to 'flow backwards', we now have already high voltage being made into extremely high voltage, and hitting the power tubes.
     

    Attached Files:

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  17. peteb

    peteb Friend of Leo's

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    Hartley Peavey is a good man.

    I expect he’s a straight shooter.
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Normally the power transformer center tap or bridge rectifier ground reference is connected to the chassis and the chassis is grounded through the safety ground and the safety ground is connected to a rod driven into the ground near the electrical service entrance to your home. Therefore if you touch a hot tube socket pin there is a path through you to the ground you're standing on and back to the power transformer secondary.

    But, if the power transformer secondary isn't connected in any way to the wall receptacle's ground or neutral terminals then yes, you can touch hot tube socket pins with no shock because you are not a pathway between any of the transformer secondary wires.
     
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  19. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Sounds like I'm definitely going to float the circuit then. I mean, I've always wanted to be able to do that. :twisted::D



    <sarcasm> :)
     
  20. peteb

    peteb Friend of Leo's

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    Thanks Rob. There is a couple of questions being answered here. I’m trying to understand why a chassis that is floating to DC, stays at zero volts DC. I’m not sure if that’s why you mention the centertap on the PT secondary, but that is a solid connection to the chassis. Is the centertap where the ungrounded chassis gets its DC ground reference?


     
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