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Thread: Weird OT issue ?

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    Weird OT issue ?

    Hello,
    I just noticed using one Hammond OT and trying to apply some nfb. Keeping common secondary lead to whatever ground point or keeping isolate didn't do any diference. I checked DC continuity in isolation condition and shows secondaries are isolated from ground, the output jack's are suspended from chassis, even the iron is physical dc isolate from its support. I injected 9db feedback in my amp and from an accident I disconnected the common ground lead from the ground. I didn't noticed any diference . I repeated more carefully, swapping signal from 100 to 10000 hz, no diferences,the feedback loop is pretty stable with, or without common lead grounded, it looks like OT was grounded by default, despite the fact it is completely isolated. In fact it not exist a physical nfb return.Now I wonder if my transformer could be as well AC coupled with my steel chassis to not see any diference in terms of grounding. It is normal , please? Thanks

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    Last edited by catalin gramada; 08-25-2019 at 10:14 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    And the circuit:

    Click image for larger version. 

Name:	20190825_131200.jpg 
Views:	37 
Size:	2.22 MB 
ID:	54890

    How nfb loop can be functional without nfb return to common OT tap, please? Thanks

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    Bent Member Chuck H's Avatar
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    It looks to me like the global FB may be getting cancelled by the anti phase of the triodes at the shared cathode because both are elevated by a 47R resistor. What happens if lift the global FB loop? Any change? My thinking is that you may not be seeing that you still have a FB loop without common ground, but rather that you DON'T have a FB loop even with a common ground.

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    Hey, thanks for you input.
    The feedback is applied only over 47ohm resistor in cathode of first stage, the 100u over shared 820ohm resistor drain ac voltage to the ground, the 47 ohm over non inverted stage is there just to keep the bias as same and is not compulsory, so nothing is cancelled. The amount of feedback works exactly as should be. Lifting the nfb wire put the voltage gain of the stage with 9 db more.
    I still not see it the issue in circuit, the global feedback works very well, except the fact lifting nfb return didn't do any diference. The OT is completely isolated by any grounding point. I don't get it....

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    This is a pic at onset clipping with 9...or 10 db nfb applied ( not sure ...did a lot of changes) with nfb return lifted. It looks pretty decent, nothing to worry, ..but how the heck it works just with one wire...?

    Click image for larger version. 

Name:	20190825_183238.jpg 
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Size:	2.46 MB 
ID:	54898

    and with nfb lifted

    Click image for larger version. 

Name:	20190825_184549.jpg 
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    Both pictured without any nfb return, without any common secondary tap physical tied to any ground. DC is infinite

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    Last edited by catalin gramada; 08-25-2019 at 05:55 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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    Bent Member Chuck H's Avatar
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    Are you sure the load is floating also? Because that could provide a return path to 0V if it's weren't.

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    Yes, I removed the output jack's from chassis to be sure and the power resistors are away on my table. It's woo-doo I tell Ya.
    What it remains to do is to remove the OT bolts and to move it away to see if the loop is not closed by induced current in chassis - if it have any sense...

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    Last edited by catalin gramada; 08-25-2019 at 07:59 PM.
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    Oh, wait a sec.... Can be the loop closed through the probes via wall outlet and far away through earth protection of the amp ? Should be a long return, but still...?

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    Bent Member Chuck H's Avatar
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    I'm interested. I'd be surprised if the OT EMF is coupling/damping at the chassis so completely that the NFB level doesn't change significantly from an actual common connection to 0V. Not much to work with. I'm out of ideas.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by catalin gramada View Post
    Oh, wait a sec.... Can be the loop closed through the probes via wall outlet and far away through earth protection of the amp ? Should be a long return, but still...?
    I thought about this, but it seemed unlikely. The probes should have considerable impedance.

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    ....no, no...Bingo! ...I just checked ...between probes and amp chassis get 0.25 ohm resistance. It is a healthy path....Thank You.

    The alligator clip which was connected to the common tap get contact direct with the chassis through 0.25 ohm resistance of earthing wiring.

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    Last edited by catalin gramada; 08-25-2019 at 08:21 PM.
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    Quote Originally Posted by catalin gramada View Post
    ....no, no...Bingo! ...I just checked ...between probes and amp chassis get 0.25 ohm resistance. It is a healthy path....Thank You.

    The alligator clip which was connected to the common tap get contact direct with the chassis through 0.25 ohm resistance of earthing wiring.
    Oh.!. You have the scope plugged in and obviously grounded. So because you're measuring AC at the load you have the scope - connected to one end of the load and the + connected to the other. Makes sense now. Glad you found it before unbolting the transformer

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Chuck H View Post
    Are you sure the load is floating also? Because that could provide a return path to 0V if it's weren't.
    Quote Originally Posted by catalin gramada View Post
    Yes, I removed the output jack's from chassis to be sure and the power resistors are away on my table.
    Quote Originally Posted by catalin gramada View Post
    ....no, no...Bingo! ...I just checked ...between probes and amp chassis get 0.25 ohm resistance. It is a healthy path....Thank You.

    The alligator clip which was connected to the common tap get contact direct with the chassis through 0.25 ohm resistance of earthing wiring.
    'Floating' load means NO other earthed equipment (like scope) be connected. This is also very problematic when testing solid-state amps that use current feedback.
    Best practice is to leave scope ground connected to amp chassis, or measure at load with a hand held multi-meter that has no earth connection.

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    Member uneumann's Avatar
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    Quote Originally Posted by g1 View Post
    'Floating' load means NO other earthed equipment (like scope) be connected. This is also very problematic when testing solid-state amps that use current feedback.
    Best practice is to leave scope ground connected to amp chassis, or measure at load with a hand held multi-meter that has no earth connection.
    True - problematic, but many dual-channel scopes allow A-B display so they provide a solution. The A and B channel probes can be connected to the two floating outputs, with their ground clips both clipped to chassis. This safely avoids the ground problem described above, and it shows exactly what the output (spkr) signal looks like.

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    Quote Originally Posted by catalin gramada View Post
    The feedback is applied only over 47ohm resistor in cathode of first stage, the 100u over shared 820ohm resistor drain ac voltage to the ground, the 47 ohm over non inverted stage is there just to keep the bias as same and is not compulsory, so nothing is cancelled. The amount of feedback works exactly as should be. Lifting the nfb wire put the voltage gain of the stage with 9 db more.
    I still not see it the issue in circuit, the global feedback works very well, except the fact lifting nfb return didn't do any diference. The OT is completely isolated by any grounding point. I don't get it....
    Why do you say so?
    The "common" end of the OT is connected DIRECTLY to ground, per your schematic .
    And the 8 ohm tap is connected to first PI cathode through a 2k2 resistor and a series 100uF capacitor.

    NFB loop, at audio frequencies, is 2k2/47 ohm

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    Hi. Yes this was the circuit. My issue was related by accidental disconnection of nfb return which You remarked in the drawing. I didn't saw it the earth grounding path because is pretty unfamiliar for me on my bench. I did my hobby for years in a building without earthing connection at wall sockets and anyhow I used to use grounding paths soldered even for experiments. Now it was happen to use a strap wire which accidental was disconnected, is not happen to often to me, especially when work over hundred volts...but was happen and didn't know where to look. I have limited experience, I didn't learn from books, just test some circuits when have time and save what I consider relevant for futures builds. I found the source of problem by determination not by analysis. I checked DC resistance in circuit removing the load, the scope was conected over my load resistor, I did not saw it.
    Got a lot of good advices over here, Thanks guys!

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    Last edited by catalin gramada; 08-28-2019 at 04:43 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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