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Thread: Humbucking Reverb Tank?

  1. #36
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    Why is the magnetic coupling between coils in a humbucking PU so low, as the coils are coupled via a per magnet?
    Is it related to the alnico bar magnet material that joins the iron cores (screws/poles) in the PAF style PU?
    Perfect coupling would mean that all the flux through one coil also runs through the other coil, in other words both coils should see the same flux. This is approximately achieved in quality transformers having closed, high permeability cores with no airgap. As soon as part of the flux runs through air (air is a weak magnetic conductor!), there will be flux leakage and reduced coupling.

    A humbucking PU has a large airgap between the poles and the partial coupling provided by a bar magnet is low, because its AC permeability (= magnetic conductivity for flux changes) is very low.

    Some permeability values:

    Air: 1
    Alnico ~5
    Ceramic magnet : ~1
    Neo: ~1
    Transformer steel: 500 and more.

    As indicated before humbucking PUs need to have low coupling for high efficiency.

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    Last edited by Helmholtz; 01-09-2020 at 02:18 PM.
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  2. #37
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    This is so interesting wrt Humbucking PU and their design.
    I've noticed, both with my own build PU, and commercial ones, that any inclusion of a steel "reflector" plate providing magnetic coupling between the coils, always increases the ambient hum from the PU. I have to think that you have just provided me with the explanation!

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  3. #38
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    Reporting in on my trials using a dummy coil.
    I took the output transducer and coil from another tank, and trialled it as a dummy coil. I connected the two output coils (stock + dummy) both in anti series and in anti parallel to compare the noise. I moved the dummy transducer/coil around the stock one. There were locations where the noise decreased significantly, and others where the noise increased. The location has to be exact, and almost impossible to locate the dummy coil exactly while the tank is in it's final position!
    While I had the 2 coils wired in anti series, I found the quietest location for the dummy, and then shorted out the dummy coil leaving me with just the stock coil. In the quietest locations, the noise signal was roughly the same either stock or stock + dummy. For any less quiet location, the stock coil produced less noise than stock + dummy.
    For this experiment, I wasn't able to get any practical EMF noise reduction benefits from using this dummy coil.

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  4. #39
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    Re. the dummy coil core, does it have to match the transducer core wrt size/amount of iron/any other factors, or does it just require a core to fill the bobbin?
    I missed this question. Answer is yes, best is identical coil and core. Also the dummy transducer should be mounted very close to the main one with same core orientation.


    Just to make sure that you don't have some other hum source, short the reverb output transducer at the tank's jack and see if you you still have hum.


    If there is enough space you could use the shell of another (defective) tank as a magnetic shield to make a closed Faraday's cage.

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  5. #40
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    I have done some more investigation on reducing the hum/noise pickup from the reverb tank.

    Firstly, after all of the excellent help and advice you have given me, I gave up on the hum bucking idea, and ordered a sheet of mu metal to shield the "output" side of the reverb tank. It arrived recently, so I dug my Oscilloscpe out of storage (house renovations!) to see if I could measure the noise reduction from using the mu metal. I couldn't. The mu metal shielding reduced the noise significantly, but I couldn't get a measure on that because the dominant 50Hz noise is all that the oscilloscope measured. But the Oscilloscope did give me some other useful info...

    Screen shots below, measured from the speaker out, Gain, Reverb and Volume controls at max, nothing plugged into amp input.

    First on with reverb switched OFF at foot switch.
    Click image for larger version. 

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    Now with reverb switched ON at foot switch.
    Click image for larger version. 

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    The level of 50Hz hum is hardly increased by switching on the reverb. However the buzzing noise that I have been trying to reduce is riding on the 50Hz. I think that it looks like diode switching noise? The PS uses UF4007 diodes, full wave.

    I'm wondering if my problem here is that the diode switching noise is radiating from my toroidal PT, and that is why I'm getting so much noise pickup from the reverb tank? I tried again moving the tank around in proximity to the amp chassis, and the noise is greatest when the reverb tank "output" side is closest the the PT.

    So perhaps the next thing for me to try is to remove/reduce this diode noise by trialling snubbers. I've done some reading today on this, and think I'll try a RC snubber across the PT secondary. If anyone has suggestions for R and C values, I'm all ears

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  6. #41
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    Just to make sure that you don't have some other hum source, short the reverb output transducer at the tank's jack and see if you you still have hum.
    Did you try this^^^?

    Some pictures of the interiors of your amp and cab might be useful.

    What kind of cables do you use to connect the tank? Maybe the shielding is not good enough.

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  7. #42
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    I did try shorting the output transducer at it's RCA jack, and the noise disappeared, the noise signal on the oscilloscope reads the same value with the shorted jack with reverb ON at footswitch as with reverb OFF at foot switch without the short.

    The cable is a commercial Fender style bought from Antique Electronic Supply, and cut in half with a stereo 1/4" plug fitted to the amp end, stock RCA plugs on the tank end.
    Click image for larger version. 

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    Here's a couple of pics of the PS section of the amp chassis. PT is an Antek toroid, with 2 secondary windings of 250V (yellow and white wires), connected as a two phase rectifier, with one of each pair of secondary wires as CT, one of each pair as HT. Two UF4007 diodes in series for each phase, giving ~ 330VDC at the first reservoir cap. The white and yellow wires enter the eyelets from under the board.

    Click image for larger version. 

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    Click image for larger version. 

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  8. #43
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    It would be interesting to see the mounting position of the tank wrt the PT.

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  9. #44
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    Apologies for the delay, here's a pic of the chassis and tank in situ, with the cover off the back the chassis. You can see the bolt that attaches the toroidal PT centre left of the chassis.
    The "output" transducer and jack is RHS of the reverb tank, with the jack facing away from camera. I hope this is helpful!

    Click image for larger version. 

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  10. #45
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    Quote Originally Posted by jimboyogi View Post
    Apologies for the delay, here's a pic of the chassis and tank in situ, with the cover off the back the chassis. You can see the bolt that attaches the toroidal PT centre left of the chassis.
    The "output" transducer and jack is RHS of the reverb tank, with the jack facing away from camera. I hope this is helpful!

    Click image for larger version. 

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    Thanks.
    There seems to be sufficient distance between the PT and the tank. I would use shielded wiring inside the chassis connecting to the reverb jack.

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  11. #46
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    Thanks Helmholtz. I'll have a look at the wring from reverb jack to gain stage grid.

    I did some trials on the PS to see if I could reduce the buzzy noise pickup (pictured ^ thread a bit) by cleaning up the diode switching noise.

    I tried -

    Between first diode/PT secondary junction and ground, for both phases -
    10nF and 1K in series - no change to noise signal voltage when reverb on and tank close to PT
    10nF and 1.5K in series - no change as above (these are the values suggested by Merlin in his Hi Fi preamp book)
    10nF and 2.2K in series - no change as above

    Between the two PT secondary phases -
    100nF cap - increased the noise signal substantially as measured on my oscilloscope!

    So from these experiments I was not able to reduce the noise pickup. And the cap across the secondary windings made the noise pickup worse. Either my component values are not optimised for effective snubbing, or else I'm barking up the wrong tree!

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  12. #47
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    the noise signal on the oscilloscope reads the same value with the shorted jack with reverb ON at footswitch as with reverb OFF at foot switch without the short.
    Is the footswitch cable (as well as the switch itself) well shielded? Also the amp internal wiring to the footswitch is critical.

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  13. #48
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    The foot switch cable is a microphone grade shielded cable. I just did the experiment of measuring the noise signal on my 'scope with the footswtich plugged into the chassis jack with Reverb ON, then unplugging the foot switch from the chassis (reverb is ON when footswitch cable unplugged). There was no difference in noise signal voltage or audible noise between these two modes. I think that this indicates that the foot switch is not contributing to the noise pickup.

    I also measured the noise signal with reverb on, with and without a mu-metal shield around the reverb tank at the "output" transducer end. Without the mu-metal = 1.1Vpp. With Mu-metal = 0.4Vpp, So the mu-metal does provide a big shielding effect for the transducer.

    WRT my theory of diode switching noise, I also scoped the HT rail in the amp at each of the filter capacitor nodes. There was no sign of any HF spikes on any of the nodes. So I think that this, combined with the failure to get any PS snubbers to improve the noise signal pickup as outlined in previous post, points to this diode noise theory being incorrect.

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  14. #49
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    It might make sense to look for external sources of magnetic interference.

    You can use a high impedance reverb transducer coil (or a single coil PU) connected to the amp's input as a "sniffer".

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