Humbucking Reverb Tank?

[Mick Bailey]

Try your setup with just the main coil to see if that signal is weak on its own.

[jimboyogi]

Try your setup with just the main coil to see if that signal is weak on its own.

I can't remember if I tried that. I was doing the experiments on the floor, in a small space. Ended up hurting my back from all of the awkward bending over!

I've still got the experimental tank, so I will try what you suggest to make certain.

[Helmholtz]

So I hacked into an existing core, cutting one end from an "EI" output core, so it resembles an "E" with elongated upper and lower horizontal edges. I then stole an extra coil from another tank, and mounted 2 coils onto this core - 1 on the central limb, and 1 on the lower limb, both mounted near the limb ends. The magnetised coil end beads then sit between these 2 limbs. This should give these 2 limbs opposing magnetic fields as the beads move between them, giving the reverse polarity required for the hum cancelation.

I don't think an E core can provide the necessary symmetry of flux distribution (especially if the limbs have different lengths).
So only partial compensation will result.

Humbucking results may improve if you cut off the "unused" upper limb.

I wonder if by opening up the laminated core, I lowered the efficiency of the transducer significantly, causing the loss of reverb signal recovery.

Yes, opening the core increases leakage and reduces transducer sensitivity. And lower transducer sensitivity means worse signal to hum ratio...

[Helmholtz]

I think I just identified an essential issue with the 2 coils on a common core arrangement: "Transformer" coupling between the coils.

Ideally the 2 coils should produce equal hum voltages but out-of-phase signal voltages, so that anti-phase wiring results in hum cancelling and signal adding.
The coupling of the coils however mutually adds part of the voltage of the other coil, increasing the hum signal but decreasing the signal components in both coils.
With 100% coupling the output should be zero: No hum and no signal.

Humbucking PUs work because the magnetic coupling between the coils is weak (e.g. 20%).

This all speaks for a separate dummy coil with own core.

[jimboyogi]

I think I just identified an essential issue with the 2 coils on a common core arrangement: "Transformer" coupling between the coils.

Humbucking PUs work because the magnetic coupling between the coils is weak (around 20%).

This all speaks for a separate dummy coil with own core.

This is fascinating info. I have some questions though -

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?

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?

[Helmholtz]

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.

[jimboyogi]

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!

[jimboyogi]

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.

[Helmholtz]

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.

[jimboyogi]

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.


Now with reverb switched ON at foot switch.


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

[Helmholtz]

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.

[jimboyogi]

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.



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.

[Helmholtz]

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

[jimboyogi]

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!

[Helmholtz]

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!

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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.