Humbucking Reverb Tank?

[jimboyogi]

Hi all,

Firstly I'm not sure that this is the best part of the Forum to post this, so feel free to move it if required.

Now a few years ago I built a home-brew tube combo amp including tube driven and recovered spring reverb. During the design and breadboarding phase the reverb tank sat well away from the chassis, and all worked perfectly. But when I eventually put the chassis and the tank into the cabinet, I had issues with EMF noise pickup in the reverb tank "output" transducer.
I started a thread here, and got excellent advice thanks! Ended up putting a steel cover on the PT, and a steel plate covering the open side of the tank. These measures reduced the noise to an acceptable level, not perfect, but acceptable.

I have just put another build together, different circuit, different cabinet dimensions, same problem! If I remove the reverb tank from the cab and move it away from the PT, the noise disappears. There is no orientation of the tank within the cab that reduces the noise to acceptable levels.

I know that I can reduce the noise with shielding like I did last time. I even tested mu metal around the tank and it was quite effective also (not my mu metal, I'd have to spend the $ to get some!)

But looking closely at the tank transducers, got me wondering if they could be converted to a hum cancelling set-up, using 2 coils per transducer, reverse connected, and mounted on opposite magnetic polarity parts of the laminations. I know it works for guitar pickups, so could it work for these transducers?
Has this been done previously?
Is it possible?
Is it safe?

Any information on previous trials of this idea, or details on the workings of the transducers would be greatly appreciated.

[Leo_Gnardo]

Interesting concept! First you wouldn't need any humbucking coil on the drive side, only on the pickup transducer. On that, I have 2 ideas. 1: Got any broken tanks around? If you had one with an open drive coil, you could remove the pickup coil, try wiring that in series with your amp's reverb tank pickup coil, and placing it inside the tank, mounted very close to that coil, and oriented the same way. Polarity? One way will give you more hum, the other way should cancel most of the hum. On this you'll have to experiment.

Idea B) similar to the "remote" humbucking coil for Strats etc. offered by Suhr and I'm sure by now others, you could create a coil using a larger geometry, also larger and easier to handle wire, mount that on a piece of cardboard or fish paper, place it near the pickup end of your tank, and experiment with polarity & placement to give you optimum hum reduction. Your HB coil would be wired in series with the pickup coil in your tank.

Safe? I don't see what could go wrong as far as safety for you or your amp.

Good luck!

[Chuck H]

I'm down to hear how it goes. Because I tried it without much luck. It's possible I didn't take the experiment far enough in the optional incarnations though because I HAVE had luck using dual inductors for EMF noise abatement for other problems.

But I didn't key up just to say I failed to get satisfaction with what you propose. In my particular case I ended up mounting the reverb tank cattywhompus on the lower rear baffle. It was the only orientation that didn't pick up hum. The point is that there is nearly always a position that won't pick up hum and you should keep looking for it. That was the first time I had to mount a tank somewhere other than the bottom of the cabinet. Usually there IS a place in the lower cabinet that will be hum free. I had one project where I mounted a reverb tank in a head cabinet. Of course it hummed. Scootching the tank around I found a quiet spot. Moving it even a half inch on any corner resulted in the hum returning, but there WAS that one quiet spot. So get the amp up and running, controls set to instigate hum and with the tank unmounted in the bottom of the cabinet. Then scootch it around. It's probable you'll find a quiet location. Try EVERYTHING. Reverse the tank from what others say is an ideal orientation if you have to. Hell, try it on it's side on the lower rear baffle in BOTH orientations.

[Mick Bailey]

With your amp build do you have sufficient LF rolloff in the recovery stage? The recovery tube coupling cap should be low enough to give a corner frequency of about 400 or 500 Hz, which will go a long way to reducing hum.

[Chuck H]

With your amp build do you have sufficient LF rolloff in the recovery stage? The recovery tube coupling cap should be low enough to give a corner frequency of about 400 or 500 Hz, which will go a long way to reducing hum.

I want to say this well enough that it's not offensive. Sometimes I fail at that, so please take this at face value.

I think that no matter what values are in the circuit now it's still a good practice to mitigate as much EMF influence as possible. So that should be addressed. Then, I agree with your design criteria in this matter. In fact, as much so as hum reduction it's very hard to keep a tank acoustically stable if there's too much LF allowed through the recovery end. I like a big sounding reverb with a little more low end on it than most modern amps and I've had no end of trouble with acoustic stability because of this.

[Helmholtz]

details on the workings of the transducers would be greatly appreciated.

It should be possible, but you would need to place the 2 coils on opposite limbs of a 2-limb core output transducer. This arrangement is necessary to produce inverse signal flux directions/polarity in the coils, while the interference flux is the same for both coils.

Better output transducers have a closed 3-limb core that picks up less hum.

[Justin Thomas]

Thinking on the effects of springs... Would a stiffer but maybe longer spring help allow more low end verb without feedback? I have no idea what I'm talking about. Just random wacky ideas.

Jusrin

[Mick Bailey]

I want to say this well enough that it's not offensive. Sometimes I fail at that, so please take this at face value. I think that no matter what values are in the circuit now it's still a good practice to mitigate as much EMF influence as possible. So that should be addressed. .

Don't think that I'd be offended - always happy to see the other side of the discussion.

Agreed - if you can get to the root of the problem that's always a better starting point than mitigating a problem later on down the line. I'm also keen to see where the humbucking idea goes, though most reverbs in most amps work well enough - even if there's relatively close proximity to the PT as with Traynors where the tray is mounted right on the amp chassis, or Marshall heads where the tray mounts right behind the upper front panel. A combination of screening, orientation and HPF usually works well enough in combination but a tray that hums badly due to physical location or proximity can be a problem. I guess those manufacturers didn't just end up with relatively hum-free reverbs by pot luck and they had to work it out through the design stages at some point.

It always struck me that Accutronics in particular didn't fit a shielding plate over the recovery transducer. I have a standalone reverb that has a slight hum that's only really noticeable because I'm listening for it and its my own design. If I fit a Cascade tray then I don't get the hum. The difference is that the cascade has that plate over the transducer (it also has a peaking capacitor across the output coil). Seems to make quite a difference in this particular situation.

[jimboyogi]

Hi all, thanks for the replies, and Happy New Year!

Leo_Gnardo - I have a few functioning tanks, and I can canibalise them for coils to experiment with. Your first idea is the one I was thinking of, I hadn't thought of the "dummy coil" idea, but it's a ripper of an idea and worth pursuing.

Chuck H - Sad to hear that you have tried this and not succeeded yet. Re finding a EMF noise free spot somewhere in the cab, I've tried but I can't find any spot where the tank fits that gives acceptably reduced noise

Mick Bailey and Chuck H - I'm on the same path as Chuck here, I have kept plenty of LF in the return circuit, because I really like the extra depth of tone in the reverb. The EMF noise sounds to me like it contains lots of harmonics of 50Hz (I'm in Australia), so any effective LF filter would have to have a fairly high corner frequency to reduce all the harmonics.
WRT acoustic feedback, the first problem I had when I put this amp into it's cab was LF acoustic feedback. It was only when I solved this that the EMF problem was revealed in all it's glory! I found a very effective cure for the feedback, using a self-adhesive butyl dampening sheet designed for automobile chassis. I stuck a strip of this onto the reverb tank, along the edge of the large face, and folding onto the long edge, and the two short ends. It completely cured the acoustic feedback, I was astonished how well it worked.

Helmholtz - This is along the lines I was thinking.
I have both Accutronics and MOD tanks.
Accutronics tanks have an open "U" shaped drive transducer laminations, with the coil on one limb. The output transducer has "EI" shaped laminations, with the outer edges continuous, and the inner central limb not connected on one side, and the coil on this inner limb.
MOD tanks use the same transducer shape for both drive and output, the same "EI" shape/style as the output transducer on the Accutronics.

To fit 2 coils onto the opposing limbs to create opposite magnetic fields in the coils, the "U" shaped laminations would be far simpler to adapt or fabricate. However is this a less advantageous shape for magnetic function? Is it akin to amp output transformers with or without an air gap?

[Helmholtz]

The output transducer has "EI" shaped laminations, with the outer edges continuous, and the inner central limb not connected on one side, and the coil on this inner limb.

THis what I meant with closed, 3-limb core. This core type has better hum rejection but is not suitable for humbucking because signal flux in the outer legs is symmetrical/in-phase.

Humbucking can only work with the U-shaped core. Requires 2 identical coils on the legs.

Both core types have airgaps. But they probably have different effect on transducer inductance/impedance and maybe sensitivity.
Also adding a second (anti-)series wired humbucking coil will increase transducer impedance by up to a factor 4, but will about double output.

You may also wire the 2 coils (anti-) parallel. This will only slightly reduce impedance and give about the same output as the single coil.

[R.G.]

Humbucking is a way to use iron and copper to pick up an equal non-signal hum and electronically subtract it from signal+hum. It's an exercise in cancellation. The classic humbucker pickup uses a magnetic+winding polarity setup to both cancel hum and double signal level. This requires a special construction core and winding techniques.

If you're willing to sacrifice the doubling of signal level, you can simply put an identical pickup coil very near the functioning pickup coil, but not feed it signal, and arrange its "output" to oppose the actual signal pickup. This dummy coil picks up only the interference and can be arranged to mostly cancel the signal coil interference.

So you might be able to remove a pickup coil from an otherwise dead tank that matches your good tank, mount it near the pickup coils in the good tank, and use the dummy coil to cancel much of the hum. It won't be perfect, but it might be enough. This will, as noted, affect the notional "impedance" of the pickup coil when it's connected in series-opposting. However, that's probably not too bad. The DC part of the impedance will be doubled - maybe up to a few K of resistance - as will the inductance. However, this just means, like with guitar pickups, that the input impedance of the input it feeds must be "high". In practice, this isn't too difficult to do. You don't need matched impedances, just enough.

Of course, a soft-iron shell wrapped around the whole pickup coil might be good enough too. Mumetal is expensive, and soft iron may be good enough.

[Mark Hammer]

The Craig Anderton-designed PAiA-manufactured "Hot Springs" reverb used an ostensibly hum-bucking design; utilizing dual reverb pans wired up reverse phase to each other. Here is someone's meagre demo of their build.

[Helmholtz]

The DC part of the impedance will be doubled - maybe up to a few K of resistance - as will the inductance.

Inductance will double only if the dummy coil has a similar core as the transducer. Using a dummy coil with core is advisable anyway as the core influences its hum sensing properties. A high permeability core focusses the interference field.

The impedance of reverb transducers is typically more than 10 times higher than the DCR, indicating that the impedance is mostly determined by the inductance.

Output transducer inductance may matter if "resonance tuning" with a parallel cap is used to improve high frequency response.

[Leo_Gnardo]

The Craig Anderton-designed PAiA-manufactured "Hot Springs" reverb used an ostensibly hum-bucking design; utilizing dual reverb pans wired up reverse phase to each other. Here is someone's meagre demo of their build.

There's a unique approach! Good ol' Craig, he would think up something like that.

In comparison, many years ago I was asked by a studio owner if I could reduce/eliminate the hum in his expensive and presumably well-engineered Klark-Teknik "studio quality" spring reverb. I got nowhere. If KT had employed a method like this, or else a dummy coil, they'd be so far ahead of the game! In the end, I told him to find an electrically "quiet" place in his building where he could park his KT reverb, and run long balanced lines there and back.

[nickb]

An alternative to high pass filters it the twin T notch. The one shown below inserted between the reverb output and the recovery grid will give you more than 30dB of attenuation at 50Hz. With a little tweaking of the values you can adjust the null to match your exact frequency.