Reverb Noise Induced By Speakers

[Fletcher Munson]

I've got a scratch build Vibrolux Reverb that has excessive hum on the reverb channel. In troubleshooting the problem I've concluded the noise is coming from the speakers, but I don't see much chatter here or elsewhere that indicates speakers as a source of reverb circuit hum. I'm wondering if anyone has experienced this or can offer suggestions.

Here's what I've done so far:

Speakers and reverb tank mounted in the cabinet in the usual way - No noise with reverb control set as to zero. The noise increases as reverb control is turned up. At high settings (8+) the reverb will go into oscillation, producing a whining sound. Swapping the tank end for end makes a small but insignificant difference.

Reverb tank removed from the cabinet and placed about 2 feet away (as far as the cables will reach) - No noise, no oscillation. The reverb circuit now sounds like a typical vintage amp.

Chassis and tank removed from the cabinet. The chassis is sitting in a cradle and the tank is quite close to the transformers and tubes (actually closer than it would be when mounted in the cabinet). The speaker cabinet is several feet away. - No noise, no oscillation.

So it seems that the noise is not coming from the transformers or the chassis, but is being induced by the speakers. Has anyone else experienced this? It seem like these particular speakers are problematic and my next test should be to try some other speakers in the cab. The speakers in question are one WGS G10C and one WGS G10A-8. The ceramic speaker in particular seems to have a very powerful magnet and is physically large, hence closer to the reverb tank.

[glebert]

How is the tank housed/mounted? I've had mechanical vibration of the cabinet create reverb tank noise. Usual course of action for me would be to decouple the tank from the cabinet (rubber mounts, etc) or in a bag, perhaps with some bubble wrap or foam to keep the housing from vibrating.

[g1]

I've had that happen when installing replacement speakers that had bigger magnets, or deeper baskets putting the magnet closer to the tank. I think the magnetic pull can set up a mechanical oscillation of the tank springs. I don't recall the hum in particular, but definitely the whining/feedback.
In those cases, the only solution was different speaker or increasing distance from speaker to tank.

[Enzo]

Reverb pans feed back acoustically. They howl, or whatever noise verb you like. This is due to the sound energy from the speaker vibrating the reverb pan or springs. SO in that sense the speaker does it. I mean the speaker is what makes the sound. Just like the closer your mic gets to the speaker, the more it feeds back. If you put a speaker in one end of a pipe, and the reverb at the pother end, That would feed back too. It isn't the speaker so much as the source of sound energy.

SO we do things:
we cover the open side of the pan - cardboard works. We run some foam strips down the long flat pan surface to prevent resonance. We put the pan in a bag to further isolate it. We don't screw it down tight to the cabinet. And we move the pan around to find the spot with the least standing waves or other acoustic sand traps.

Reverb pans can also be noisy, the transducer on the output end picks up hum. The power transformer in an amp has a strong magnetic field. And if you every walked up to your amp with a guitar strapped on, you can pick up that field with the guitar. A metal shield over the output end - or whole open bottom - helps shield ift. Also we want to mount the pan as far from the transformer as possible.

Note feedback and hum are separate problems. each has its own solutions.

You put the pan two feet away, yes, it is far from the transformer for hum, and far from the speaker for feedback.

If your pan is not reacting to the transformer, count your blessings. The speaker drives acoustic energy into the reverb pan, which is a mechanical assembly. I can think of no method by which a speaker magnet can cause hum. I could see it attracting magnetic parts in a reverb and making it more sensitive to feedback, but look carefully at what is and is not magnetic in there.