Insulating the reverb pan?

[Chuck H]

I'm having some poor luck battling acoustic feedback with a combo cab/reverb pan relationship. I've tried a few things already. Any tips on what has worked in problematic cases???

[Leo_Gnardo]

I'll give it a go Chuck. The old Fender method of stuffing the tank into a "sock" made of spare Tolex is a good starter, and of course covering the open side of the tank. Sometimes I get some relief from feedback howl by wrapping the tank in a layer of packing foam, sheet type about 1/2 inch thick. For the toughest cases - get yourself a roll of "Camper mounting foam" from your local Home Despot or similar store. It's about 1/8 in thick and 1.25 or 1.5 inch wide. Should cost @ 8 bucks. Stick strips of camper foam on every inch of the tank's external surface leaving the RCA jacks peeking through. Even cover the ends. Cover the open side with the typical sheet of corrugated cardboard, then put a layer of foam on that. Now it's completely wrapped like a mummy & will just barely fit in the "sock." This has worked for me on a couple of occasions when reverb howl-around was vexing me awfully. Hope it works for you.

A roll of camper tape will cover 2 or 3 long tanks. I make sure I keep some in the workshop all the time now. Note that it cuts easily with a fresh razor blade. Try it with a worn blade & you'll be struggling to make a cut.

Sometimes an old tank will be extra sensitve to all vibration - every footstep will set it off. Don't have a solution for this except replacement.

[Chuck H]

Thanks Leo. I've been mummifying the tank in steps. Mostly cotton fabrics. Not much joy. I've used foam in the past with some success and was planning on trying it again. Since I have a piece of 1" memory foam (that is about to be discarded, minus the exact amount needed for wrapping a reverb pan) I'll try that next/again. I'll wrap it in the foam and then wrap the foam in tape to squeeze it down a little. Then maybe I can stuff it into the bag like a big reverberating sausage.

I'm still open to success stories since this amp is NOT with me. When I do get to where it is I'll want my game in check. So... Foam = good. But was already on the list. Anything else???

[Leo_Gnardo]

There are more expensive sheet foams intended to keep auto panels from rattling. They're denser than the camper-top gasket foam I mentioned, but a good deal more expensive. Parts Express sells these and you may be able to find panel-deadening foam at a big auto store or an outfit that sells/installs x-pensive car stereos. Some of the hi tech ones even have a layer of sheet lead, yes lead the deadly metal. I haven't gone this route yet because I'm a cheap so and so.

You're trying to "sound proof" the walls of the tank, also mechanically decouple the tank from the board it's mounted to.

Yes, any other ideas out there? I'm tapped out.

[Enzo]

Leo has good advice, I offer my standard response that overlaps his.

Reverb pan must not have solid contact with cab, ie don't screw it down tight. It should have a cardboard cover over the open side, and then should be inside a reverb bag. Always check inside the pan that the long springs are intact and that the four short corner suspension springs are OK. The innards are suspended, and can potentially hop up on top of the posts that are there to limit motion. Check that.

It matters where inside the cab we are mounted, sometimes it only takes an inch to the side... And sometimes it helps to turn the thing upside down.

The top flat surface of the pan can resonate, so glue a strip of self-adhesive weatherstripping foam down the centerline of the top surface to dampen any of that. car makers do that inside large body panel walls to kill sheet metal noise. It doesn't much matter what you use, I guess you could run a bead of silicone rubber down the top too.

If you are insulating it with foam sheet, then I'd think the last thing you want to do is "squeeze it down a little." The fluffier it is, the less sound energy gets to the pan.

It helps to determine just how the feedback occurs. Isolate the problem. For example, we can fight the pan all we want and it won't help at all if it is really the recovery tube being microphonic. Does this happen if the pan is sitting outside of the cab? Assuming it is not yet bagged, set the pan in the cab bottom and get it howling. Now just lay your hand on the top surface. Does that damp off the howl, or does it keep going? Does flipping it over make a difference? Does sitting it on edge make a difference?

[Mick Bailey]

I had a similar problem and used acoustic wadding that's used in hi-fi speaker cabinets. Actually I found out it was hollow dacron fibre used in pillow stuffing as well, and not specific to the hi-fi market, so bought the cheapest pillow I could get for a good supply. I thought it was going to be random fibres, but the stuff I got was in a layered sheet so was easy to wrap around the tray. Here in the UK it has to be fire retardant too. I found that it transmits much less vibration than foam, which didn't cure the problem. The wadding worked a treat, but who's to say anything will work in another situation.

[R.G.]

I agree that it's worth figuring out whether it's really the pan or not. Get some hifi RCA extension cords and move the pan outside the enclosure several feet and see if the feedback changes. If it does, then working on the pan will help. If it doesn't, it's not the pan, or not primarily the pan.


If it's the pan, what you're trying to do is to attenuate the amount of sound that's moving the reverb pan relative to the inertia-mediated position of the springs. You can either do broad-spectrum deadening, or attenuate just the frequencies the pan transmits. In metal sheets, you're generally trying to dampen the many resonances of the metal sheet, and these resonances may be insanely high-Q. Damping the whole panel may not be needed.

Damping is literally trying to eat energy out of the panel, rather than letting it ring with the exciting sound. Padding and stuffing primarily lower the incoming sound intensity to the degree that they can. Car body dampening foams work on the mass/damping principle of putting a lossy material (often vinyl or rubber foam or low-viscosity gell) on one side of a high density sheet of metal, often aluminum, for reasons having to do with price.

The premium damping/acoustic stopping material is lead sheet. It's high mass, it's highly dissipative, and has no significant resonances of its own because it's so NOT rigid. It used to be used for all kinds of sound damping things in buildings until lead-phobia hit the world. Plumbing supply houses sometimes still carry it. If you had some vinyl sheet (shower-pan liner, also plumbing supply houses) and some lead sheet, you could concoct a premium damping panel additive by (1) spraying some pressure-sensitive adhesive on the pan, then putting a sheet of vinyl or rubber sheet or foam on the pan, then (2) spraying some PSA on some lead sheet and pressing that onto the dissipative sheet.

The mass of the lead drops the frequency of resonance dramatically, usually to sub audio, and the dissipative nature of vinyl or rubber foam eats the excess energy in ringing, damping the ringing.

Of course, by using lead in any way, you're using a material known to the State of California to cause cancer and reproductive defects, and also a known human accumulative toxin. So if you do this, you probably should not eat, lick, or otherwise ingest any portion of your reverb pan, and once you're completely done with your amp in a few decades, you should take it to an environmentally safe hazardous materials disposal facility...

[bob p]

I agree that it's worth figuring out whether it's really the pan or not. Get some hifi RCA extension cords and move the pan outside the enclosure several feet and see if the feedback changes. If it does, then working on the pan will help. If it doesn't, it's not the pan, or not primarily the pan.

reading this made me think of the expression, "it won't pan out," but I'm resisting the temptation to go to far with a really bad pun.

chuck, how did your experiment go with the new production pans from MOD? just wondering if the MOD units panned out for you (there ! I said it!) or if the MOD pan is the one that's giving you resonance headaches.

[olddawg]

The rubber mounting grommets for the tanks in old amps get rock hard. You can find replacement grommets but I have always had better results just using a 1" thick piece of furniture foam and using longer screws. The down side is you have to replace the foam every couple of years.

[g1]

It matters where inside the cab we are mounted, sometimes it only takes an inch to the side...

Can't overemphasize this. Trial and error has shown me that the magnetic pull from the speaker can wreak havoc with the tank. You can get them to start oscillating on their own without any signal applied just due to proximity with the speaker magnet. So I would think even if not oscillating on it's own, the right proximity could put it "one the edge" of oscillation, just waiting for signal to start the feedback.
So as well as insulation from vibration, consider magnetic pull/fields from the speaker.

[trobbins]

I'd be guessing that, like microphonics of a tube, the moving mass of the tank chassis itself has to be isolated from the walls of the combo by a compliance that has a natural resonance below the audio range. The 'dacron' option discussed may well be the only path suggested that gives a suitably low natural resonant frequency. Imagine the tank sitting on a foam that you want to use - the foam is the spring - that system needs to have a resonant frequency below audio, such that audio frequency accelerations of the combo wall or floor or whatever the mounting is are attenuated by the spring system from getting to the tank chassis. I doubt many foams would allow a suitably low natural resonance. Measuring the compression of the 'spring' when the tank is attached is a simple way to determine the mechanical stiffness characteristic and hence natural frequency. Suspending a pan on say four soft springs that meet the resonance aim may be another path to take (but perhaps not too practical).

I can also see that in this situation any air vibration should also be suppressed from getting to the inner sprung sub-chassis and spring assemblies.

[Tom Phillips]

I’ll offer one additional thing that can cause feedback which is sagging reverb springs that are contacting the solid cover on the bottom of the tank. This can be caused by:
1) Stretched or otherwise “too long” reverb springs.
2) Misplaced or broken suspension springs.
3) A bottom cover that doesn’t have the spacer strips like Fender traditionally uses. Especially true if the cardboard used for the bottom cover is warped or otherwise pushed up into the interior of the tank when it is mounted or shoved into a tank bag.

Basically any situation that causes the springs to touch the bottom cover. (Keep in mind that they sag more as the ambient temperature rises.)

Tom

[Chuck H]

Wowzer! you guys really come through. I know I wasn't very specific about what I've done already, and much of this discussion I already have under control. Like isolating the problem, tank location and the physical principals at work. The damping material ideas are what I was after and I got that in spades. With a special nod to Enzo for emphasizing reduction of resonances in the tank itself. As mentioned, I started by using materials I had on hand with no luck. I'll bring materials for this specific problem next time I know I'll be where the amp is. Excellent gents.

Bob, this IS the MOD pan that I ordered. It's a three spring (six spring?) model. Which are more prone to this problem anyway. It's also in a 2X12 combo cab with a 60W amp. The reverb level can do full BF wash. So I'm not being gentle to the situation. I'll get this worked out. The good news is that the pan sounds pretty good. It doesn't have the splash of the old Accutronics tanks. But the reverb note is representative of the note going in, the frequency spectrum vibes in a good way with basic amp tone giving a very full effect and the decay is pleasant and works well with a variety of tones. One could always alter the recovery response to emphasize more splashy top end. Overall I'm giving MOD tanks the thumbs up.

[bob p]

thanks! I've been thinking about putting the 3 spring long version in my super twin reverb. it puts 180W into a 2x12 twin reverb cabinet, and I imagine that it's going to be a feedback machine, so i'll pay close attention to how you solve the feedback problem.

[ReadyTeddy]

If it in't heresy, has anyone tried the Belton units?
Results for Reverb

They sure sound easy to use, don't use much power, and are quite small.