Reverb Oscillation

[PfeifferElectronics]

I have a new build of a blackface pro reverb. If I turn the reverb up past 4 or 5 it begins to oscillate. Ideas on what would be causing this?

Thanks

[olddawg]

I have a new build of a blackface pro reverb. If I turn the reverb up past 4 or 5 it begins to oscillate. Ideas on what would be causing this?

Thanks

How are you mounting the tank? Try using an RCA cord and barrel connectors to locate the pan outside of the cab temporarily. The tank should be loosely mounted on thick rubber grommets or on a piece of 1" foam. Otherwise it will become microphonic from the the vibrating cabinet.

[TimmyP1955]

It's important that the tank and the amp have the correct grounding scheme on the RCAs.

On the tank for an oldie Fender, the tank's input RCA should be floating, and the output RCA should be grounded to the tank (per the specs on the tank - the amp schematic does not detail this). Looking at an amp schematic, it appears that the send RCA is grounded to the chassis, and the return RCA is floating (it seems to me that this should not work - curious). However the schematic also shows that the shields of both cables are grounded. A 65RI schematic shows both amp RCAs to be grounded, which makes sense.

[The Dude]

Just got in a brand new Deluxe Reverb RI with the same problem. I've discovered that removing the tank from the chassis corrects the problem, relating primarily to the tank's proximity to the speaker. I believe the speaker magnet is causing the coils in the tank to vibrate, as I can stop it by holding my fingers on the coils. Further inspection shows that the coils are extremely loose and "floppy"- far more so than I remember in previous tanks I've had out. I've inserted some glue in between the coils and the core they're mounted on to stop them from vibrating. I'll report back on how it works after the glue dries.

[The Dude]

Amp fixed. The "glue-job" worked.

[Alexander]

The metal pan itself needs to be damped- they get to singin' all on their own...long strips of old inner-tube rubber super glued on helps dull the ringing of the tank.

[The Dude]

The tank had foam strips on the bottom and was enclosed in one of the typical Fender "baggies". It wasn't the pan that was singing in my case. It was the little transformers. As I said, I could stop the oscillation by touching the coils. They were extremely loose on the core. Hard to describe, but I've not seen this on Accutronics tanks before. Maybe a bad batch? Anyway, your pan tip is still a good one!

[Chuck H]

I've done about everything (other than Dude's coil snuggie) to stop vibration from getting to a reverb pan. Rubber strips, location adjustments, loose bagging, memory foam wrap, you name it!

Persistence is the key. Read and try until it works. My most difficult one was an independent reverb amp. A twelve watt tube amp in an 18"x22"x9" cabinet dedicated to reverb. It's enough reverb to give a full wet sound to a clipping Marshall Getting the three spring, long pan to stay stable up to full volume was a real grind.

My latest is a 60W twin twelve combo. I tried all my experienced moves and it still let's go a little (moans quietly at about 500Hz). I may need to try Dude's "touch the transducer" test.

A good tip I got along the way (but haven't used yet) is to sub in a three spring short tank. Supposedly sounds just great but a lot less prone to acoustic feedback. I've heard a few short three springers and they do sound good (not just good enough).

I think Belton makes a digital box that plugs right in where a pan would go. I'm game if it sounds good.

Lot's of options and stuff to try. The bad news is that reverb tanks/pans aren't getting any better and they're not as good as they were.

[pdf64]

There's a possibility it could be a bad cathode bypass cap on V4.
Unplug the tank and see if it still oscillates.

[Leo_Gnardo]

The metal pan itself needs to be damped- they get to singin' all on their own...long strips of old inner-tube rubber super glued on helps dull the ringing of the tank.

I've done similar, using foam tape meant for mounting camper tops on pickup trucks. It's about 1.5 in wide and 1/8 thick. Under $10 a roll at Home Despot, enough to mummify 3 tanks. Stick it on front back sides, every metal surface. Cover the open side of the tank with cardboard like Fender, then slide the whole meshugah into a tank "sock". If THAT doesn't do it, it's well and truly doomed.

There are tanks that just never sound good no matter what you do. And that was the case even 25 years ago when Accutronics was king.

[Jazz P Bass]

Amp fixed. The "glue-job" worked.

I check for 'floppy coils' on all repairs that have an Accutronics tank.
Reglue as needed.

If all of the above does not cure the ringing, sometimes it helps to put a piece of squishy foam directly underneath the springs.

[Randall]

I wish I could visualize the glue job, but I'm not understanding it exactly.

[Jazz P Bass]

Transducer image.



You do not want the transducer 'wiggling' on the laminated steel.
On the attached pic, the glue bead can be run along the plastic plate & the laminate. (right above where it says "coil")
You have to look at each transducer to decide where to bond it, as they are setup differently.

Good reading: http://www.oberlin.edu/staff/thinder...rb/reverb.html

[52 Bill]

The OP said the circuit was oscillating, I took it to mean whistle not tank feedback, but who knows.

[g1]

I've done about everything (other than Dude's coil snuggie) to stop vibration from getting to a reverb pan. Rubber strips, location adjustments, loose bagging, memory foam wrap, you name it!

One more thing you can try for tank feedback, the 4 springs that hold the inner pan to the outer chassis are adjustable. The support springs can be moved to different holes in the outer chassis. Sometimes you can make gains this way.