Do you see anything in this circuit that could cause reverb ringing?

[mort]

Just built this up and got it running over the weekend. It sounds great and really happy with the over all sound. One problem though, if the reverb level is turned up to 3/4 or above it will slowly start ringing and get louder and louder until the reverb is turned down for a few seconds and then it goes back to normal as long as you stay below 3/4. Gonna try a new 12at7 this evening but thought I'd throw this out there and see if anyone has any circuit corrections/ideas to suggest. Thanks, as always, for looking.

[J M Fahey]

You are feeding back the reverb recovery signal into the reverb drive input.
You are using a simple passive mixer which gives it a "backwards" path.



Short simple kludge is to attenuate signal being fed back with a fixed resistive divider so "10" now becomes "6" or whateverīs necessary not to reach ringing levels.

Full solution is to do it the Fender way, cloning the full Reverb circuit, not just part of it.

[mort]

So you're saying that if the reverb doesn't start ringing until the pot gets below say 22k, then to put a 22k across the input and wiper to make sure the control never actually sees less than 22k?

What about placing a diode in series between the reverb output and the mixing resistor? Would that stop it from feeding back?

[Enzo]

Or it is acoustic feedback, not uncommon. Is this a combo amp? If so, pull the reverb pan out of the cabinet, and get it as far from the speaker as the cables allow. Does it still do it? If that stoped it, you have acoustic feedback. That gets my vote, as the 2.2M mixers are not far from the 3.3M Fender used.

Acoustic feedback just means the reverb sound out the speaker drives the springs. Unplug the drive cable from the INPUT jack on the pan, turn the reverb control to max. Does it still feed back?

Never screw the reverb pan down tight to the cabinet, always let it float. Screws in only far enough so it won't flop around loose.

In any case, the reverb pan should be in a vinyl bag - anyplace selling reverbs usually sells the bags. Cut a piece of cardboard to fit the bottom of the pan and tape it over the open side. These block the sound partly from the springs. SOmetimes just the position of the pan in the cab makes a difference. Try moving it around if you can. SOmetimes the flat metal top surface of the pan resonates, so we run a strip of adhesive foam insulation down the centerline. The inch wide foam strip you might seal around a garage door with.

[mort]

That sounds like good advice too, and I'll test for that.

A couple of notes: this circuit has been put into a gutted blues Jr cab & chassis and while the tank is a new/different one, it's the exact same size and placement as the blues Jr, although mine is a tube Reverb so it might be more sensitive to feedback(?). I did tighten the screws down firmly so I'll test that angle as well.

[g1]

Swapping in speakers with bigger magnets can also cause this issue. If so, in a cab that small you may not find room to move the tank out of the problem zone.

[mort]

I didn't mention it but it's also the same speaker [and transformers] as the Blues Jr.

Gonna test it out here shortly. It's good to be off work a little early and no one home but me. Rare treat around here.

[mort]

Enzo you were correct about the acoustic feedback. If I set the tank outside of the cabinet the ringing does not occur at full max on the reverb pot. Moving it to various distances from the speaker it's obvious that it's the close proximity of the tank to the speaker. Right now it's just screwed down with the foam strips between the pan and floor of the cabinet. Would the vinyl bag really help against a magnetic field?

[Leo_Gnardo]

Would the vinyl bag really help against a magnetic field?

A tank sock will help a little, so will closing the open side of the box with a sheet of corrugated cardboard. In extreme cases I've "mummified" reverb tanks with weatherstrip tape. Home Despot sells a roll for under $10 that's intended for pickup truck camper caps, to keep the wind from whistling thru between cap and truck body. It's about 1 1/4 inch wide, 1/8 inch thick, enough to cover 3 tanks front back and sides. Nothing exceeds like excess! I'm sure you can find some uses for the leftovers. If that doesn't do it completely, you can sandwich your mummified & be-socked tank between layers of foam rubber & mount that with a couple long screws to the "floor" of your combo amp. Yes sometimes it takes all that to get the infernal howling to stop. Awooooo!

[mort]

lol


In this case it begins so subtly and slowly that I suspect it won't take much to curb it out.

[J M Fahey]

Cool
And of course donīt

I did tighten the screws down firmly so I'll test that angle as well.

pan is separated from screws with oversize rubber grommets precisely to UN-couple it from cabinet walls.

[mort]

Well the cardboard and vinyl bag didn't quite do the trick so I'm gonna put a governor on the pot like Juan suggested. No matter, there's plenty of reverb to spare anyway. I'll just know that when I build this circuit in my own cabs to make the cab just a smidge larger, or if building it into a Blues Jr cabinet to buy a vertical mount tank and stick it to the side wall, farther away from the speaker.

[J M Fahey]

So you're saying that if the reverb doesn't start ringing until the pot gets below say 22k, then to put a 22k across the input and wiper to make sure the control never actually sees less than 22k?

Not exactly.
I suggest an extra resistor in series with Reverb return pot.
Start with 100k; adjust as needed.

Or try first with a 1M Log pot in series, connected as a variable resistor, and set it so you have max reverb possible without ringing or feedback, then substitute with a fixed resistor, the value you just found.

Fender Champ 12 drives speaker tank straightbfrom speaker out (with a series resistor), recovers signal with a FET and sends it back to preamp ... a sure firerecipe for disastrous feedback ... only they add hidden (to end user) volume trimmer in series with lanel reverb control, and instructions are: "set reverb to 10, trimmer to 0, rise until howling and then back down enough to guarantee not howling at any preamp setting."

Being a cheap bass tard I have used that myself a few times for quick and cheap reverb addiion to amps which did not have any.

What about placing a diode in series between the reverb output and the mixing resistor? Would that stop it from feeding back?

It will heavily distort the Audio signal so donīt.
keep attenuation resistive .

[Enzo]

Bag not to shield from magnetic field, it is there to block acoustic energy from the speaker cone.

[olddawg]

I always just used a half inch or inch thick pad of upulstery foam and used longer screws to slightly compress the pad with the tank.