Reverb Feedback

[Toxophilite]

Hi all
In one of my deluxe ab&63 builds I have an add on reverb circuit (AB763)It works great and occasionally I use it full bore as an effect
As I love JBLs, and clean headroom I popped a K120 into the amp
It's a NICE sounding speaker, if you like cleansHowever it has a GIANT alnico magnet and now the reverb wants to start a little upper mid-rangy feedback past about 4-5 on the dialThe tank is flat on the amps bottom and is in a double thickness naugahyde bag
From past experience mounting it vertically (on edge) makes the situation worse


Is there any other solutions to this sort of problem( besides ditching the JBL, which I'm enjoying)

[tubeswell]

Could need more acoustic damping. Cut a rectangle of corrugated-centre carton cardboard to cover the open side of the pan and then wrap the whole lot in bubble wrap before popping it back in the bag.

[Toxophilite]

Hmm I like the bubble wrap idea
Right now the tank is in one naugahyde bag(it's the thicker suff with a soft white lining) which is inside another bag of the same material. It has a 3/8" thick high density foam bottom (like those interlocking floor matts)


I should also say that the feedback happens all by it's lonesome
I don''t have to be playing guitar or have one plugged in
Past about 5 it slowly starts to howl and it's not a squeal, it's a mid range (maybe lower mids actually ) howl

[Willabe]

Could need more acoustic damping. Cut a rectangle of corrugated-centre carton cardboard to cover the open side of the pan and then wrap the whole lot in bubble wrap before popping it back in the bag.

All the old Fenders with reverb tanks had corrugated cardboard on the bottom of the verb tank.

Hmm I like the bubble wrap idea
Right now the tank is in one naugahyde bag(it's the thicker suff with a soft white lining) which is inside another bag of the same material. It has a 3/8" thick high density foam bottom (like those interlocking floor matts)

I've done some sound proofing for recording studios and it's very interesting how different materials 'decouple' differently at different frequencies.

You might think this material 'should' work, like 3/8" thick high density foam and get it done but the truth is, in the end, after trying different things what works.

What 'should' work and what does work, in the end might be different.


               Brad       

[Toxophilite]

I think it's the giant K120 magnet
If I slowly slide the reverb unit in it's bags out of the amp(with the bottom back panel off)
It stops howling when it's out from beneath the JBL...grr
It also wasn't howling with the D123 or the 12" Roa C12N copy I had in there
the K120 sounds pretty dam fine though


I'll experiment with other materials

[Toxophilite]

turning the tank upside down seems to stop the howl
however as it's too late to play at any volume in my apartment I can't tell if it's just helping by inhibiting the reverb

[Willabe]

turning the tank upside down seems to stop the howl
however as it's too late to play at any volume in my apartment I can't tell if it's just helping by inhibiting the reverb

1st, Volume matters with verb feed back.

There's 3 things;

1. Acoustic feed back.

2. Magnetic feed back/interference.

3. Verb tanks are made to be positioned a certain way. (It's the way they aline the springs weight at their connection to the verb transducer.)   
 
The verb tank is most sensitive at it's output. The input to the tank is fairly strong but the output of the tank is very, very low. So it could be that the magnetic field from the speaker is messing with it.

Fender amps with verb have always put the verb output opposite the PT, otherwise the amp had noise.

Try flipping the verb end to end in the cab.     

[Toxophilite]

I did try flipping it end to end. I had to switch the cables too of course as one of mine is shorter than the other
That didn't make a difference, though I appreciate the suggestion(thought my response was a little stiff sounding, apologies)


It was originally a vertical mounted tank though I had mounted it that way when I first added the reverb unit and it squealed like a stuck pig
That was with a Rola C12N type speakers with a big strong ceramic magnet
Laid flat with that speaker (or the D123) it didn't howl




I just remounted it vertically (to the back panel)and it seems to be okay with the K120....UNTIL I tilted it back on it's arms and then squeal!


I read somewhere that the transducer being loose affects the quality/noisiness of the reverb and a fellow was suggesting a drop of crazy glue...I have about 4 tanks and ALL the transducers are loose so I don't know about that idea
Anybody tried it??

[thermion]

I had this same problem in a pro reverb, applying dense self-adhesive foam weather stripping to both sides of the tank itself did the trick. 

[Toxophilite]

Upside seems to make a big difference, though it seems wrong


With the weather stripping foam was it applied to isolate the tank acoustically(maybe a couple strips down the length top and bottom or did you completely cover the tank?

[tubeswell]

Bubble wrap and corrugated cardboard on the pan works for me.


On combos I often use weather strip seal between the amp chassis and the cabinet, or sometimes also between the baffle and the cab, or between the back panels and the cab to kill some of the pathways for acoustic vibration getting back into the tubes.