Reverb Feedback

Bo Borland · Post by **Bo Borland** » 6 Feb 2000 10:02 am

I have a Peavey Session 400 mid 70's vintage with a JBL on board. My problem is a reverb feedback when I get it above 6 . I realize that those setting are a bit high, but I do not have a delay and I am trying for a wetter sound during practice.
Any suggestions ??

Jon Light (deceased) · Post by **Jon Light (deceased)** » 6 Feb 2000 10:26 am

I had some sever problems with my Session 400 Ltd. feeding back. I discovered that one of the screws holding the reverb down to the cabinet had torn loose and the tank was magnetically attached to the speaker!! It was hardly surprising that it was squealing like a pig. So I fixed it. And you know what? It still was feeding back at high volume/reverb levels. I pulled the tank and got a Boss RV-3. Even if the fix is simple, I just didn't want to deal with it.

J Fletcher · Post by **J Fletcher** » 7 Feb 2000 9:11 am

The reverb pan will feedback if it's not acoustically isolated from the speaker. That's why the pans are usually in a thick vinyl bag. If it's not in a bag, that's the problem. If it's in a bag, it may need to be loosened up a bit. If it's in there too tight, then you're not getting enough isolation from the speaker...Jerry

Frank Parish · Post by **Frank Parish** » 7 Feb 2000 10:12 am

Recently got an Evans SE 200 and had to replace the reverb pan. The reverb is weak next the session 400 and the more you turn it up the more it hums. What's this?

Steven Welborn · Post by **Steven Welborn** » 7 Feb 2000 10:47 am

A similar question to Franks': I recently acquired an Evans fet500HV and it's reverb is weak compared to my Webb's, though they both use identical(visibly) type accutronics units. I'm wondering if a replacement would cure that or, could failing amp circuitry affect how much reverb derived from unit?[This message was edited by Steven Welborn on 07 February 2000 at 10:50 AM.]

J Fletcher · Post by **J Fletcher** » 7 Feb 2000 12:08 pm

If the reverb is weak sounding it could be that the springs aren't being driven hard enough by the rev curcuit, or it could be that the reverb spring recovery stage isn't eq'd to your liking. To correct these problems would require modifying the amp. Could be a bad pan too, or maybe one of the springs has jarred loose in the pan. Worth looking inside the pan to see if that's the case...Jerry[This message was edited by J Fletcher on 07 February 2000 at 12:10 PM.]

Jack Stoner · Post by **Jack Stoner** » 7 Feb 2000 12:14 pm

The Accutronics reverb tanks (what most manufacturers use)all look alike physically, but there are several models, electronically (impedence). If someone replaced one and it had the wrong impedence it could be weak or not operate at all.

This was discussed in another thread (may have been lost with the system crash) but the fact the signal is a very low signal and then amplified, in the amp, accounts for a lot of the hum in reverb units.

There are other factors, such as bad interconnect cables (grounds lost, bad shielding, etc), placement of the reverb in relation to AC power sources, etc.

You have to take each case individually and try and determine what is causing it. What fixes one reverb hum problem may not be the problem in another reverb hum case.

Steven Welborn · Post by **Steven Welborn** » 7 Feb 2000 3:27 pm

Bo, I just discovered why Webb attaches a piece of cardboard box material to the open bottom of the reverb pan: not a dust cover, but a accoustic insulator, sandwiched between pan and cabinet. The bag was not enough. I made one for the Evans reverb pan and it solved the feedback problem. Just cut one out of brown cardboard using the pan shape and tie it to two kittycorners of bottom of pan...back in the bag and cabinet.
I doubled up the layers for good measure.

Bill Crook · Post by **Bill Crook** » 8 Feb 2000 6:11 am

Frank and Steve......

I had a problem like that once.
It seems that the cables to the reverb pan was pluged in reversed !! This efectily cut the output of the reverb unit in half.

The impedience of the input transducer is of a lower Q than the output transducer. ( If you measured the input and output of the reverb unit, there should be a reasonable high difference beween them.)

This would also explain the hum problem that is noticed when the volumne is turned up high. ( In the reverb section )

Hope this helps a bit.