Still having cab issues

What amp are you using with this cab? How is your eq dialed in? What bass? Eq on that? Any pedals?

 

How many drivers does the cab have?
It could also be one of the lead-wires that run to the voice-coil, sometimes (with cheap) drivers they rattle against the cone.....

 

These rattles can sometimes drive you nuts. It could be the cab itself, the speaker or the way that it is mounted, the handles, the wiring harness, the jack plate, or even related to the casters. The problem needs to be broken down and each possibility looked at. A repair shop might be able to help as you've got someone with a new perspective that can assess it.

With older plywood cabs I've encountered loose internal laminations that require injecting a glue (I use CA) to stabilize the wood. Sometimes you can get noise at the joints. Again, a glue can help.

If the rattle is in the cab, playing through it to create the rattle and having someone applying pressure at various locations can help track it down. Take whatever you can out of the picture. If the grille frame can be removed, do so. Then continue your tests with what is left. Same for the castors, etc.

 

You can use an old piece of garden hose as a stethoscope, or take a trip to Harbor Freight and pick up the real deal for cheap.

If possible, hook the cab to your computer with a sound generator app (TrueRTA or similar) and find the frequency that creates the buzz.
Isolate with the stethoscope.

Trashed drivers will buzz, as will handles, grille's and cab panels themselves.
Any panel section that produces noticeable vibration to your touch, is wasting speaker energy.
The easy fix for flexing panels is gluing a pair of blocks on internal opposing sides where the buzz is occurring.
Once the blocks are in place, you can glue a brace to the blocks.
This method avoids having to make a precise cut of the brace to fit between the walls.
If you use a good sized brace, i.e. 1/2" x 3" width, you can Swiss-cheese the brace for weight reduction.

 

Maybe it is time for a new cabinet, how much time/money is a 20 year old cabinet worth to you? You can find used Ampeg cabinets all day long for dirt cheap, I would just upgrade if your getting this much of a headache. Time is worth money.

To each their own.

 

The problem may be with the baffle.

Check to see if the baffle is loose enough to vibrate. Some cabs have the baffle attached with screws, others are glued.

Next pill the speaker and take a look at the plies around the speaker cutout. Look for any voids or loose plies. If everything checks out, tap around the area where it is buzzing and listen for a void. It will sound hollow or different in comparison to where it is solid. It could also feel soft when you push.

 

Yes, the speaker has to be removed and the cabinet inspected from the inside for broken joints, failed glue, separated plywood. I would have been inside this cabinet with bright lights, my head, cameras, etc.

I worked on a customer's Super Reverb that had a rattle no one could find. After a lot of looking and testing I found the magnets had shifted on three of his four speakers. (Jensen alnico reissues.) I was able to use small wedges I made, to move two of the three magnets back into the right place and then secure them with industrial epoxy to stop the voice coil rub the shifted magnets had caused. One I couldn't save, so he had to buy one replacement. The rattle sounded exactly like a cabinet buzz. Two other techs had tried to fix this amp. I assume it was a bad run of speakers.

Don't discount the possibility of voice coil rub. It can sound like a cabinet problem.

I would fix it rather than replace the cabinet. I hate to see musical equipment discarded. But that's easy for me to say, as I don't have to pay anyone to repair my stuff.

There have been plenty of complaints about vibration problems with this cabinet.

http://www.talkbass.com/forum/f15/cabinet-vibration-ampeg-svt-15e-787524/