Kustom K 100 Runaway hum in reverb caused by mechanical coupling to cab - fixes?

[lassoharp]

I just acquired a Kustom K-100 head that was missing the original reverb tank. I installed a replacement from MOD and the reverb itself works fine. However when I place the head on top of a speaker cab and turn the reverb up it's mechanically coupling to the speakers and gives a runaway hum problem. I don't have the original tuck and roll cab and have tried it on a couple different cabs I have and the nature of the hum does change slightly on different cabs, but it's still pretty bad. Makes me wonder if the original tuck and roll cab provided enough insulation to prevent this problem. Anyone else ever experience this issue? Any suggestions for preventing this?

[Leo_Gnardo]

If you don't have a spare swatch of good ol' Kustom tuck'n'roll, and who does, you could "nest" that reverb on a slice of foam rubber or similar material. I've used bubble wrap for instance.

In really bad cases I've "mummified" the tank with weatherstrip foam intended for mounting camper-caps to pickup trucks. Home Despot sells it, reasonably cheap for a roll, under $10. It's about 1.25 inches wide, 1/8 inch thick, long enough to treat 3 tanks. DO cover the bottom of the tank with corrugated cardboard or thin wood, then cover as much of the tank as you can with weatherstrip. Including the bottom cover.

[52 Bill]

How did you mount the new tank?

The original ones were mounted to two of the aluminum cross bars that tie the front and back sides of the chassis together. They used rubber bushings to isolate the metal tank from the cross bars. If you tighten the tank to the chassis there will be a direct vibration link from the chassis to the tank shell, which would cause the feedback that you are describing.

There was also a foam cover for the open top side of the tank and the flat metal side of the tank was dampened with a foam strip, similar to what Leo described.

[Enzo]

Standard techniques:

Not inside your Kustom head likely, but in most amps, we put the pan in a reverb bag.

Cut a piece of cardboard to cover the open side of the pan and tape it on. The pan should not be screwed tight to anything, it should float on those large gummy washers. Screwing it down tight allows vibration to pass from the amp chassis into the pan.

Got four short springs at the corners supporting the innards. Some times moving those a step inward to loosen them a little can help.

Sometimes the wide flat top surface of the pan can resonate, so we stick a stripe of self-adhesive foam weatherstripping down the center line to dampen such vibration.

[olddawg]

Or... Use a section of 1" thick upholstery foam and set the tank on it. Mount the tank and the foam to the bottom of the cab or wherever it fits with a couple of long screws.