If your amp doesn't have a grounded plug, then this mod will make it both safer and quieter. I reused a computer cord. This hasn't completely isolated the amp from line (the heater circuit mentioned in previous references), but it should be safe enough to use.
A grounded AC plug (in the USA) has the following color codes:
Route the black wire through a fuse holder to the power switch. The white wire is connected directly to the transformer. Connect the green GRD to the chassis ground. This means screwed or riveted directly to the chassis--not just soldered to an existing grounded wire.
The fuse holder (and fuse) was added, not being OEM for this amp. So a 1/2 hole was drilled for mounting.
A larger rubber grommet replaced the original cord grommet. The 'stress relief' consists of two zip ties, and could be better...
Here's an in-depth explaination of the mod: http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html
For folks outside the US, the international power cord color codes:
--BROWN : Hot
--BLUE : Neutral
--GREEN/YELLOW STRIPE : Ground