Polytone Mini-brute Electric Buzz

[cmarks]

Hi all,
I bought a used Polytone Mini-brute that was in excellent condition at purchase.  I played an electric violin through it for several months before the problem developed.

At the point where I cross a certain low frequency on the instrument, the amp goes into some sort of feedback loop.  It emits a very loud, deep, erratic electric buzzing that sounds like a ray gun from a saturday morning cartoon.  Does anybody know what's going on?

[J M Fahey]

Post some MP3.
Does it vary with the controls (Volume, tone, etc.)
Does it disappear if you bump on the amp or knock some panel?