FEEDBACK

[WALLS@xxxxxxxxxxxxx]

     I play through a Shure Green Bullet (Bingo) Microphone to a Fender
Bassman Amp hooked to a 12 inch speaker in a closed cabinet.  If I turn
the amp up above 3 on the 1 to 10 range, I get feedback.  This also
happens using a Shure 56 Microphone.  I have a little Fender Amp and
have tried other amps as well.  I always get feedback between a setting
of 3 and 4.  Guitars playing through the same amps can go right on
up to a louder setting with no feedback.  I've played with treble,
bass, and mixer settings to no avail.  Even though I keep away from
the front of the speaker, I guess playing with the acoustic microphone
is the culprit.  It is not that I need to be a lot louder, but it is
annoying to have to always be playing just at the upper limit.
     Does anybody have a suggestion as to how I can eliminate the
feedback?