Mark I feedback howl problem

[scelerat]

I have a Mark I combo. When the master and overall volume is high, there is a microphonic howling feedback which occurs, independent of whether an instrument is plugged in. It doesn't happen if I unplug the internal speaker and use an external cabinet.

I've tried swapping out preamp and power amp tubes, to no avail. The speaker trick makes me think this has something to do with internal vibrations. Any ideas?

 

[mark1406]

Does the amp have reverb?

My old Boogie had reverb and it use to feedback when I turned the reverb up. The springs in the reverb use to react with the speaker.

The howling, is the something new or has the amp always done it?

Mark