There should be a small allen screw on the bottom side of the knob you can tighten. The same thing happened to my Forte 44 and that fixed the problem
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The set screw (not necessarily allen head) should be in the knob to tighten it to the shaft. You should not need to open the case. If you find a small threaded hole, perpendicular to the shaft, then the screw fell out. If the allen screw is tight, then there may be a metal insert inside the knob, which is no longer gripping the inside of the knob. In that case, loosen the set screw, take the knob off, glue the insert into the knob and then refasten
I see the set screws you're talking about. However, it isn't just the knob which is loose. Or, rather, it isn't the knob--the knob is fastened securely to a metal piece which is supposed to be inside the preamp, presumably connected to a rheostat somehow. So the knob together with the metal column to which it is attached slides out.
I did find out that my output knob is unfastened--this will take one of the two remedies that Swampwalker mentioned.
Thanks--I very much appreciate the feedback.