Q. I have a book on tube-type amplifiers that talks about “motorboating.” What is this?
A. In tube-type amplifiers, when the filter capacitors get old they lose some of their capacitance and develop leakage between their plates. They no longer do as good a job of bypassing AC on the positive side of the power supply to ground. As a result, the supply voltage can sag when the output stages draw current. When they stop conducting the power supply voltage comes back up.
These changes in supply voltage are coupled back to the sensitive voltage amplifiers in the input stages of the amplifier. The result is a kind of oscillation that comes in bursts. In a speaker connected to the amplifier these bursts make a “putt-putt” noise that sounds very much like an old-time motorboat.